The circumventricular organs: An atlas of comparative anatomy and vascularization

B R A I N R E S E A R C H R E V I E W S 5 6 ( 2 0 0 7 ) 1 1 9 – 1 4 7

ava i l ab l e a t www.sc i enced i rec t . com

www.e l sev i e r. com/ loca te /b ra in res rev

Review

The circumventricular organs: An atlas of comparativeanatomy and vascularization

Henri M. Duvernoya,⁎, Pierre-Yves Risoldb

aUniversité de Franche-Comté, Faculté de Médecine et de Pharmacie, 12 Chemin des Relançons, 25000 Besançon, FrancebEA3922, IFR 133 France

A R T I C L E I N F O

⁎ Corresponding author. Tel.: +33 381 502 724E-mail address: henri.duvernoy@wanadoo

0165-0173/$ – see front matter © 2007 Elsevidoi:10.1016/j.brainresrev.2007.06.002

A B S T R A C T

Article history:Accepted 4 June 2007Available online 18 June 2007

The circumventricular organs are small sized structures lining the cavity of the thirdventricle (neurohypophysis, vascular organ of the lamina terminalis, subfornical organ,pineal gland and subcommissural organ) and of the fourth ventricle (area postrema). Theirparticular location in relation to the ventricular cavities is to be noted: the subfornical organ,the subcommissural organ and the area postrema are situated at the confluence betweenventricles while the neurohypophysis, the vascular organ of the lamina terminalis and thepineal gland line ventricular recesses. The main object of this work is to study the specificcharacteristics of the vascular architecture of these organs: their capillaries have a walldevoid of blood–brain barrier, as opposed to central capillaries. This particular arrangementallows direct exchange between the blood and the nervous tissue of these organs. This workis based on a unique set of histological preparations from 12 species of mammals and 5species of birds, and is taking the form of an atlas.

© 2007 Elsevier B.V. All rights reserved.

Keywords:Circumventricular organsVascularizationNeuroanatomyNeuroendocrinologyBlood–brain barrier

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1192. The neurohypophysis (Figs. 2–21) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1203. The vascular organ of the lamina terminalis (Organum vasculosum laminae terminalis—OVLT) (Figs. 22–40) . . . . 1214. The subfornical organ (SFO) (Figs. 41–51) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1225. The subcommissural organ (SCO) (Figs. 52–56). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1226. The pineal gland (epiphysis cerebri) (Figs. 52 and 56–61) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1227. The area postrema (Figs. 62–75) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1238. Discussion/conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

120

..fr (H.M. Duvernoy).

er B.V. All rights reserved.

mailto:[email protected]

http://dx.doi.org/10.1016/j.brainresrev.2007.06.002

120 B R A I N R E S E A R C H R E V I E W S 5 6 ( 2 0 0 7 ) 1 1 9 – 1 4 7

1. Introduction

The circumventricular organs (CVO) are peculiar brain structuresthat are located in the walls and often protrude in the lumen ofthe third and fourth ventricles (Hofer, 1958). No clear agreementcan be found in the literature on the number of these organs inmammals (Ganong, 2000; Hofer, 1958; Kuenzel and Van Tienho-ven, 1982; Mestres and Rascher, 1994; Mikami, 1975; Oldfield andMcKinley, 2004). However, they show no uniformity of structureor of known functions (Dall’Aglio et al., 2006; Oldfield andMcKinley, 2004; Tsuneki, 1986). In the present work, we consideras common distinctive criteria, their location with regard to theventricles, their vascular network formed of special capillarieswith fenestrated walls (Bouchaud et al., 1989; Petrov et al., 1994;Wislocki and King, 1936; Wislocki and Leduc, 1952). In theseorgans, the nervous tissue and the blood compartment aremoreeasily accessible to each other than in the central nervoussystem in general. According to these characteristics, six organswill be described here in the following order: the neurohypo-physis, the vascular organ of the lamina terminalis, thesubfornical organ, the subcommissural organ, the pineal glandand the area postrema (Fig. 1). The choroid plexuses oftenassumed to belong to this group will not be described here. If wetake into account their suspected functions, these circumven-tricular organs are often divided into sensitive organs (vascularorgan of the lamina terminalis, subfornical organ and areapostrema), and secretory organs (the neurohypophysis andpineal gland) (Cottrell and Ferguson, 2004; Johnson and Gross,1993; Kizer et al., 1976; McKinley et al., 1990; Weindl, 1973). Thesubcommissural organ which seems to have no fenestratedcapillaries has, at present, imprecise functions.

Several mammals and birds were studied here:

– MAMMALS (nomenclature fromNowak and Paradiso, 1983):Rodentia and Lagomorpha

Fig. 1 – Sagittal section of the brain (domestic cat). Thecircumventricular organs are in relation with the third (III V)and fourth (IV V) ventricles. 1—Neurohypophysis(1’—adenohypophysis); 2—Vascular organ of the laminaterminalis (organum vasculosum laminae terminalis OVLT);3—Subfornical organ (SFO) (3’—columns of fornix);4—Subcommissural organ (SCO) (4’—posterior commissure);5—Pineal gland; 6—Area postrema; 7—Optic chiasma;8—Anterior commissure; 9—Corpus callosum;10—Mammillary body; 11—Mesencephalon; 12—Pons;13—Medulla; 14—Cerebellum.

Common or Black bellied Hamster: Cricetus cricetusDomestic albino rat: Rattus norvegicus albinusGarden dormouse (Lerot): Eliomys quercinusMerion (Jird): Meriones crassusOld World or domestic rabbit: Oryctolagus cuniculusCarnivoraDomestic cat: Felis catusDomestic dog: Canis lupus familiarisOld World badger: Meles melesRed fox: Vulpes vulpesArtiodactyla (Suidae)Domestic pig: Sus scrofaPrimatesHuman: Homo sapiensVervet monkey: Cercopithecus pygerythrus

– BIRDS:Buzzard: Buteo buteoDomestic pigeon: Columba livia domesticaMagpie: Pica picaMuscovy duck: Carina moschata

All the circumventricular organs were not cited in eachspecies. The cat was chosen as a model because all CVO havebeen more accurately observed, but each organ is describedin several species belonging to several mammalian/birdorders.

The preparations presented here were obtained afterintravascular injection of India ink and plastic fluid (mercox).In certain cases, silver impregnation (Bodian) was used. Thehistological material studied here (except for more recentstudies in human) belong to a collection obtained in the1960s and not easily available (Duvernoy and Koritké, 1961,1963, 1964, 1965, 1968; Duvernoy et al., 1969a,b, 1972a,b, 2000;Duvernoy, 1971, 2006).

Several of the species examined in this review are nowprotected in Europe, and would now be difficult to study.

The aim of this work is to allow the readers to have anoverview of the circumventricular organs. Their main func-tions will only be cited here as they are the subject of manyresearches which are synthesized in the work of Oldfield andMcKinley (2004).

2. The neurohypophysis (Figs. 2–21)

(Domestic cat: Figs. 2–4; domestic rabbit: Figs. 5, 6; pig: Figs. 7,8; domestic dog: Figs. 9, 16; vervet monkey: Fig. 10; albino rat:Figs. 11–13; old world badger: Figs. 14, 15; duck: Figs. 17, 18;domestic pigeon: Fig. 19; magpie: Fig. 20; buzzard: Fig. 21).

The hypophysis is divided into two different parts accord-ing to their architecture and functions: the adenohypophysis(anterior lobe) and the neurohypophysis. Their relative size isvariable: for example, in birds, the neurohypophysis is verysmall by comparison with a far larger adenohypophysis.

Only the neurohypophysis belongs to the circumven-tricular organs as it originates from the floor of the thirdventricle.

The neurohypophysis is divided into a distal part, theneural lobe and a proximal part, the median eminencewith an external and an internal layer. An extension of the

121B R A I N R E S E A R C H R E V I E W S 5 6 ( 2 0 0 7 ) 1 1 9 – 1 4 7

third ventricle, the hypophysial recess, limits the medianeminence.

The morphology of the neurohypophysis varies consider-ably according to different species: in some cases, thehypophysial recess extends as far as the neural lobe which isthus in contact, as in the case of the median eminence, withthe ventricular cavity (cat, dog, pigeon and duck).

With regards to their nervous connections and vascular-ization, the two parts of the neurohypophysis show dissimilaraspects.

The nervous lobe is vascularized by an intense capillarynetwork characteristic of other circumventricular organs. Thiscapillary network receives hypothalamic neurohormones(vasopressin and ocytocin) by an important nervous tractoriginating in themagnocellular paraventricular and supraop-tic nuclei. This tract, the hypothalamoposthypophysial tract,courses through the median eminence to reach the bloodvessels of the nervous lobe.

The median eminence is divided into an internal layercontaining the above mentioned tract and an external layerreceiving the tuberohypophysial fibers from the hypothala-mus which carry the neurohormones towards the glandularcells of the adenohypophysis. This hormonal one-way trafficis possible because of a special vascular system: the hypophysialportal system. This system is composed of a primary plexusreceiving the hypothalamic neurohormones and portal ves-sels reaching a secondary plexus which in turn vascularizesthe endocrine cells of the adenohypophysis.

We shall only describe the primary plexus as it vascularizesthe median eminence that belongs to the circumventricularorgans. It consists in a superficial capillary network and a deepcapillary network.

The superficial capillary network covers the surface of themedian eminence. It is formed of capillaries anastomosed intoa dense mesh-like network. Some of these meshes penetratethe external layer of the median eminence to form the shortcapillary loops. The superficial network receives through thetuberohypophysial tract, the hypothalamic neurohormones.

