Environmental Health PerspectivesVol. 18, pp. 5-12, 1976

Comparative Placental Morphology and

by Felix Beckac

The distinction between histiotrophic nutrition (in which local macromolecules are chieflyresponsible for the maintenance of the embryo) and hemotrophic nutrition (which results from atransfer of material between the maternal and fetal circulations) is made. Placentation in anumber of commonly used laboratory animals and in man is described, and it is shown thatdependence upon histiotroph and hemotroph varies greatly, not only between species but also atdifferent stages of gestation in a single species. These facts are likely to be reflected in con-siderable differences in response to certain teratogens; they must be carefully considered whenexperimental results are extrapolated between species. The significance to man of an agent whichhas been shown to be teratogenic in a single species of experimental animals should be evaluatedin terms of possible differences in placental function between man and that species. This is par-ticularly so if there is a suspicion that the potential teratogen may affect the fetal membranes.

IntroductionA discussion of comparative placental mor-

phology and function is conveniently introducedby classification of the extraembryonic mem-branes involved in placental formation. This isbest done by studying the relevant tissues inaquatic oviparous animals and developed by ref-erence to more complex forms ending with theplacenta of various eutherian mammals. Usingthis taxonomic approach we are better able toappreciate the biological potential of differentfetal membranes and their capacity for modi-fication (in concert with maternal tissue) to pro-

duce the extraordinary range of placental formsseen in the Eutheria. There is obviously an evo-lutionary corollary to this subject but it is com-

plex and outside my present terms of reference.From the material to be presented here no con-clusions can be drawn on that matter.

The most primitive fetal membrane is the yolksac (Fig. 1). Early cleavage in bony and cartilag-inous fish results in a blastoderm perched on a

*Department of Anatomy, University of Leicester,Leicester, U. K.

mass of yolk. Gradually the ectoderm, mesoderm,and underlying periblast extend from the blasto-derm around the yolk mass and surround it in avascularized trilaminar yolk sac. Embryonic nu-trition is due to absorption of yolk by the peri-blast and transfer of nutrients to mesodermalblood vessels for transmission to the embryo. Therequirements of terrestrial life lead to the devel-opment of further fetal membranes by reptiles(Fig. 2). The shell membranes are modified toprevent desiccation, but this and other factorsinhibit gaseous interchange and excretion. Adiverticulum of the hind-gut-the allantois-isformed. Its cavity is lined by endoderm, and it iscovered by vascularized mesoderm. This largesac receives and stores fetal urine and lies in theextraembryonic coelom. The latter develops as aresult of a split in the extraembryonic mesodermwhich separates an outer chorion (ectodermaltrophoblast lined by avascular mesoderm) fromthe endodermal yolk sac covered by vascularizedmesoderm. Allantoic mesoderm fuses with chori-onic mesoderm to form the chorio-allantois, andby this means the allantoic circulation becomesgreatly concerned with gaseous interchange. Aswill be seen later the chorio-allantois forms the

December 1976 5

YOLK SAC LlKMESODPERIBLAST

VITELLINE VESSELS

FIGURE 1. Diagrammatic section through an idealized largeyolked anamniote conceptus.

basis of the mature eutherian placenta, and itsfunction of gaseous interchange is thus greatlyextended to embrace the passage of solutes be-tween mother and fetus. In addition to all theabove modifications, the reptilian embryo devel-ops in an ectodermally lined fluid filled sac-theamniotic cavity (Fig. 2).The development of viviparity confers great

advantages upon the conceptus among which sta-bility of environment is clearly the greatest. Intruly viviparous forms, the embryo absorbs ma-terials of maternal origin, and thus a reduction inthe amount of egg yolk present occurs. This situa-tion reaches its highest state of development ineutherian mammals. Newly born animals can thushave a far higher protein content than the fer-tilized ova.Mossman's (1) definition of the placenta as an

intimate apposition or fusion of fetal organs tomaternal tissues for physiological interchange isstill valid, and we must pause to consider which ofthe extraembryonic membranes are used for thispurpose in Mammalia. In Metatheria it is princi-pally the yolk sac, though some species do makerestricted use of the chorio-allantois. Eutherianmammals, sooner or later during the course ofgestation, use the chorio-allantois as their prin-

AC Chorio-allantois

Chorihorn

FIGURE 2. Diagrammatic section showing the extraembryonicmembrane of an amniote: (AC) amniotic cavity; (YS) yolksac; (EEC) extraembryonic coelom; (AL) Allantois.

