Fine structure and innervation of the avian adrenal gland

Z. Zellforsch. 146, 385402 (1973) © by Springer-Verlag 1973

Fine Structure and Innervation of the Avian Adrenal Gland*

IV. F i n e S t r u e t u r e of I n t e r r e n a l Cells

K. Unsicker

Department of Anatomy, University of Kiel (Head: Prof. Dr. Drs. h.c.W. Bargmann)

Received July 26, 1973

Summary. Cords of inf~rrenal-cells of birds resemble "tubes without lualina", which are lined by columnar cells arranged in double rows. A subeapsular and an inner zone of the interrenal gland may be distinguished according to the structure of their mitochondria, the existence of smooth and rough surfaced endoplasmic reticulnm, and the lipid contents. The cells of the inner zone are clearly polarized. The extrusion of lipid by exocytosis is discussed. I t is often difficult to decide, whether ultrastructural details are species - specific or indicate a certain state of function.

Key words: Interrenal cells - - Avian adrenal gland - - Lipid exocytosis - - Electron microscopy.

Introduction

Recent investigations of the fine structure and innervation of avian adrenal chromaffin cells (Unsicker, 1973a, b, c) showed a mutual dependence of the distribution of nerve fibres and the pattern of innervation on the one hand and the arrangement of ehromaffin cells on the other hand. Furthermore, these reports under l ined once again the value of a compara t ive s t u d y of different species and orders and revealed sl ight bu t d is t inc t differences in the fine s t ruc ture of ehromaffin cells of var ious b i rd species.

This and a following pub l ica t ion (Unsicker, 1973e) deal wi th the fine struc- tu re and innerva t ion of the av ian interrenM gland. The fine s t ruc ture will be regarded for three reasons: 1. in fo rmat ion on the u l t r a s t ruc tu re of in te r rena l cells of birds is only scarce and, wi th some except ions, confined to the species Gallu~ dome~ticu~ (Fuj i ta , 1961 ; F u j i t a et al., 1963 ; Sher idan et al., 1963 ; Bel t et al., 1965; Grignon et al., 1966; Harr ison, 1966; K j a e r h e i m and Kondics , 1966; Kondics and Kjae rhe im, 1967; Kjae rhe im, 1968a, b). 2. Though the cy to logy of the in te r rena l gland, especial ly in mammals , has been near ly comple te ly revealed (cf. Ide lman , 1966; a.o.), a number of quest ions have remMned open up to now, concerning bo th fundamen ta l p roblems as to the format ion, storage, and release of s teroids and the possible role of l ipid drople ts in this respect , and the i n t e rp re t a t i on of cytological pecul iar i t ies which m a y be species- or funct ion- dependent . So, for instance, Rhod in (1971) suggested t h a t l ipid drople ts leave adrena l cor tex cells of the r a t b y exocytosis . Observat ions suppor t ing this view can be given now in ano ther class of ver tebra tes . 3. As was a l r eady the case in

* This work was supported by a grant from the "Deutsche Forschungsgemeinschaft" (Un 34/1).

386 K. Unsicker

adrena l ehromaff in t issue i t cannot be excluded f rom the ve ry beginning t h a t the a r r angemen t and d i s t r ibu t ion of in te r rena l cells are responsible for cer ta in character is t ics of the i r innerva t ion . Therefore a shor t descr ip t ion of the h is to logy and cy to logy of the in te r rena l g land of 15 b i rd species will be given now.

Materials and Methods

1. Animals Studied (/or Number, Sex, Maturity, and Date of Fixation See Unsielcer, 1973a)

Domestic Fowl, Domestic Goose, Peking Duck, Larus ridibundus (Black-headed Gull), Larus argentatus (Herring Gull), Larus marinus (Black-backed Gull), Rissa tridactyla (Kittiwake), Uria aalge (Guillemot), Domestic Pigeon, Corvus/rugitegus (Rook), Corvu~ monedula (Jackdaw), Turdus merula (Blackbird), Sturnus vulgaris (Starling), House Sparrow Fringilla coelebs (Chaffinch).