The deep capillary network is composed of long capillaryloops. In themost typical cases, each capillary loop consists inan afferent branch stemming from the superficial network.The afferent branch crosses the external and internal layers ofthe median eminence to reach the top of the capillary loopwhich spreads out near the hypophysial recess. The efferentbranch of the loop, often of a large caliber, returns to thesuperficial plexus and then to the portal vessels. In manycases these typical loops are replaced by a large subependymalnetwork. The long capillary loops cross the external andinternal layers of the median eminence and are in contactwith the tuberohypophysial and hypothalamohypophysialtracts, but their functions are still obscure. The maincharacteristics of the deep capillary network are their relation-ships with the hypophysial recess. This fact suggestsexchanges between the blood of the portal system and theventricular cerebrospinal fluid (Talanti and Kivalo, 1961).However, this role of the deep capillary network (andespecially of the long capillary loops) is not well accepted atpresent time because the special ependymal layer may notallow these exchanges. If this is the case, special ependymalcells, the tanycites may establish the link between the

ventricular cavity and the median eminence (Assenmacher,1952; Benoit and Assenmacher, 1951; Green and Harris, 1947;Merchenthaler et al., 1982; Mezey et al., 1985; Page, 1986; Singhand Dominic, 1975; Talanti and Kivalo, 1961; Wislocki andKing, 1936; Xuereb et al., 1954).

3. The vascular organ of the lamina terminalis(Organum vasculosum laminae terminalis—OVLT)(Figs. 22–40)

(Domestic cat: Figs. 22–24; fox: Figs. 25, 26; domestic rabbit:Figs. 27, 28; domestic dog: Fig. 29; old world badger: Fig. 30; pig:Fig. 31; vervet monkey: Fig. 32; garden doormouse: Fig. 33;hamster: Fig. 34; human: Figs. 35–37; pigeon Figs. 38, 39; duck:Fig. 40).

The cavity of the third ventricle is rostrally limited by thelamina terminalis which extends from the anterior edge ofthe optic chiasma ventrally to the area of the anteriorcommissure dorsally. However, its dorsal extension variesconsiderably according to species: in rodents, it is limited tothe suprachiasmatic region whereas in birds it reaches theanterior commissure.

Transversely, the lamina terminalis is limited to a sagittalfold which protrudes in the ventricular cavity above the opticchiasma with which it delimits an extension of the thirdventricle, the supraoptic recess. For this reason, the laminaterminalis was called for a long time the supraoptic crest.

In the cat, the vascular organ of the lamina terminalis isorganized in a typical fashion. It consists of a superficialcapillary network which lines the deep fold of the laminaterminalis and looks like a sagittal lamina. The deepnetwork isformed of capillary loops issuing from the superficial network.In the cat, the top of these loops does not reach the ventricularcavity which is bordered with a fine subependymal network.Therefore, the superficial and deep networks of the laminaterminalis, bear a strong resemblance with the primary plexusof themedianeminence fromwhich they aremerely separatedby the optic chiasma. The typical feature of the vascular organof the lamina terminalis is that it is in relation with the sub-arachnoidal spaces of the prechiasmatic cistern through itssuperficial capillary network and with the ventricular cavitythrough its deep network.

The variations according to species depend on the mor-phology of the lamina terminalis: in small rodents, the OVLT isonly a limited superficial network but it is more developed inrabbits and even more in other mammals, but shows a max-imum development in birds. In human, the lamina terminalisis a fine flattened lamina. Therefore, the superficial capillarynetwork appears directly. Despite the thinness of the lamina, adeep network of the capillary loops can be observed.

The blood vessels of the OVLT bear the same features as inother circumventricular organs devoid of a blood–brainbarrier. Thus neurons and nervous fibers are in direct relationwith the blood borne substances. According to the level ofangiotensin II, it would seem that the OVLT plays a part incontrolling the release of vasopressin elaborated in thehypothalamic paraventricular and supraoptic nuclei andwould therefore act as an osmoregulator. Besides, fibers ofhypothalamic origin containing LHRH (Luteotrophin Hormone


Releasing Hormone) reach the OVLT which might thusparticipate in the control of the cyclic production of thehypophysial gonadotrope hormones (Barry, 1955; Behnsen,1927; Landas and Phillips, 1987; Larsen et al., 1991; Larsen andMikkelsen, 1995; Mergner, 1959; Mergner, 1961; Szabo, 1983;Wenger and Leonardelli, 1980; Yamaguchi et al., 1993).

4. The subfornical organ (SFO) (Figs. 41–51)

(Domestic cat: Figs. 41–44; old world badger: Fig. 45; domesticrabbit: Fig. 46; vervet monkey: Fig. 47; merion Fig. 48; gardendoormouse: Fig. 49; human: Figs. 50, 51).

The subfornical organ is a small median structure attachedto the inferior surface of the cerebral fornix (hence its name) atthe level of the origin of the right and left columns of thefornix. This is why it was called intercolumnar tubercle byPutnam (1922).

Its main feature is that it is situated at the meeting point ofthe lateral ventricles with the third ventricle. Its vascularstructure is similar to that of other circumventricular organs.In cat, the capillary network is composed of large intermingledcapillaries forming large meshes from which a few capillaryloops can be observed in the vicinity of the ventricular cavity.In the cat, the inferior or rostral pole of the SFO is vascularizedby a fine capillary network as opposed to the main network.

The capillary network is very similar in the differentspecies of mammals that we have studied. In our limitedcollection of bird samples, we have not been able to observe asimilar organ. However, several authors have mentioned asubseptal organ corresponding to the subfornical organ.

The functions of the subfornical organ are closely related tothose of the vascular organ of the lamina terminalis. It wouldseem that both of them belong to the primitive lamina term-inalis forming the anterior wall of the median prosencephalicvesicle and that they were separated by the development ofthe anterior commissure.

Thanks to the special permeability of capillaries, neurons inthe subfornical organs react to the level of plasmatic angio-tensin II. Most of the fibers issuing from these neurons reachthe paraventricular and supraoptic hypothalamic nuclei andthus regulate the release of vasopressin. Itwould seemthat thesubfornical organ induces the sensation of thirst caused bydehydration (Akert et al., 1961; Allen et al., 2000; Dellmann,1998; Dellmann and Simpson, 1979; Ferguson and Bains, 1997;Janssen and Stephan, 1956; Legait and Legait, 1958; Mark andFarmer, 1984; Miselis et al., 1979; Putnam, 1922; Schmid, 1995;Smith, 1898).

5. The subcommissural organ (SCO)(Figs. 52–56)

(Domestic cat: Figs. 52, 53; old world badger: Fig. 54; pig: Fig. 55;vervet monkey: Fig. 56; human: Fig. 57).

The subcommissural organ is composed of large cuboidalependymal cells which cover the anterior and inferior surfacesof the posterior commissure. It is situated above the opening ofthe cerebral aqueduct and lines the roof of this canal as far asthe recessus mesocoelicus. In the cat, a network of distended

and anastomosed capillaries extends between the ependymallayer and the posterior commissure. In this species, the capil-lary network is composed of numerous capillary loops, andhasthe same aspect as in the other circumventricular organs.However, it is now agreed that the vessels of the subcommis-sural organ have no fenestrated walls unlike in the othercircumventricular organs. Thus the blood–brain barrier isfunctional.

It is to be noticed that the intensity of its vascularizationvaries considerably according to species: in the cat but also inbadger and pig, there is a dense vascularization while inrodents, it is very limited. In monkeys and in human, thevascularization of the subcommissural organ is even absent.In birds the same organ has been described. Since it does notappear clearly on the preparations at our disposal, it will notbe described here.

The ependymal cells of this organ secrete a glycoproteinwhose condensation constitutes the Reissner’s fiber whichextends along the cerebral aqueduct as far as the fourthventricle. It is possible that this fiber facilitates the flow of thecerebrospinal fluid in this narrow aqueduct. The subcommis-sural organ has been credited with many other functionswithout definite evidence. Descriptions of these functionswhich are outside the limits of the present study, can be foundin the bibliography (Afifi, 1964; Bargmann and Schiebler, 1954;Gobron et al., 1999; Kumar and Kumar, 2000; McKinley et al.,1990; Meiniel, 2007; Oksche, 1961, 1969; Palkovits and Wetzig,1962; Rodriguez et al., 1987, 1992, 1998, 2001; Severs et al., 1987;Talanti, 1958).

6. The pineal gland (epiphysis cerebri) (Figs. 52and 56–61)

(Domestic cat: Fig. 52; vervet monkey: Fig. 56; human: Figs.57–61).

The pineal gland is often excluded from the group of thecircumventricular organs for several reasons: its action ismore endocrine in nature as the gland which is mainly com-posed of pinealocytes, elaborates melatonin; besides, therelations of the pineal gland with the ventricular cavity varyconsiderably according to species: thus, in birds and amphi-bians the pineal gland extends as far as the top of the skull. Anopposite argument would be that during the brain develop-ment, the pineal gland grows as a thickening of the ependymaof the posterior wall of the third ventricle and that the pinealcells migrate further in a dorsocaudal direction. Finally, as weshall see further, most of the capillaries in the pineal bear thesame aspect as in other circumventricular organs.

In the present work, the only species studied are cats,monkeys and human. In human, the pineal gland has anovoid shape, like a pine cone, and is in contact with thethird ventricle through two expansions of the posterior wallof the third ventricle, the pineal recess and the suprapinealrecess.

The vascular structure of the pineal gland is generallycomposed of large caliber capillaries anastomosed in a densenetwork. However, the fenestrated aspect of these capillariesand therefore the lack of a blood–brain barrier is not acceptedby all researchers.


In fact, the capillary network of the pineal gland variesaccording to species and even within the same species: forinstance, in human, the capillary network of the pineal glandcan be divided into several areas. In the central and the dorsalparts of the gland, the pinealocytes are assembled in lobules:each of them is surrounded with a network of large capillariesfrom which capillary loops identical with those of the othercircumventricular organs, stem and penetrate the center ofthe lobule.

On the other hand, in the inferior region of the gland, thepinealocytes show a diffuse lay-out and are vascularized bysmall caliber capillaries. Therefore, it may well happen that inthe same organ some vessels are devoid of a blood–brainbarrier while others are less permeable (Chunhabundit andSomana, 1991; Duvernoy et al., 2000; Fazio and Perria, 1940;Hodde, 1979; Kleiter and Lametschwandtner, 1995; Wislockiand Leduc, 1952).

7. The area postrema (Figs. 62–75)

(Domestic cat: Figs. 62, 63; domestic dog: Fig. 64; red fox: Figs.65–67; old world badger: Fig. 68; vervet monkey: Fig. 69;domestic rabbit: Fig. 70; human: Figs. 71–73; duck: Fig. 74;pigeon: Fig. 75).

The area postrema is located at the caudal end of the brain(hence its name) at the junction of the medulla and the spinalcord. Thus, it is the most distal of all the circumventricularorgans.

The area postrema consists in two right and left massesattached to the inferior angle of the floor of the fourth ven-tricle and linked together along the median line on the obex.

These two lateral masses are clearly visible in the cat thatwe have chosen as a model but also in most mammals withthe exception of rodents and lagomorphs where the areapostrema is often limited to a median mass attached to theobex.

In birds, the area postrema is highly developed as a longstrip of capillaries stretching from the obex on themedian lineto the lateral angles of the fourth ventricle. In birds, itsresemblance with the lamina terminalis has already beenpointed out.

Whatever its morphology, the area postrema is relatedwith the opening of the central canal in the fourth ventricle.

In human, the area postrema (already described by Retziusin 1896) is directly visible from the back through the aperturamedialis of Magendie which forms a hole in the tela choroideaof the fourth ventricle. Thus, the area postrema is connecteddirectly with the subarachnoidal medullocerebellar cistern.However this pattern seems to be limited to human.

The vascularization of the area postrema is typical of thatof circumventricular organs: its large caliber capillaries areanastomosed in a dense network with numerous capillaryloops. It is agreed that the vessels of the area postrema aredevoid of the blood–brain barrier, thus enabling blood bornesubstances to reach directly the neurons in the area postrema.

With the neighboring nucleus of the solitary tract andmotor dorsal vagal nucleus, the area postrema forms thedorsomedial medulla or vagal complex, an important area ofthe autonomic system which plays a part in cardiovascular

and respiratory regulation. The area postrema might also bethe origin of the emetic reflex which controls the vomitingcenter situated in the reticular formation of the medulla(Böhme, 1972; Brizzee and Neal, 1954; Cammermeyer, 1947;Dempsey, 1973; Faraci et al., 1989; Ferguson and Bains, 1997;Gross, 1991; Joy, 1971; Larsen and Mikkelsen, 1995; Leonhardt,1980; Leslie and Gwyn, 1984; Miller and Leslie, 1994; Miselis etal., 1987; Moll and Hilvering, 1951; Morato et al., 1959;Porzionato et al., 2005; Retzius, 1896; Shapiro and Miselis,1985; Srinivasan et al., 1993; Sun and Spyer, 1991; Van DerKooy and Koda, 1983; Wilson, 1906; Wislocki and Putnam,1924).

8. Discussion/conclusion

This review, which takes the form of an atlas, is based onancient works published in often poorly accessible and oftennon-English journals, and concerns the gross morphology ofthe circumventricular organs, their capillary network andtheir relation with the ventricular compartment. Its aim wasto reassess these data in the light of recent publications moreconcerned with known functions of these organs (Anderssonet al., 1995; Bourque and Oliet, 1997; Catt, 1995; Cottrell andFerguson, 2004; Goren et al., 2006; Johnson and Gross, 1993;Oldfield and McKinley, 2004; Vigh et al., 2004). Two classes ofcircumventricular organs are now described.

The sensory circumventricular organs which are sensibleto blood-borne substances; because of the special permeabilityof capillaries, molecules may directly affect neurons incircumventricular organs that in turn innervate specificnervous centers. The area postrema and the SFO are twomembers of this class of circumventricular organs.

The secretory circumventricular organs are recipients of anabundant innervation that discharges neurohormones intothe blood stream, again because of the special permeability ofcapillaries. The main example is the neurohypophysis.

However, several circumventricular organs do not utterlyfit in this classification: for example the OVLT might be a“double” organ as it is sensible to circulating molecules, butalso receives a dense terminal input from the hypothalamusthat reach the capillary bed (Barry, 1955; Oldfield andMcKinley, 2004). However, the secretory functions of theOVLT are not yet fully ascertained. Another example is theSCO that is still a very poorly understood structure, but manyauthors exclude it from the circumventricular organs alto-gether because it does not have fenestrated capillaries. Themain characteristic of this organ is that it produces theReissner’s fiber. Thus, the secretory product from the SCOflows in the ventricle and not in the vascular system.

The current thinking about circumventricular organs andcerebrospinal fluid (CSF) is that they do not affect each other.This notion is sustained by two facts: first the ependymawhich covers the circumventricular organs is less permeablethan the ependyma of the surrounding nervous tissue; theependymocytes over circumventricular organs are bound bytight junctions (Oldfield and McKinley, 2004). Then, as theblood–brain barrier is not present in the circumventricularorgans, it is hypothesized that the ependyma form a barrier toisolate the ventricular compartment from these organs


(blood–CSF barrier). Another point is that the CSF circulationdue to the systolic pressure is of a very low amplitude withinthe ventricles, which does not fit well with a theory based on adiffusion of substances from the circumventricular organsinto the CSF (Baledent et al., 2001; Enzmann and Pelc, 1991;Greitz et al., 1992; Henry-Feugeas et al., 1993).

However, from our observations in the brains of severalmammals and birds, we cannot rule out the possibility thatcircumventricular organs may develop strong functional linkswith the ventricular compartment. The first of these dataconcerns the special position of the circumventricular organswith regard to the ventricular cavities. On this criterion, cir-cumventricular organs can be divided into two groups: someare associated with ventricular recesses, like the OVLT withsupraoptic recess and the posterior pituitary in the bottom ofthe hypophyseal recess. Others are strategically located inrelation to the ventricular junction: the SFO in the third ven-tricle between the two foramen communicating with thelateral ventricles, the SCO at the opening of themesencephalicaqueduct, and the area postrema in the fourth ventricle at theopening of the central canal. The second evidence resultingfrom the observation of our material is that the blood vesselsand capillaries loops are always oriented toward the ventri-cular cavities, often coming almost up to the ventricularsurface. These observations corroborate the hypothesis thatspecific molecular or physical interactions can take placebetween the circumventricular organs and the CSF compart-ment. Several data from the literature also support this hypo-thesis. We have seen that tight junctions between ependy-mocytes at the level of the circumventricular organs providethe basis of the concept of blood–CSF barrier. However, theluminal surface of circumventricular organs is specialized as itis not ciliated (unlike surrounding ventricular surface), butoften exhibits microvilli. For example microvilli are describedat the surface of the area postrema and SFO (Klara and Brizzee,1975, 1977; Phillips et al., 1974). The choroid plexus whichproduces the CSF, presents many similar anatomical arrange-ments. In addition to being largely folded, this organ is alsocovered by microvilli on the ventricular surface, and is alsocharacterized by a blood–CSF barrier and fenestrated capil-laries. However, experimental data that would show a pu-tative secretory activity of other circumventricular organs inthe CSF is lacking, except for the SCO. Very few data tend toshow that information may flow from the CSF to circumven-tricular organs. It may be pointed out that the lamina term-inalis (including the OVLT and SFO)may have an essential rolein the response to changes in the concentrations of Na+ andANG II in the CSF (May et al., 2000; Watson et al., 2004). Fur-thermore, electrical stimulation of the area postrema reducesblood flow in the choroids plexus, but leaves cerebral bloodflow unchanged (Williams et al., 1991). If proven to be true,these interactions are the indication that some circumven-tricular organsmonitor the CSF composition. The secretion bythe choroid plexus of the CSF is controlled by branches fromthe vagal nerve or from the superior cervical ganglion (Lindvallet al., 1978; Lindvall and Owman, 1981; Tamamaki and Nojyo,1987). SFO, OVLT, and area postrema are involved in well-established circuits connecting the hypothalamus, the dorsalvagal complex and preganglionic neurons in the intermediatecolumns of the spinal cord. Then, these circuits may very well

control through polysynaptic or neuroendocrine pathways thesecretion of the CSF by the choroid plexus. Thus, parallel to thecircuits involved in the control of blood osmolarity andpressure, circumventricular organs and hypothalamus couldbe involved in pathways controlling the CSF osmolarity andpressure. In this context, it is critical to intensify research oncircumventricular organs that are in a good position tomonitor blood and CSF composition.

Acknowledgments

The authors thank G. Brezard for editing the manuscript,M. Gaudron and Dr G. Viennet for their tremendous help inthe illustrations, and Dr C. Colard for her decisivesecretarial assistance.