2. Methods

Paraffin sections (7-10 ~m thick) of adrenal glands of the domestic fowl, duck, black-headed gull, black-backed gull, kittiwake, guillemot, pigeon, rook, starling, sparrow, and chaffinch were stained with Heidenhain's azan and chrome alum hematgxylin-phloxin according to Gomori-Bargmann.

Semithin sections of ArMdite-embedded material were stained with azure II-methylene blue (Richardson, modified).

The distribution of autofluorescent pigment bodies was studied in freeze-dried, paraffin- embedded sections under the fluorescence microscope (Carl Zeiss).

Adrenal glands of all above mentioned species were studied under the electron microscope. The animals were perfused with Macrodex®, phosphate-buffered glutaraldehyde, and phosphate buffer, and postfixed in unbuffered 2% OsO 4. Blocks were then dehydrated with ethanol and embedded in Araldite. Thin sections were stained with uranyl acetate (saturated solution in 70% methanol) and lead citrate and observed through a Siemens 101 or a Zeiss EM 9A. For further details see Unsicker (1973a).

Observations

Domest ic Fowl. Convent iona l ly s ta ined paraff in sections show in te r rena l cells wi th a s l ight ly vacuol ized cy top la sm a r ranged at b ranching and anas tomosing cords. Often these cords resemble " tubes wi thou t l umina" , covered b y a s imple or pseudos t ra t i f i ed co lumnar ep i the l ium (Fig. 1). The nuclei are somet imes s i tua ted close to the basa l lamina, sometimes, however, t h e y can be observed as well near the apices of the ceils. Benea th the capsule of the g land in te r rena l cells are of ten a r ranged in solid complexes showing no vacuol iza t ion of cy toplasm, and small , dense nuclei (el. B lack-backed Gull, Fig. l a ) .

Avian interrenal cells 387

The light microscopical observations on the arrangement of interrenal cells can be confirmed by electron microscopy (el. Herring Gull, Fig. 2). Interrenal cells of the inner zone 1 are bipolar.

They rest upon a basal lamina which marks the border to vessels, connective tissue, and chromaffin cells. Either the apical surfaces are attached to one another, or the opposite cells leave between them a small gap.

Nuclei which can be found both basa]ly or apiea]ly located are round or oval in shape, and 3-7 ~m in diameter. Indentations of the nuclear membrane are not seldom evoked by large lipid inclusions.

The Golgi apparatus is well developed (el. Black-headed Gull, Fig. 3a; Jack- daw, Fig. 5 a) and commonly situated in the apical part of the cell. I t is composed of parallel eisternae, light vesicles up to the diameter of small lipid droplets, coated vesicles, and dense-cored vesicles which also can be demonstrated within small basal processes of the cells. In the subcapsular zone the Golgi complex is smaller than in the inner zone.

Dense bodies are a regular constituent of the Golgi region (el. Black-headed Gull, Fig. 3a). As in ehromaffin cells (Unsicker, 1973a) they are larger and more numerous in adult than in juvenile animals. They are nearly always surrounded by a unit membrane and often associated with lipid inclusions.

Mitochondria of the inner zone belong nearly without exception to the tubulus/ saceulus-type (Figs. 3a, 5b). Cristae, which are seldom embedded into the dense matrix (cf. Jackdaw, Fig. 5a), are preferentially located in central parts of the mitochondrion. They can be packed so densely, that the narrow space between the two membranes seems to disappear. Profiles of mitochondria are round or oval and measure 0.2-1.3 ~m. In the subcapsu]ar zone mitochondria are elongated and belong predominantly to the crista-type. Bowl-shaped or dumb-belLlike mitoehondria are seldom found.