R E F E R E N C E S

Afifi, A.K., 1964. The subcommissural organ of the camel. J. Comp.Neurol. 123, 139–145.

Akert, K., Potter, H.D., Anderson, J.W., 1961. The subfornical organin mammals. J. Comp. Neurol. 116, 1–13.

Allen, A.M., Oldfield, B.J., Giles, M.E., Paxinos, G., McKinley, M.J.,Mendelsohn, F.A.D., 2000. Localization of angiotensin receptorsin the nervous system. In: Quiriou, R., Bjorklund, A., Hokfelt, T.(Eds.), Handbook of Chemical Neuroanatomy, vol. 16. ElsevierScience, Amsterdam, pp. 79–124.

Andersson, B., Eriksson, S., Rundgren, M., 1995. Angiotensin andthe brain. Acta Physiol. Scand. 155, 117–125.

Assenmacher, I., 1952. La vascularisation du complexehypophysaire chez le canard domestique. Arch. Anat. Microsc.Morphol. Exp. 41, 69–152.

Baledent, O., Henry-Feugeas, M.C., Idy-Peretti, I., 2001.Cerebrospinal fluid dynamics and relation with blood flow. Amagnetic resonance study with semiautomatedcerebrovascular fluid segmentation investigative. Radiology36, 368–377.

Bargmann, W., Schiebler, T.H., 1954. Histologische undcytochemische Untersuchungen am subkommissural Organvon Saügern. Z. Zellforsch. 37, 583–596.

Barry, J., 1955. Les voies extra-hypophysaires de la neurosécrétiondiencéphalique. C. R. Assoc. Anat. 89, 264–276.

Behnsen, G., 1927. Uber die Farbstoffspeicherung im Zentralnervensystem der weissen Maus in verschiedenenAlterszustanden. Z. Zellforsch. 4, 515–572.

Benoit, J., Assenmacher, I., 1951. Etude préliminaire de lavascularisation de l’appareil hypophysaire du canarddomestique. Arch. Anat. Microsc. Exp. 40, 27–47.

Böhme, G., 1972. Untersuchungen an der Area postrema vonGallus domesticus: II. Das morphologische Problem derExistenz einer Bluthirnschranke. Acta Neuropathol. 21,308–315.

Bouchaud, C.C., Le Bert, M.M., Dupouey, P.P., 1989. Are closecontacts between astrocytes and endothelial cells aprerequisite condition of a blood–brain barrier? The ratsubfornical organ as an example. Biol. Cell 67, 159–165.

Bourque, C.W., Oliet, S.H.R., 1997. Osmoreceptors in the centralnervous system. Annu. Rev. Physiol. 59, 601–619.

Brizzee, K.R., Neal, L.M., 1954. A re-evaluation of the cellularmorphology of the area postrema in view of recent evidence fora chemoreceptor function. J. Comp. Neurol. 100, 41–61.

Cammermeyer, J., 1947. Is the human area postrema aneurovegetative nucleus? Acta Anat. (Basel) 2, 294–320.


Catt, K.I., 1995. Angiotensin II receptors. In: Robertson, J.I.S.,Nichols, M.G. (Eds.), The Renin–Angiotensin System:Biochemistry, Physiology, Pathophysiology, Therapeutics, vol.1. Gower Medical Publishers, London, pp. 12.1–12.4 (chap. 12).

Chunhabundit, P., Somana, R., 1991. Scanning electronmicroscopic study on pineal vascularization of the commontree shrew (Tupaia glis). J. Pineal Res. 10, 59–64.

Cottrell, G.T., Ferguson, A.V., 2004. Sensory circumventricularorgans: central roles in integrated autonomic regulation. Regul.Pept. 117, 11–23.

Dall’Aglio, C., Ceccarelli, P., Pascucci, L., Brecchia, G., Boiti, C., 2006.Receptors for leptin and estrogen in the subcommissural organof rabbits are differentially modulated by fasting. Brain Res.1124, 62–69.

Dellmann, H.D., 1998. Structure of the subfornical organ: a review.Microsc. Res. Tech. 41, 85–97.

Dellmann, H.D., Simpson, J.B., 1979. The subfornical organ. Int.Rev. Cytol. 58, 333–421.

Dempsey, E.W., 1973. Neural and vascular ultrastructure of thearea postrema in the rat. J. Comp. Neurol. 150, 177–200.

Duvernoy, H., 1971. The vascular architecture of the medianeminence. In: Knigge, K.M., Scott, D.E., Weindl, A. (Eds.),Brain–Endocrine Interaction. Median Eminence. Structure andFunction, Int. Symp. Karger, Munich, pp. 77–108.

Duvernoy, H.M., 2006. Comments on the microvascularization ofthe brain. Cerebrovasc. Dis. 21, 423–424.

Duvernoy, H., Koritké, J.G., 1961. L’organe vasculaire de la lameterminale et ses relations avec la substance nerveuse. C. R.Assoc. Anat. 47, 294–318.

Duvernoy, H., Koritké, J.G., 1963. Les fibres nerveuses de la lameterminale et leurs rapports avec l’organe vasculaire de la lameterminale. C. R. Assoc. Anat. 48, 562–576.

Duvernoy, H., Koritké, J.G., 1964. Contribution à l’étude del’angioarchitecture des organes circumventriculaires. Arch.Biol. (Liège) 75, 849–904.

Duvernoy, H., Koritké, J.G., 1965. Recherches sur la vascularisationde l’organe subfornical. J. Med. Besançon 2, 115–130.

Duvernoy, H., Koritké, J.G., 1968. Les vaisseaux sous-épendymairesdu recessus hypophysaire. J. Hirnforsch. 10, 228–245.

Duvernoy, H., Gainet, F., Koritké, J.G., 1969a. Sur la vascularisationde l’hypophyse des oiseaux. J. Neurovisc. Relat. 31, 109–127.

Duvernoy, H., Koritké, J.G., Monnier, G., 1969b. Sur lavascularisation de la lame terminale humaine. Z. Zellforsch.102, 49–77.

Duvernoy, H., Koritké, J.G., Monnier, G., 1972a. Sur lavascularisation du tuber postérieur chez l’homme et sur lesrelations vasculaires tubéro-hypophysaires. J. Neurovis. Relat.32, 112–142.

Duvernoy, H., Koritké, J.G., Monnier, G., Jacquet, G., 1972b. Sur lavascularisation de l’area postrema et de la face postérieure dubulbe chez l’homme. Z. Anat. Entwicklungsgesch. 138, 41–66.

Duvernoy, H., Parratte, B., Tatu, L., Vuillier, F., 2000. The humanpineal gland: relationships with surrounding structures andblood supply. Neurol. Res. 22, 747–789.

Enzmann, D.R., Pelc, N.J., 1991. Normal flow patterns ofintracranial and spinal cerebrospinal fluid defined withphase-contrast cine MR imaging. Radiology 178, 467–474.

Faraci, F.M., Choi, J., Baumbach, G.L., Mayhan, W.G., Heistad, D.D.,1989. Microcirculation of the area postrema. Permeability andvascular responses. Circ. Res. 65, 417–425.

Fazio, C., Perria, L., 1940. Primo contributo allo studio dellavasculorizzazione della pineale dell’uomo. Riv. Patol. Nerv.Ment. 56, 79–104.

Ferguson, A.M., Bains, J.S., 1997. Electrophysiology of thecircumventricular organs. Front. Neuroendocrinol. 17, 440–475.

Ganong, W.F., 2000. Circumventricular organs: definition and rolein the regulation of endocrine and autonomic function. Clin.Exp. Pharmacol. Physiol. 27, 422–427.

Gobron, S., Creveaux, I., Meiniel, R., Didier, R., Dastugue, B.,

Meiniel, A., 1999. SCO-spondin is evolutionarily conserved inthe central nervous system of the chordate phylum.Neuroscience 88, 655–664.

Goren, O., Adorjan, I., Kalman, M., 2006. Heterogeneous occurrenceof aquaporin-4 in the ependyma and in the circumventricularorgans in rat and chicken. Anat. Embryol. 211, 155–172.

Green, J.D., Harris, G.W., 1947. The neurovascular links betweenneurohypophysis and adenohypophysis. J. Endocr. 5, 136–146.

Greitz, D., Wirestam, R., Frank, A., Nordell, B., Thomsen, C.,Stahlberg, F., 1992. Pulsatile brain movement and associatedhydrodynamics studied by magnetic resonance phaseimaging. The Monro–Kellie doctrine revisited. Neuroradiology34, 370–380.

Gross, P.M., 1991. Morphology and physiology of capillary systemsin subregions of the subfornical organ and area postrema. Can.J. Physiol. Pharmacol. 69, 1010–1025.

Henry-Feugeas, M.C., Idy-Peretti, I., Blanchet, B., Hassine, D.,Zannoli, G., Schouman-Claeys, E., 1993. Temporal and spatialassessment of normal cerebrospinal fluid dynamics with MRimaging. Magn. Reson. Imaging 11, 1107–1118.

Hodde, K.C., 1979. The vascularization of the rat pineal organ. In:Ariens Kappers, J., Pevet, P. (Eds.), The Pineal Gland ofVertebrates Including Man. North Holland Biomedical Press,Elsevier.

Hofer, H., 1958. Zur morphologie der circumventriculären Organedes Zwischenhirnes der Säugetiere. Verh. Dtsch. Zool. Ges.202–251.

Janssen, P., Stephan, H., 1956. Recherches sur le cerveau del’élephant d’Afrique: III. L’organe subfornical. Acta Neur. Psych.Belgica 11, 789–812.

Johnson, A.K., Gross, P.M., 1993. Sensory circumventricular organsand brain homeostatic pathways. FASEB J. 7, 678–686.

Joy, M.D., 1971. The intramedullary connections of the areapostrema involved in the central cardiovascular response toangiotensin II. Clin. Sci. 41, 89–100.

Kizer, J.S., Palkovits, M., Brownstein, M.J., 1976. Releasing factors inthe circumventricular organs in the rat brain. Endocrinology98, 311–317.

Klara, P.M., Brizzee, K.R., 1975. The ultrastructural morphology ofthe squirrel monkey area postrema. Cell Tissue Res. 160,315–326.

Klara, P.M., Brizzee, K.R., 1977. Ultrastructure of the feline areapostrema. J. Comp. Neurol. 72, 409–431.

Kleiter, N., Lametschwandtner, A., 1995. Microvascularization ofthe pineal gland in the freshwater turtle, Pseudemys scriptaelegans (reptilian): a scanning electron microscopic study ofvascular corrosion casts. J. Pineal Res. 19, 93–102.

Kuenzel, W.J., Van Tienhoven, A., 1982. Nomenclature andlocation of avian hypothalamic nuclei and circumventricularorgans. J. Comp. Neurol. 206, 293–313.

Kumar, P., Kumar, S., 2000. The ultrastructure of thesubcommissural organ ependyma of the goat. Anat. Histol.Embryol. 29, 307–311.

Landas, S., Phillips, M.I., 1987. Comparative of the organumvasculosum of the lamina terminalis. In: Gross, P. (Ed.),Circumventricular Organs and Body Fluids, vol. 1. CRC Press,Boca Raton, pp. 131–156.

Larsen, P.J., Mikkelsen, J.D., 1995. Functional identification ofcentral afferent projections conveying information of acute“stress” to the hypothalamic paraventricular nucleus.J. Neurosci. 15, 2609–2627.

Larsen, P.J., Moller, M., Mikkelsen, J.D., 1991. Efferent projectionsfrom the periventricular and median parvicellular subnuclei ofthe hypothalamic paraventricular nucleus tocircumventricular organs of the rat: a Phaseolusvulgaris-leucoagglutinin (Pha-L) tracing study. J. Comp. Neurol.306, 462–479.

Legait, H., Legait, E., 1958. Paraphyse et organe subfornical dans lasérie des vertébrés. C. R. Assoc. Anat. 99, 427–435.


Leonhardt, H., 1980. Ependym and circumventricular organ. In:Nervensystem. In: Neuroglia, I., Oksche, A. (Ed.), Handbuch derMicroscopischen Anatomie des Menschen, vol. 4. Springer,Heidelberg, pp. 177–666.

Leslie, R.A., Gwyn, D.G., 1984. Neuronal connections of the areapostrema. Fed. Proc. 43, 2941–2943.

Lindvall, M., Owman, C., 1981. Autonomic nerves in themammalian choroid plexus and their influence on theformation of cerebrospinal fluid. J. Cereb. Blood Flow Metab. 1,245–266.

Lindvall, M., Edvinsson, L., Owman, C., 1978. Sympathetic nervouscontrol of cerebrospinal fluid production from the choroidplexus. Science 201, 176–178.

Mark, M.H., Farmer, P.M., 1984. The human subfornical organ: ananatomic and ultrastructural study. Ann. Clin. Lab. Sci. 14,427–442.

May, C.N., McAllen, R.M., Mckinley, M.J., 2000. Renal nerveinhibition by central NaCl and ANG II is abolished by lesions ofthe lamina terminalis. Am. J. Physiol.: Integr. Comp. Physiol.279, R1827–R1833.

McKinley, M.J., McAllen, R.M., Mendelsohn, F.A.O., Allen, A.M.,Chai, S.Y., Oldfield, B.J., 1990. Circumventricular organs.Neuroendocrine interface between the brain and the hemalmilieu. Front. Neuroendocrinol. 11, 91–127.

Meiniel, A., 2007. The secretory ependymal cells of thesubcommissural organ: which role in hydrocephalus? Int. J.Biochem. Cell Biol. 39, 463–468.

Merchenthaler, I., Vigh, S., Petrusz, P., Schally, A.V., 1982.Immunocytochemical localization of corticotropin-releasingfactor (CRF) in the rat brain. Am. J. Anat. 165, 385–396.

Mergner, H., 1959. Untersuchungen am Organon VasculosumLaminae Terminalis im Gehirn einiger Nagetiere. Zool. Jahrb.(Jena) 77, 290–355.

Mergner, H., 1961. Die Blutversorgung der Lamina terminalis beieinigen Affen. Z. Wiss. Zool. 165, 140–185.

Mestres, P., Rascher, K., 1994. The ventricular system of the pigeonbrain: a scanning electronmicroscope study. J. Anat. 184, 35–58.

Mezey, E., Kiss, J.Z., Mueller, G.P., Eskay, R., O’Donohue, T.L.,Palkovits, M., 1985. Distribution of pro-opiomelanocortinderived peptides, adenocorticotropic hormone, α-melanocytestimulating hormone, and β-endorphin (ACTH, α-MSH,β-END) in the rat hypothalamus. Brain Res. 328,341–347.

Mikami, S., 1975. A correlative ultrastructural analysis of theependymal cells of the third ventricle of japonese quail,Coturnix coturnix japonica. In: Knigge, D., Scott, E., Weindl, A.(Eds.), Brain–Endocrine Interaction: II. The Ventricular System.Karger, Basel, pp. 80–93.

Miller, A.M., Leslie, R.A., 1994. The area postrema and vomiting.Front. Neuroendocrinol. 15, 301–320.

Miselis, R.R., Shapiro, R.E., Hand, P.J., 1979. Subfornical organefferents to neural systems for the control of body water.Science 205, 1022–1025.

Miselis, R.R., Shapiro, R.E., Hyde, T.M., 1987. The area postrema. In:Gross, P. (Ed.), Circumventricular Organs and Body Fluids, vol.11. CRC Press, Boca Raton, FL, pp. 185–207.

Moll, J., Hilvering, C., 1951. An area postrema in birds? Proc. K. Ned.Akad. Wet., Ser. C 54, 301–307.

Morato, M.J.X., Teixeira, I., Texeira-Pinto, A.A., 1959. Nouvellesrecherches sur les aspects morphologiques de l’area postremachez les oiseaux et les mammifères. C. R. Assoc. Anat. 45,575–580.

Nowak, R.M., Paradiso, J.L., 1983. Walker’s Mammals of the World,4th edition. The Johns Hopkins Univ. Press, Baltimore andLondon. 2 vol.

Oksche, A., 1961. Vergleichende Untersuchungen über diesekretorische Activität der Subkommissuralorgans und dergliacharacter seiner Zellen. Z. Zellforsch. Mikrosk. Anat. 54,549–612.

Oksche, A., 1969. The subcommissural organ. J. Neurovisc. Relat.Suppl. 9, 111–139.

Oldfield, B.J., McKinley, M.J., 2004. Circumventricular organs, In:Paxinos, G. (Ed.), The Rat Nervous System, third edition.Elsevier, Amsterdam, pp. 389–406.

Page, R.B., 1986. The pituitary portal system. In: Granten, D., Pfaff,D. (Eds.), Current Topics in Neuroendocrinology, vol. 7.Springer-Verlag, Berlin, pp. 1–47.

Palkovits, M., Wetzig, H., 1962. Untersuchungen uber dasSubcommissuralorgan und den Recessus mesocoelicus bei derSturmmöve Larus canus L. Z. Mikrosk. -Anat. Forsch. 68, 612–626.

Petrov, T., Howarth, A.G., Krukoff, T.L., Stevenson, B.R., 1994.Distribution of the tight junction-associated protein ZO-1 incircumventricular organs of the CNS. Brain Res. Mol. Brain Res.31, 235–246.

Phillips, M.I., Balhorn, L., Leavitt, M., Hoffman, W., 1974. Scanningelectron microscope study of the rat subfornical organ. BrainRes. 80, 95–110.

Porzionato, A., Macchi, V., Morsut, L., Parentia De Caro, R., 2005.Microvascular pattern in human medullary tegmentum at thelevel of the area postrema. J. Anat. 206, 405–410.

Putnam, T.J., 1922. The intercolumnar tubercle: an undescribedarea in the anterior wall of the third ventricle. Bull. JohnHopkins Hosp. 38, 181–182.

Retzius, G., 1896. Das Meschenhirn. Norsted, P.A. and Söner,Stockholm.

Rodriguez, E.M., Oksche, A., Rodriguez, S., Hein, S., Peruzzo, B.,Schoebitz, K., Herrer, H., 1987. The subcommissural organ andReissner’s fiber: fine structure and cytochemistry. In: Gross, P.(Ed.), Circumventricular Organs and Body Fluids, vol. II. CRCPress, Boca Raton, FL, pp. 3–41.

Rodriguez, E.M., Oksche, A., Hein, S., Yulis, C.R., 1992. Cell biologyof the subcommissural organ. Int. Rev. Cytol. 135, 39–121.

Rodriguez, E.M., Rodriguez, S., Hein, S., 1998. The subcommissuralorgan. Microsc. Res. Tech. 41, 98–123.

Rodriguez, E.M., Oksche, A., Montecinos, H., 2001. Humansubcommissural organ with particular emphasis on itssecretory activity during fetal life. Microsc. Res. Tech. 52,573–590.