Endop]asmie reticu]um (ER) occurs both in its granular and smooth form. The quantitative distribution of both forms may serve as a criterion for the distinc- tion of the two interrenal zones. Cells of the subeapsular zone are rich in rough- surfaced ER and show smaller amounts of smooth ER than cells of the inner zone. Ribosomes are more often freely distributed within the cytoplasm than attached to Eg-membranes. Both forms of the ER are correlated with the specialization of mitoehondria: cells with a well developed smooth-surfaced ER have mitoehondria of the tubulus-type; cells with a large amount of rough-surfaced ER exhibit mitochondria of the erista-type. Modifications of this scheme sometimes occur in the transitional regions of subeapsular and inner zone. The smooth- surfaced ER can be organized either as a meshwork of short anastomosing tubules (ef. Jackdaw, Fig. 5a; Black-headed Gull, Fig. 6b) or in the manner of parallel, concentrically arranged double membranes which surround lipid droplets, aggregates of vesicles or ribosomes, or mitoehondria (cf. Jackdaw, Figs. 3c, 4e). In spite of a good state of preservation of the tissue we observed "light" cells in the

1 Concerning the nomenclature of the different zones of the avian interrenal gland we follow Kondics and Kjaerheim (1966) calling the "outer zone" subcapsular and the region where cords and tubes occur, "inner zone". The latter corresponds to the "central zone" of Gosh (1962).

L

Fig. 2. Lavus argentatus. Columnar cells in the inner zone of the interrenal gland. The cells are clearly polarized. The nuclei are si tuated basally; the bulk of lipid droplets (1), cytosomes (cy), and the Golgi apparatus (g) can be found in the apical par t of the cell. At the lateral cell surfaces dilations of the intercellular space can be seen, which contain remnants of membranes

(->). Mitochondria (mr). Basal lamina (Bin). Capillal T endothelium (/ce). ×5400

Fig. 1 a--f . Adrenal chromaffin cells (A) and interrenal (I) cords in the adrenal gland of a. Larus ma~inus (paraffin section), b. Larus ridibundus (paraffin section), and House Sparrow (semithin section), a. Note the "pseudotubule"-s t ructure of I-cords in the inner zone (iz)! I-cells are columnar and exhibi t nuclei in a basal position and a l ight cytoplasm. In the subcapsular zone (sz) tile tubule-like arrangement of dark I-cells is only recognizable in places ( ~ ). Nerve fibers (nb). Capsule (]c). b. Interrena] cords of the inner zone. Note the terminal bar (sn) l c. I-cells arranged as tubules (s). They are moderately loaded with lipid. Nuclei are s i tuated in a compact area of cytoplasm (/cc). Capillaries (]c) d ~ . Varying stages of lipid content in the inner zone of the interrenal gland of d. Uria aalge, e. a juvenile, and f. an adul t animal of

Larus argentatus (semithin sections)

Fig. 3 a--c


adrenal glands of two immature animals (el. Rook, Fig. 3b), which can only be distinguished from other cells by the low density of the hyaloplasm.

Lipid droplets are more numerous and larger in the inner than in the subcapsular zone. The diameter varies from 0.3 to 2.5 ~zm; the contents may be translucent or highly electron-dense (el. Jackdaw, Fig. 3e). Lipid droplets can accumulate either in the basal or in the apical part of the cell. Regularly we found lipid droplets which communicate with the intercellular space and contain pale remnants of membranes (Fig. 5b). The cells are connected by short cytoplasmic processes, zonulae oeeludentes, adhaerentes and two types of desmosomes on the lateral surfaces of cell apex. Type I desmosomes correspond to the commonly found macula adhaerens, type I I is remarkable for its large accumulations of electron-dense particles. Virus-like particles can be observed in the surroundings of interrenal cells and inside of them. In the following chapters only those observations will be mentioned which differ from those in the domestic fowl.

Domestic Goose. The pseudotubule-strueture of interrenal cords is even clearer than in the chicken. The nuclei, which are mainly basally situated, look like dark islands within the highly vaeuolized cytoplasm. The size distribution of mitochondria varies significantly more than in the chicken. Large aggregates of mitochondria lie beneath the nuclei. The smooth-surfaced ER is better developed than in the domestic fowl; on the other hand, the granular EI~ has dimi- nished.