Schmid, H.A., 1995. Regional differences in the blood–brain-barrierof the subfornical organs of the rats and ducks (Anasplatyrhynchos). J. Hirnforsch. 36, 565–574.

Severs, W.B., Dundore, R.L., Balaban, C.D., 1987. Thesubcommissural organ and Reissner’s fiber: fine structure andcytochemistry. In: Gross, P. (Ed.), Circumventricular Organsand Body Fluids, vol. II. CRC Press, Boca Raton, FL, pp. 43–68.

Shapiro, R.E., Miselis, R.R., 1985. The central neural connectionsof the area postrema of the rat. J. Comp. Neurol. 234,344–364.

Singh, K.B., Dominic, C.J., 1975. Anterior and posterior groups ofportal vessels in the avian pituitary: incidence in forty ninespecies. Arch. Anat. Microsc. Morphol. Exp. 64, 359–374.

Smith, G.E., 1898. Further observations upon the fornix, withspecial reference to the brain of Nyctophilus. J. Anat. Physiol.(London) 32, 231–246.

Srinivasan, M., Bongianni, F., Fontana, G.A., Pantaleo, T., 1993.Respiratory responses to electrical and chemical stimulation ofthe area postrema in the rabbit. J. Physiol. 463, 409–420.

Sun, M.K., Spyer, K.M., 1991. GABA-mediated inhibition ofmedullary vasomotor neurons by area postrema stimulation inrats. J. Physiol. 436, 669–684.

Szabo, K., 1983. The vascular architecture of the developingorganum vasculosum of the lamina terminalis (OVLT) in therat. Cell Tissue Res. 233, 579–592.

Talanti, S., 1958. Studies on the subcommissural organ in somedomestic animals, with reference to secretory phenomena.Ann. Med. Exp. Biol. Fenn. 36, 1–97.

Talanti, S., Kivalo, E., 1961. The infundibular recess in the brain ofCamelus dromedarius with particular reference to its


neurosecretory pathways into the third ventricle. Experientia470, 1–4.

Tamamaki, N., Nojyo, Y., 1987. Intracranial trajectories ofsympathetic nerve fibers originating in the superior cervicalganglion in the rat:WGA-HRP anterograde labeling study. BrainRes. 437, 387–392.

Tsuneki, K., 1986. A survey of occurrence of about seventeencircumventricular organs in brains of various vertebrates withspecial reference to lower groups. J. Hirnforsch. 27, 441–470.

Van Der Kooy, D., Koda, L.Y., 1983. Organization of the projectionsof a circumventricular organ: the area postrema in rat. J. Comp.Neurol. 219, 328–338.

Vigh, B., Manzano e Silva, M.J., Franck, C., Racz, G., Lukats, A., Szel,A., 2004. The circumventricular organs of the brain: do theyrepresent a cerebrospinal fluid-dependent regulatory system?Med. Hypotheses Res. I, 77–100.

Watson, A.M.D., Mogulkoc, R., McAllen, R.M., May, C.N., 2004.Stimulation of cardiac sympathetic nerve activity by centralangiotensinergic mechanisms in conscious sheep. Am. J.Physiol.: Integr. Comp. Physiol. 286, R1051–R1056.

Weindl, A., 1973. Neuroendocrine aspects of the circumventricularorgans. In: Ganong, W.F., Martini, L. (Eds.), Frontiers inNeuroendocrinology. Oxford Univ. Press, London, pp. 3–32.

Wenger, T., Leonardelli, J., 1980. Circadian and cyclic LHRHvariations in the organum vasculosum of the lamina

terminalis of female and male rats. Neuroendocrinology 31,331–337.

Williams, J.L., Thebert, M.M., Schalk, K.A., Heistad, D.D., 1991.Stimulation of area decreases blood flow to choroid plexus.Am. J. Physiol. 260, H902–H908.

Wilson, J.T., 1906. On the anatomy of the calamus region in thehuman bulb; with an account of a hitherto undescribed“nucleus postremus”. J. Anat. Physiol. 40, 210–241; 40, 357–386.

Wislocki, G.B., Leduc, E.H., 1952. Vital staining of thehematoencephalic barrier by silver nitrate and trypan blue,and cytological comparisons of the neurohypophysis, pinealbody, area postrema, intercolumnar tubercle and supraopticcrest. J. Comp. Neurol. 96, 371–413.

Wislocki, G.B., King, L.S., 1936. The permeability of thehypophysis and hypothalamus to vital dyes, with a studyof the hypophyseal vascular supply. Am. J. Anat. 58,421–472.

Wislocki, G.B., Putnam, T.J., 1924. Further observations on theanatomy and physiology of the area postrema. Anat. Rec. 27,151–156.

Xuereb, G.B., Prichard, M.M.L., Daniel, P.M., 1954. The hypophysialportal system of vessels in man. Q. J. Exp. Physiol. 39, 219–230.

Yamaguchi, K., Morimoto, A., Murakami, N., 1993. Organumvasculosum lamina terminalis (OVLT) in rabbit and rat:topographic studies. J. Comp. Neurol. 330, 352–362.

Fig. 2 – Sagittal section of the hypophysis (domestic cat). Thehypophysis is composed of the neurohypophysis (1) and theadenohypophysis (anterior lobe, 1’). The neurohypophysis,themain circumventricular organ, may be divided, accordingto its functions and vascularization into two parts: 2—theneural lobe (distal neurohyphysis) and 3—the medianeminence (proximal neurohypophysis); 4—the hypophysialrecess, an expansion of the third ventricle, is limited in mostspecies to the median eminence but can extend as far as theneural lobe as in the cat. The neural lobe receives fibers fromthe paraventricular and supraoptic nuclei. Through this path,vasopressin elaborated in these nuclei may reach the largeand dense capillaries (5) of this lobe supplied by inferiorhypophysial arteries and veins (6). The median eminencereceives fibers from several hypothalamic nuclei whichsynthesize the releasing and inhibiting hormones. Thesehormones reach and control the glandular cells of theanterior lobe (1’) by way of a special vascular system: thehypophysial portal system. This system supplied by superiorhypophysial arteries (7), is made up of a primary plexus (8)drained by portal vessels (9) to a secondary plexus (10) intothe anterior lobe (10’ venous drainage of the anterior lobe).The primary plexus is composed of a superficial network (11)covering the median eminence and a deep network withinthemedian eminence. The short capillary loops (12) belong tothe superficial network. The deep network contains the longcapillary loops (13). The top of the long capillary loopsreaches the ventricular cavity (see Fig. 4), a characteristic ofthe circumventricular organs.

Fig. 3 – Sagittal section of the hypophysis after intravascularinjection corresponding to the drawing in Fig. 2 (domestic cat,bar: 2 mm). 1—Neural lobe; 1’—anterior lobe; 2—medianeminence with capillary loops; 3—hypophysial recess. Thearrow explains the endoventricular view in Fig. 4.

Fig. 4 – Endoventricular view of the floor of the hypophysialrecess, domestic cat (bar: 2 mm). The tops of the longcapillary loops are in relation with the ventricular cavity.


Fig. 5 – Section of the hypophysis, domestic rabbit (bar:1.5 mm). 1—Neural lobe; 2—anterior lobe; 3—medianeminence; 4—vascular organ of the lamina terminalis;5—optic chiasma; 6—hypothalamic paraventricular nucleus;7—mammillary body.

Fig. 6 – Sagittal section of the median eminence, domesticrabbit (bar: 400 μm). 1—Long capillary loops of the primaryplexus; 2—hypophysial recess; 3—anterior lobe.

Fig. 7 – Sagittal section of the hypophysis, pig (bar: 1 mm).1—Neural lobe; 2—median eminence; 3—anterior lobe;4—third ventricle; 5—mammillary body; 6—Optic chisma.

Fig. 8 – Sagittal section of the median eminence, pig (bar:285 μm). The primary plexus of the portal system iscomposed of: 1—superficial network; 2—short capillaryloops; 3—long capillary loops; 4—portal vessels;5—hypophysial recess.


Fig. 9 – Sagittal section of the hypophysis, domestic dog (bar:1 mm). 1—Neural lobe; 2—median eminence; 3—anteriorlobe; 4—third ventricle (hypophysial recess); 5—mammillarybody.

Fig. 10 – Sagittal section of the hypophysis, vervet monkey(bar: 1.3 mm). 1—Neural lobe; 2—median eminence;3—anterior lobe; 4—hypophysial recess; 5—optic chiasma;6—posterior tuber; 7—mammillary body; 8—pons.

Fig. 11 – Sagittal section of the hypophysis, albino rat (bar:350 μm). 1—Neural lobe; 2—median eminence with capillaryloops; 3—anterior lobe; 4—portal vessels; 5—hypophysialrecess; 6—mammillary body. The arrow A explains theventral view in Fig. 12 and the arrow B the endoventricularview in Fig. 13.

Fig. 12 – Ventral view of the hypophysis according to thearrow A in Fig. 11, albino rat (bar: 270 μm). Vascular injectionwith resin (mercox) and scanning electron microscope view.1—Superficial network of the primary plexus of the portalsystem (note its “coat of mail” aspect); 2—portal vessels;2′—lateral portal vessels; 3—secondary plexus of the portalsystem within the anterior lobe; 4—superior hypophysialarteries.


Fig. 13 – Endoventricular view of the median eminenceaccording to the arrow B in Fig. 11, showing the top of thecapillary loops. Vascular injection with resin (Mercox) andscanning electron microscope view, albino rat (bar: 46 μm).

Fig. 14 – Sagittal section of the median eminence, old worldbadger (bar: 350μm). This view shows the high density of thelong capillary loops in the median eminence. 1—Medianeminence; 2—long capillary loop; 3—hypophysial recess;4—anterior lobe.

Fig. 15 – Enlargement of Fig. 14, old world badger (bar:130 μm). This view shows in more detail the structure of along capillary loop: 1—stem of the long capillary loop; 2—topof the loop. Its long course is in relationwith the hypophysialrecess (3); 4—intermediary branch within the medianeminence. In this species, the short capillary loops aredifficult to identify (5).

Fig. 16 – Structure of a long capillary loop, domestic dog (bar:115 μm). 1—Afferent ascending branch; 2—top of the loop inrelation with the hypophysial recess (3); 4—large efferentdescending branch towards the anterior lobe (5);6—intermediary branch of the loop.


Fig. 17 – Sagittal section of the third ventricle (1), duck (bar:1.75mm). 2—Neural lobe of the hypophysis; 3—anterior lobe;4—portal vessels; 5—hypophysial recess; 6—optic chiasma;7—vascular organ of the lamina terminalis; 7’—its extensioninto the suspected subseptal organ; 8—anterior commissure;9—posterior commissure; 10—cerebellum;11—mesencephalon; 12—paraventricular organ.

Fig. 18 – Enlargement of Fig. 17, duck (bar: 520 μm).1—Neural lobe; 2—anterior lobe; 3—superficial capillarynetwork of the primary plexus of the portal system coveringthe median eminence; 4—median eminence. In the majorityof birds, the median eminence is thin and almost devoid ofdeep vascular network; 5—portal vessels reaching thesecondary vascular plexus in the anterior lobe (2) by anindependent and clearcut path; 6—Hypophysial recess.

Fig. 19 – Sagittal section of the hypophysis, pigeon (bar:330 μm). 1—Neural lobe; 2—anterior lobe; 3—superficialcapillary network of the primary plexus of the portal systemlining the median eminence (4); 5—the portal vessels form adistinct pedicle reaching the secondary plexus in the anteriorlobe; 6—hypophysial recess; 7—optic chiasma.

Fig. 20 – Sagittal section of the hypophysis, magpie (bar:1 mm). 1—Neural lobe; 2—anterior lobe; 3—the medianeminence is, as opposed to the preceding species (duck andpigeon) vascularized by vascular capillary loops belonging tothe primary vascular plexus of the portal system;4—hypophysial recess; 5—optic chiasma.


Fig. 21 – Sagittal section of the hypophysis, buzzard (bar:600μm). 1—Neural lobe; 2—anterior lobe, note the reduced sizeof the neural lobe in comparison to the anterior lobe, as in theother birds studied; 3—hypophysial recess; 4—optic chiasma.

Fig. 22 – Drawing of a sagittal section of the vascular organ ofthe lamina terminalis in the domestic cat. 1—the laminaterminalis is a thin structure fixed to the anterior border of theoptic chiasma (2). It forms the anterior wall of the thirdventricle (3). In cat, the lamina terminalis overhangs thesuperior aspect of the optic chiasma and delimits thesupraoptic recess (4), a small extension of the third ventricle.The lamina terminalis contains the vascular organ of thelamina terminalis. The vascular architecture of this organlooks inmany aspects like that of themedian eminence of thehypophysis: 5—superficial capillary network in relation to theprechiasmatic subarachnoid cistern (6); 7—deep capillarynetwork with capillary loops. In cat, these loops do not reachthe ventricular cavity which is lined by a subependymalvascular network (8); 9—arterial supply; 10—venous drainage.

Fig. 23 – Sagittal section of the vascular organ of the laminaterminalis, intravascular injection corresponding to thedrawing in Fig. 22, domestic cat (bar: 350 μm). 1—Vascularorgan of the lamina terminalis; 2—optic chiasma; 3—thirdventricle; 4—supraoptic recess; 5—superficial capillarynetwork; 6—prechiasmatic subarachnoid cistern; 7—deepcapillary network with capillary loops; 8—subependymalvascular network.


Fig. 24 – Sagittal section of the vascular organ of the laminaterminalis in the domestic cat. Enlargement of Fig. 23 (bar:140 μm). 1—Superficial capillary network; 2—deep capillarynetwork with capillary loops; 3—subependymal network;4—third ventricle.

Fig. 25 – Sagittal section of the vascular organ of the laminaterminalis, red fox (bar: 290 μm). 1—Superficial capillarynetwork; 2—deep capillary network. In this species, thewell-developed capillary loops extend throughout the laminaterminalis; 3—third ventricle.

Fig. 26 – Sagittal section of the vascular organ of the laminaterminalis, red fox (bar: 25 μm). Intravascular injection andsilver impregnation (Bodian). 1—A small bundle of nervousfibers is in close relationship with the capillary loops (2)belonging to the deep capillary network.

Fig. 27 – Sagittal section of the vascular organ of the laminaterminalis, domestic rabbit (bar: 220 μm). 1—Superficialcapillary network; 2—deep capillary network (capillaryloops); 3—third ventricle; 4—optic chiasma; 5—blood vesselsin the prechiasmatic subarachnoid cistern.


Fig. 28 – Sagittal section of the vascular organ of the laminaterminalis, domestic rabbit (bar: 77 μm). 1—Superficialcapillary network; 2—deep capillary network: the largecapillary loops protrude into the ventricular cavity (3)(arrows); 4—supraoptic recess; 5—optic chiasma.

Fig. 29 – Sagittal section of the vascular organ of the laminaterminalis, domestic dog (bar: 125 μm). 1—Superficialcapillary network; 2—deep capillary network with manycapillary loops; 3—third ventricle.

Fig. 30 – Sagittal section of the vascular organ of the laminaterminalis, old world badger (bar: 160 μm). 1—Superficialcapillary network; 2—the deep capillary network iscomposed of complex capillary chains; 3—third ventricle.

Fig. 31 – Sagittal section of the vascular organ of the laminaterminalis, pig (bar: 160 μm). 1—Superficial capillarynetwork; 2—the deep capillary network is devoid of capillaryloops and limited to a subependymal network; 3—thirdventricle; 4—supraoptic recess.


Fig. 32 – Sagittal section of the vascular organ of the laminaterminalis (1), vervet monkey (bar: 135 μm). 2—Superficialcapillary network; 3—subependymal network; 4—thirdventricle.

Fig. 33 – Sagittal section of the vascular organ of the laminaterminalis, garden doormouse (bar: 90 μm). In small rodents,the vascular organ of the lamina terminalis is reduced to asuperficial capillary network (1). The deep capillary networkshows only few and small capillary loops (2). 3—Thirdventricle; 4—optic chiasma.

Fig. 34 – Sagittal section of the vascular organ of the laminaterminalis, hamster (bar: 133 μm). 1—Superficial vascularnetwork; 2—deep vascular network with small vascularloops; 3—third ventricle; 4—optic chiasma.


Fig. 35 – (a) Frontal view of the lamina terminalis in human (bar: 3.2 mm) after removal of blood vessels and pial lamina.1—Lamina terminalis with a triangular aspect; 2—the vascular organ forms a slight thickening in the center of the laminaterminalis (dotted line); 3—optic chiasma; 4—optic nerves. (b) Frontal view of the lamina terminalis in human (bar: 2 mm) seenfrom the prechiasmatic subarachnoid cistern. 1—Superficial capillary network of the vascular organ of the lamina terminalis;2—anterior cerebral arteries; 3—anterior communicating artery; 4—anterior cerebral vein; 5—optic chiasma; 6—optic nerve;7—hypophysial stalk.

Fig. 36 – Drawing of the deep capillary network of the laminaterminalis in human. The deep capillary network of thevascular organ of the lamina terminalis forms a capillary ring(1) from which capillary loops stem (2) oriented toward thecenter of the ring.

Fig. 37 – Lamina terminalis, human (bar: 200 μm). The deepcapillary network of the vascular organ inside the laminaterminalis, is seen in transparency after clearing.1—Capillary ring; 2—capillary loops.


Fig. 38 – Lamina terminalis, pigeon (bar: 330 μm). In birds,the lamina terminalis is a very long structure (1). It forms asagittal fold. This fold is covered by an extensive superficialcapillary network (2). This plexus extends as far as theanterior aspect of the anterior commissure (3). From thissuperficial network, stem small capillary loops forming in thepigeon a reduced deep plexus (4). 5—Supraoptic recess;6—left lateral side of the third ventricle.

Fig. 39 – Lamina terminalis, pigeon (bar: 100 μm).Enlargement of a segment of the lamina terminalis visible inFig. 38. 1—Superficial capillary network of the vascular organof the lamina terminalis; 2—deep capillary network withsmall capillary loops; 3—third ventricle.

Fig. 40 – Lamina terminalis (1), duck (bar: 100 μm). In ducks,the vascular organ of the lamina terminalis shows a typicalaspect as in mammals: 2—superficial capillary network. Thedeep capillary network is made up of capillary loops (3) andsubependymal vessels (4) lining the third ventricle cavity (5).

Fig. 41 – Sagittal section of the subfornical organ, domesticcat (bar: 235 μm). 1—The subfornical organ is a smallrounded structurewhich protrudes into the third ventricle (2).The subfornical organ is fixed to the origin of the right and leftcolumns of the fornix (3). 4—Choroid plexus of the thirdventricle.