Peking Duck. The tubular character of interrenal cords is still clearer than in the goose. Nuclei have a basal position without exception. The cells are not as rich in lipid as in the goose. Mainly the basal regions exhibit compact areas of cytoplasm. Subcapsnlar and inner zone cannot be distinguished as easily as in the chicken. Auto fluorescent pigment bodies are confined to the subeapsular zone. Ultrastrueturally there are no differences compared with the goose.

Larus r idibundus (Black-headed Gull). The tubule-like arrangement of interrenal cells is not as distinct as in the goose and the duck. The lipid content is low (Fig. l b). Interrenal cells show a large amount of eytosomes (Fig. 3a). They arc predominantly found in the Golgi region and resemble those of adrenal chromaffin cells of the same species (Unsicker, 1973a), without reaching their diameters. Bodies with a foamy inner structure prevail; they can be derived both from de- generating mitoehondria and from dense bodies which develop within the Golgi apparatus. Cytosomes enlarge and become more numerous in old age. Among mitochondria giant forms occur; their tubules partly show a hexagonal arrangement (Fig. 6b) and may sometimes contain lipid droplets.

In the pericapsnlar tissue fragments of cords and tubes can be observed. Their simple columnar epithelium only shows a weak differentiation with sparsely distributed mitoehondria (crista-type), lipid inclusions, numerous free ribosomes,

Fig. 3a. Larus ridibundus. Golgi field (g]) of an interrenal cell of the inner zone surrounded by numerous cytosomes with a foamy inner structure (cs), which can be derived from smaller and less vacuolized dense bodies (db). Mitoehondria (mr). Lipid droplets (l). × 7,200. c. Corvus ]rugilegus. Mitochondria which incorporate dark lipid droplets (lm). Light lipid droplets (hl).

b. Corvus monedula. Dark (dl) and light (hl) lipid droplets. × 18000

Fig. 4a - -c . Interrenal cells of a. Turdus merula b. Domestic Fowl, c. Corvus monedula. a. Bundle of fi laments (]) which par t ly cross, bu t have no relation ~dth the nucleus, b. Junc- t ional complexes of interren~l ce]]s: 1. microvilli, 2. zonula occludens, 3. desmosome type I, 4. desmosome type I I (see text), c. Concentrically arranged continuous whorls of double membranes (dm). Cytoplasmic lamellae contuin dense-cored vesicles (dv). These formutions

obviously originate from elements of the smooth-surfaced EI~ (->). × 18000


and filaments. A connection of these cords which remarkably resemble medullary cords and tubes in the ovary of the pig (Unsieker, 1971) with interrenal cords does not exist.

Larus argentatus (Herring Gull). The two animals of this species investigated strongly differ in the diameter of interrenal cords and the amount of lipid in- corporated. While the cells of the adult animal are heavily loaded with lipid inducing an extension of the cords, the lipid contents of the juvenile animal correspond to tha t of the chicken. Cytosomes in juvenile animals already reach the diameter which was measured in adult individuals of Larus ridibundus.

Larus marinus (Black-backed Gull). The diameters of interrenal cords equal those of the adult animal of Larus argentatus, but the cords do not show the pseudotubular pat tern and contain less lipid. In the perieapsnlar tissue tubes occur which resemble those described in the black-headed gull; however, the columnar cells often bear cilia (Fig. 7).

Rissa tridaetyla (Kittiwake). No variations in the fine structure and arrangement of interrenal cells compared with the black-backed gull were detected.

Uria aalge (Guillemot). The varying diameter of interrenal cords, which contain nearly no lipid droplets, is striking. A clear separation of subcapsular and inner zone is only possible when regarding the fine structure of mitochondria.

Domestic Pigeon. The pseudotubular structure of interrenal cords which sometimes branch is obvious. Interrenal cells of the subcapsular zone are polygonal and intensively basophflie. They exhibit fewer but larger lipid droplets than elements of the inner zone do.