Fig. 42 – Sagittal section of the subfornical organ, domesticcat (bar: 100 μm). Enlargement of Fig. 41 showing the densityof the large and sinusoidal capillary network in the superiorand middle part of the organ (1) whereas in its inferior part,the vascular network is composed of fine and sparsecapillaries (2). 3—Third ventricle.

Fig. 43 – Horizontal (axial) section of the subfornical organ,domestic cat (bar: 210 μm). The subfornical organ (1) forms aprotusion at the junction between the right and left lateralventricles (2) with the median third ventricle (3). Note itsdense capillary network in comparison with that of theneighboring structures.

Fig. 44 – Endoventricular view of the subfornical organ,domestic cat (bar: 190 μm). The subfornical organ forms arounded protrusion (1) fixed to the fornix (2) and bordered bythe left and right interventricular foramina with choroidplexuses (3).

Fig. 45 – Sagittal section of the subfornical organ, old worldbadger (bar: 135 μm). 1—Subfornical organ; 2—choroidplexus of the third ventricle; 3—fornix.

Fig. 46 – Sagittal section of the subfornical organ, domesticrabbit (bar: 125μm). 1—Subfornical organ; 2—fornix; 3—thirdventricle.


Fig. 47 – Sagittal section of the subfornical organ, vervetmonkey (bar: 220μm). Inmonkey, the subfornical organ (1) ismuch reduced in size. 2—Choroid plexus; 3—fornix.

Fig. 48 – Sagittal section of the subfornical organ, merion(bar: 83 μm). The subfornical organ (1) is a well-developedstructure. The choroid plexuses (2) are fixed on its superiorpole. 3—Third ventricle; 4—fornix.

Fig. 49 – Sagittal section of the subfornical organ, gardendoormouse (bar: 80 μm). 1—Subfornical organ; 2—thirdventricle; 3—fornix.

Fig. 50 – Coronal (frontal) section of the subfornical organ,human (bar: 250 μm). The subfornical organ (1) forms aprotusion into the third ventricle (2). Its vascular network ismade up of sinusoidal capillaries organized in large meshes(3). The most superficial capillaries are seen on anendoventricular view in the following figure (Fig. 51).


Fig. 51 – Endoventricular view of the subfornical (1), human(bar: 570 μm). Some capillaries of the subfornical organ areseen through the ventricular ependyma (2). 3—Fornix;4—choroid plexus in the ventricular foramen.

Fig. 52 – Sagittal section of the pineal gland andsubcommissural organ, domestic cat (bar: 330 μm). 1—Pinealgland, note its intense vascular network; 2—subcommissuralorgan; 3—posterior commissure; 4—third ventricle.

Fig. 53 – Subcommissural organ, enlargement of Fig. 52,domestic cat (bar: 45 μm). 1—The subcommissural organ is athickening of the ependymal layer. In the cat, this organ ischaracterized by a rich vascular network with sinusoidcapillaries and capillary loops (2); 3—posterior commissure;4—third ventricle.

Fig. 54 – Sagittal section of the subcommissural organ, oldworld badger (bar: 136 μm). 1—Subcommissural organ;2—posterior commissure; 3—recessus mesocoelicus;4—third ventricle; 5—pineal gland.


Fig. 55 – Sagittal section of the subcommissural organ, pig(bar: 530μm). In the pig, the posterior commissure is a foldedstructure (1) and the subcommissural organ iswell developedwith a rich vascular network (2). 3—Recessus mesocoelicus;4—third ventricle.

Fig. 56 – Sagittal section of the pineal gland andsubcommissural organ, vervet monkey (bar: 365 μm).1—Pineal gland; 2—habenular commissure; 3—posteriorcommissure; 4—subcommissural organ; 5—third ventricle;5’—pineal recess; 5’’—suprapineal recess.

Fig. 57 – Sagittal section of the pineal gland andsubcommissural organ, human (bar: 1.6 mm). The pinealgland (1)may be divided according to its vascular architectureinto pineal lobules (2), homogeneous pineal tissue (3) andcalcareous concretions (4). 5—Habenular commissure;6—subcommissural organ; 7—posterior commissure;8—recessus mesocoelicus; 9—third ventricle; 10—pinealrecess; 11—suprapineal recess.

Fig. 58 – Frontal (coronal) section of the pineal gland (1),human (bar: 1.7 mm). 2—Each pineal lobule is surrounded bya dense capillary network (see Fig. 59); 3—posteriorcommissure.


Fig. 59 – Section of a pineal lobule (1), human (bar: 180 μm).A pineal lobule is surrounded by peripheral large capillaries(2) and centered by capillary loops (3) similar to thosefound in other circumventricular organs.

Fig. 60 – Superior view of the pineal gland. Intravascularresin injection (mercox) and scanning electromicroscopeview, human (bar: 1.2 mm). 1—The pineal gland has an ovoidshape like a pine cone. Its dense vascular network isbordered by two right and left lateral pineal veins (2).

Fig. 61 – Axial section of the head, human (bar: 2.3 cm). Thepineal gland (1) is situated in the center of the brain andcommunicates with the third ventricle (2). These earlyobservations were the basis of Descartes’ theory (1654) thatdescribed this gland as the seat of the soul (see Duvernoy etal., 2000). 3—Frontal lobe; 4—temporal lobe; 5—occipital lobe;6—hippocampus; 7—thalamus; 8—lentiform nucleus;9—caudate nucleus; 10—frontal horn of the lateral ventricle;11—temporal horn of the lateral ventricle.

Fig. 62 – Endoventricular view of the area postrema,domestic cat (bar: 450 μm). 1—Left and right lateral parts ofthe area postrema linked together on the obex (2); 3—floor ofthe fourth ventricle.


Fig. 63 – Drawing of a transverse section of the right lateralpart of the area postrema, domestic cat (1). 2—Dense capillarynetwork; 3—subependymal capillary loops; 4—arterialsupply; 5—deep venous drainage; 6—fourth ventricle; 7—telachoroidea of the fourth ventricle; 8—fine capillary network inthe medulla.

Fig. 64 – Endoventricular view of the area postrema,domestic dog (bar: 520 μm). The two lateral parts of the areapostrema are well developed (1) with a dense and tortuouscapillary network (2); 3—floor of the fourth ventricle; 4—obex.

Fig. 65 – Endoventricular view of the area postrema, red fox(bar: 365 μm). The left and the right lateral parts of the areapostrema (1) are joined together on the obex (2). Note thedense vascular network of the area postrema in comparisonwith that of the floor of the fourth ventricle (3).

Fig. 66 – Transverse section of the area postrema, red fox(bar: 310 μm). The two lateral parts of the area postrema (1)protrude into the cavity of the fourth ventricle (2); 3—telachoroidea of the fourth ventricle; 4—medulla.


Fig. 67 – Transverse section of the right lateral part of thearea postrema, red fox (bar: 110 μm). The area postrema isrichly vascularized by a dense network composed of largediameter and anastomosed capillaries (1). A narrowavascular zone (2) separates the area postrema from thesubjacent medulla (3) (note its fine capillary network).4—Fourth ventricle.

Fig. 68 – Median sagittal section of the area postrema, oldworld badger (bar: 400 μm). As in fox (Fig. 64) the two lateralparts of the area postrema are linked together by a medianpart on the obex. This median part (1) is visible in thissection, overhanging the aperture of the central canal (2) intothe cavity of the fourth ventricle (3). Note the high density ofthe capillary network of the area postrema in comparisonwith that of the medulla (4) and cerebellum (5).

Fig. 69 – Endoventricular view of the area postrema, vervetmonkey (bar: 660 μm). 1—Lateral parts of the area postrema;2—obex; 3—aperture of the central canal; 4—floor of thefourth ventricle.

Fig. 70 – Endoventricular view of the area postrema,domestic rabbit (bar: 400 μm). As in rodents, the areapostrema in the rabbit (1) is limited to a central part fixed onthe obex (2) at the inferior angle of the fourth ventricle (3).


Fig. 71 – Posterior view of the medulla (after removal of thecerebellum and the arterial network), human (bar: 4.2 mm).1—Area postrema, right and left parts; 2—floor of the fourthventricle; 3—choroid plexus; 4—posterior superficial veinson the posterior aspect of the medulla.

Fig. 72 – Endoventricular view of the right part of the areapostrema, human (bar: 525 μm). The vascular network of thearea postrema (1) has the characteristic aspect of those of thecircumventricular organs: the capillaries are dense, irregular,sinusoidal with many capillary loops, clearly visible in thisendoventricular view. 2—Fourth ventricle.

Fig. 73 – Transverse section of the area postrema, human(bar: 450 μm). The large and sinusoidal capillaries of the areapostrema (1) may be identified in comparison with the finecapillary network of the dorsal motor vagal nucleus (2). Thearea postrema (1), the dorsal motor vagal nucleus (2) and thenucleus of the solitary tract (3) form together the dorsomedialmedulla or vagal complex with autonomic functions.4—Fourth ventricle.

Fig. 74 – Endoventricular view of the area postrema, duck(bar: 700 μm). The area postrema forms two long bands oftissue (1) extending from the obex in the midline (2) to thelateral angles of the fourth ventricle (3). Note its densevascularization. 4—Floor of the fourth ventricle; 5—dorsalaspect of medulla.


Fig. 75 – Endoventricular view of the area postrema, pigeon(bar: 640μm). As in the duck (Fig. 74) the area postrema in thepigeon is made up of two right and left parts (1) encircling theborders of the fourth ventricle floor (2). 3—Obex; 4—dorsalaspect of medulla.


Documents

The circumventricular organs: An atlas of comparative anatomy and vascularization