Corw,s/rugilegus (Rook). Both zones of the interrenal gland are clearly dis- cernible. Cells of the subcapsular zone are rich in pigment bodies, but show only few lipid inclusions. Within the inner zone lipid droplets are a little bit more numerous but do not reach the lipid content of the chicken or the pigeon. In some places only, interrenal cells stand in double rows perpendicular to the course of the cords; nuclei mostly exhibit an apical (!) position. In cells of the subcapsular zone electron micrographs show whorls of smooth-surfaced ER, which include lipid droplets, mitoehondria or cytosomes. The double membranes which con- stitute the whorls are fenestrated and change in the periphery to the tubular network of smooth-surfaced ER.

Predominantly in interrenal cells of the subcapsular region hexagonally arranged mitoehondria were observed (Fig. 6a), which are surrounded by single plates of smooth-surfaced ER. Within cells of the inner zone mitochondria with lipid or lipid-like inclusions of high electron density are regularly encountered Fig. 3b).

Corvus monedula (Jackdaw). There are no variations compared with the rook. Turdus merula (Blackbird). Interrenal cords are irregularly formed and

branched. Subcapsular and inner zone can be distinguished according to their lipid content. The fine structure corresponds to a great extent to that of corvidae. Some interrenal cells show parallel or crossed bundles of filaments without special affinity to the nneleus.

Sturnus vulgaris (Starling), House Spa~cow, 2'ringilla coelebs (Chaffinch). The interrenal glands of these species do not grossly differ from the other passeriform species investigated.

27 Z. Zellforsch., Bd. 146

394 K. Unsicker

Fig. 5a. Interrenal cells of Corvus monedula. Golgi apparatus (g) with vacuoles which sometimes reach the diameter of lipid droplets (l). Dense-cored vesicles (dr). Cytosomes (cy). Mitochondria with tubules and a few cristae (~). Agranular E R (gr). Granular ER (ge). b. Inter-


Discussion

Interrenal cords of birds often resemble "tubes without lumina" which are covered by a columnar epithelium. Comparing the different species studied we are able to say tha t this arrangement is only to some extent a common feature of the avian interrenal gland. The pseudotubular character of the cords appears especially clearly in the goose and in the duck, quite distinctly in the herring gull and the pigeon (el. Miller and Riddle, 1939), less obviously in passeriform species, chicken (of. Vincent, 1898: Gallus bankiva) and the remaining gull species. The position of nuclei in the cells fluctuates even within one and the same interrenal gland between apical and basal, nuclei in a predominantly apical position were found in the rook, comparable to the descriptions of Har tman and Brownell (1949: brown pelican). In the goose and the duck nuclei mostly took the basal position. In the interrenal cords of two chickens a small cleft was recognizable, which had been already mentioned by Rabl (1891) in the pigeon.

In most species two zones of the interrenal gland could be discerned: a subcapsular zone characterized by a low lipid content (chicken, pigeon, passeriform birds), a preponderance of solid complexes instead of interrenal cords and a great number of autofluorescent pigments (chicken, duck, starling, sparrow, chaffinch), and an inner zone with a higher lipid content. Only in the guillemot was it difficult to distinguish both zones.

Morphological, histochemical, and physiological observations indicate that the zonation of the avian interrenal gland means a functional organization comparable to that of the mammalian adrenal cortex (Latimer and Landwer, 1925; Miller and Riddle, 1942; Chester Jones, 1957; Dearie, 1962; Gosh, 1962; Arvy, 1963; Bhat taeharyya and Gosh, 1963; Bhattacharjee and Gosh, 1964; De I~oos, 1961; De Roos and De Roos, 1964; Kondics, 1964; P6czely, 1964, 1966; Kondics and Kjaerheim, 1966; Moens and Coessens, 1970). However, the inter- pretation of single features of interrenal organization and fine structure as cha- racteristic for certain species seems to be difficult. Therefore, our observations may only be regarded as a first contribution to a funetionM morphology of the avian interrenal gland, which has to be followed by investigations considering the influence of different surroundings and of stress.

All species investigated exhibit obvious ultrastructural differences in subcapsular and inner zone cells, even though a light microscopical differentiation is sometimes difficult. Cells of the inner zone are evidently polarized. Kjaerheim (1968a) has already drawn our attention to this phenomenon, which can be at tr ibuted to the constant apical position of the Golgi apparatus and the formation of an apical terminal bar. The distribution of lipid droplets depends on the position of the nucleus. That means, one cannot state either for the chicken (Kjaerheim, 1968a) or for other bird species, that the majori ty of lipid droplets is stored basally. Above all interrenal cells of the goose and the duck show compact regions

renal cells of the domestic fowl. Lipid droplets (1) communicate with the intercellular space and contain pale remnants of membranes (n~m). Coated vesicles (sb). Mitochondria with

tubular cristae (rot). X 18000

27*

Fig. 6a. InterrenM cell in the subcapsular zone of Corvus/ruyilegus with hexagonMly arranged mitochondria (-~) which are surrounded by single plates of agranul~r EI~ (gr). Lipid droplets (l). b. Interrenal cell in the inner zone of Larus ridibundu~. Gian~ mi~ochondrion with hexagonally arranged tubules (rot). Cytosomes (cy). Lipid droplets (1). Arganular ER (gr). × 18000


Fig. 7. Larus marinus. Fragment of a tube in the pericapsular tissue of the adrenal gland, which resembles medullary tubes in the ovary of the pig. Lumen (18). Desmosomes (d). Cyto-

somes (cy). Cilia (z). Basal lamina (bin). × 5400

of cytoplasm, which are rich in mitochondria, near the basal surface. Comparing juvenile and adult animals of the herring gull we can say tha t neither the amoun t nor the distr ibution of lipid is species-dependent.

I n accordance with Kjaerheim (1968a) we often found small lipid droplets in close relation to the Golgi apparatus, which, following Rhodin (1971; rat), m a y be involved in the format ion of lipid droplets. The communicat ion of lipid drop-

398 K. Unsicker

lets and round cavities of the same size, which often contain weak remnants of membranes, with the intercellular space may be interpreted by some as artifacts. Several reasons, however, perhaps speak in favour of secretion by exocytosis:

1. The phenomenon was observed in all species investigated; 2. interrenal glands fixed both by perfusion and by immersion exhibited

lipid droplets which open into the intercellular space; 3. identical pictures have been described by Rhodin (1971) in the rat adrenal

cortex and called "modified apocrine secretion"; 4. like other gland cells which release their products by exocytosis (Smith,

1971), adrenal cortex cells need extracellular calcium for the secretion of eortieo- steroids (Birmingham et al., 1953 ; Rubin et al., 1969). This may be a hint for the exsistence of exocytosis in adrenocortical cells.

This hypothesis would include that corticosteroids are contained in the lipid. Certainly, it has been proved that the lipid droplets contain the majority of adrenocortical cholesterol (Moses et al., 1969) ; however, we do not know, whether they contain considerable quantities of cortieosteroids as well. So, our specu- lations must imply that corticosterone for instance--one of the most important glucocorticoids in birds (De Roos, 1961 ; Donaldson and Holmes, 1965; Whitehouse and Vinson, 1967)--pass over from mitochondria into the lipid, before secretion takes place, The morphological condition for such a step (short distances) is fulfilled in birds as in mammals (cf. Fawcett st al., 1969). The second implication would be that the special structure of the lipid membrane which is still in discussion (cf. Barnard, 1969 ; Blanchette, 1969 ; Bloodworth and Powers, 1968 ; Fujita, 1961; Fuijta et al., 1963 (chicken); Long and Loner, 1967; Palade and Schidlowsky, 1958; Parks, 1967) allows a fusion with the plasmalemma. I t is worth mentioning that in the Harderian gland of the rabbit (Kfihnel, 1971) lipid inclusions leave the cells by exoeytosis.

The other ultrastructural details of the avian interrenal gland shall only be mentioned in so far as they differ from or complete the results of Kjaerheim (1968) in the normal chicken.

While the smooth-surfaced ER is usually organized in short, anastomozing tubules, we saw in the rook, but not in the jackdaw, great numbers of concentrically arranged fenestrated double membranes (whorls), which have been known to exist in several steroid producing glands (interstitial cells of Leydig: Carr an Cart, 1962; Christensen, 1965; Christensen and Fawcett, 1966; Murakami, 1966, 1968; Merkow etal., 1968a, b: Black and Christensen, 1969; hflus cells of the ovary: Merkow etal., 1970; Unsicker, 1970; lutein cells: Blanchette, 1966; Schmidt, 1969 Dahl, 1971a, b; adrenal cortex: Nickerson et aL, 1970; Gorgas, 1971). Kjaerheim (1968a) noticed whorls in chicken interrenal cells after stimulation with ACTtI. Similar accumulations of smooth surfaced ER occur in the glomerulosa zone of mouse and oppossum when treated with a sodium-diet. (Sheldon and Jones, 1969; Long and Jones, 1970). These observations lead to the conclusion that whorls are the result of certain types of stress; and additional genetic component as assumed by Gorgas (1971).

As in many mammalian species (Fawcett etal . , 1969) the avian interrenal gland can be divided into zones according to the struetm'e of mitoehondria (cf. Xjaerheim, 1968b, as well). Specific forms and arrangements of mitoehondria


were observed in the b lack-headed gull and in the rook. I n the gull there are gigant ic mi tochondr ia wi th hexagona l ly a r ranged tubules and l ipid incorporat ions . Such mi toehondr ia have been descr ibed in inac t ive in ter renal cells of Rana temporaria (Pehlemann and Hanke , 1968) and in the fetal and adu l t adrenal cor tex ( Idelman, 1966; Lindner , 1966; Luse, 1967; and others).

The s t r ic t ly geometr ical a r r angemen t of mi toehondr ia and smooth-surfaced El% m a y be the morphologica l equiva len t of funct ional re la t ions be tween these two elements which are involved in the s teroid synthesis .

Dense bodies ly ing in the Golgi region, the apical cy top lasm and smal l processes ex tend ing towards the basal l amina are a common fea ture of av ian interrena l cells. These electron-dense bodies of different shape and size which are bordered b y a un i t membrane , have hyd ro ly t i c enzymes ( Idelman, 1966; Szab5 et al., 1966; Nussdoffer , 1969) and m a y therefore be considered to be lysosomes. Lipofuscin p igments are commonly found as well; t h e y increase in size and number wi th age.

Bundles of f i laments (Turdus merula) have been de mons t r a t e d in m a n y s teroid producing glands ( In te rs t i t i a l ceils of Leydig : Fa w e e t t and Burgos, 1960; M u r a k a m i et al., 1968; hilus cells of the ovary : Unsieker , 1970; in te r rena l cells: Peh lemann , 1968; Nickerson et al., 1970; Volk, 1972; and others). I t is an open quest ion whether t hey es tabl ish contac ts wi th the nucleus and m a y be rega rded as a sign for amitos is (Pehlemann, 1968; Boddingius , 1970).

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Unsicker, K.: Fine structure and innervation of the avian adrenal gland. II. Cholinergic innervation of adrenal ehromaffin cells. Z. Zellforseh. (in press) (1973b)

Unsicker, K.: Fine structure and innervation of the avian adrenal gland. III. Non-cholinergic nerve fibres. Z. Zellforseh. (in press) (1973c)

Unsicker, K.: Fine structure and innervation of the avian adrenal gland. Innervation of in~errenal cells. Z. Zellforseh. (in press) (1973e)

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Priv.-Doz. Dr. Klaus Unsicker Department of Anatomy University of Kiel D-2300 Kiel Federal Republic of Germany

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Fine structure and innervation of the avian adrenal gland