cipal placental tissue, though it is often precededand accompanied by some yolk sac-mediated nu-trition; this arrangement is also seen in vivip-arous reptiles. It must be stressed however that(unlike viviparous reptiles) Eutheria possess abewildering profusion of. placental types, bothtopographically and histologically. Many struc-tures other than the chorio-allantois and the un-modified yolk sac are used, and various labora-tory mammals consequently differ greatly in pla-cental function both as regards their histophys-iology and the temporal sequence of events tak-ing place at the placental site. In the mother,great differences in the local (endometrial) andsystemic (mainly endocrine) response to pregnan-cy are also manifest and must naturally be con-sidered in any attempt at a holistic view of theproblems of placentation.

Embryonic NutritionA fundamental distinction should be made be-

tween histiotrophic and hemotrophic nutrition.Histiotrophic nutrition results from the intra-cellular breakdown of maternal macromoleculesin the fetal membranes. These biopolymers arederived from endometrial cells, glandular secre-tion, and maternal blood or its transudate. Hemo-trophic nutrition, by contrast, consists mainly ofan interchange of solutes between maternal andfetal circulations across the so-called placentalbarrier (vide infra). While not wishing to statethe obvious, it must be realized that hemato-trophic nutrition cannot begin until a fetal cir-culation perfuses the extraembryonic mem-branes. At this stage organogenesis must bereasonably advanced, at least to the point of de-velopment of a beating embryonic heart. Spacedoes not permit even a brief discussion of the fac-tors which govern the passage of materials acrossthe placental barrier (2). The majority pass bysimple diffusion along a chemical or electrochem-ical gradient, some cross by facilitated diffusionand yet others by virtue of active transport mech-anisms. Grosser's classification (3) which relatedthe ease of passage of solutes between the mater-nal and fetal circulations to the number of celllayers between them is now considered merely amorphological convenience, since it does not re-flect the thickness of the barrier or its cyto-physiological characteristics. The Fick constant,functional surface area of the placental mem-brane concerned in the exchange process, sizeand charge of the solute under consideration, aswell as its binding capacity to carriers on the fetal

Environmental Health Perspectives6

and maternal sides are among the many factorswhich must be considered.

Histiotrophic nutrition usually involves thebreakdown of maternal macromolecules in thevacuolar system of cells in the extraembryonicmembranes. Many examples involving phagocy-tosis, pinocytosis, and micropinocytosis areavailable. The principle of heterosis involved is il-lustrated in Figure 3. Besides supplying the earlynidus with its metabolic needs, heterosis is in-volved in the process of implantation and in manyspecies continues to be an important factor in em-bryonic nutrition well after the full establishmentof the chorio-allantois, often until full term. Inspecial circumstances, such as the transfer of yglobulins from mother to young, whole macro-molecules pass the placental barrier without theintervention of hydrolytic enzymes (4).

Comparative Placental Morphology and Functionin Laboratory Animals: Correlation WithEmbryonic Development

and actively invades the decidualized uterinestroma as well as the walls of the maternalcapillaries. Some histiotroph is therefore takenup at this stage even though little apparent em-bryonic development occurs. Gastrulation beginsat 8.5 days (Fig. 4c), at which stage a processknown as entypy (inversion) of the yolk sac hasdeveloped because of growth of an ectoplacentalcone composed of extraembryonic ectoderm. Atthis stage nutrition of the embryonic disc is prob-ably largely due to the ability of the extraembry-onic endoderm to degrade macromolecules in itsvacuolar system. The histiotroph appears to comefrom three sources; the maternal serum locatedin lacunae between trophoblastic cells,endome-trial cells broken down by trophoblast, and secre-tions of the uterine glands.As pregnancy advances, the uterine cavity in

the region of the implanted blastocyst is oblit-erated to reappear some days later on the an-timesometrial aspect of the implantation site.After the appearance of the embryonic head, tailand lateral folds (at about 20 somites) the laby-rinthine chorio-allantoic placenta develops as

The RatImplantation in the rat occurs at 5.5 days, 3

days before the beginning of gastrulation.Trophoblast penetrates the uterine epithelium

.. Al

p

am.

HETEROPHAGOSOME- IAPPARATUS

,PRIMARY LYSOSOME(Containing hydrolytic enzymes)

BIOPOLYMERSHETEROLYSOSOME -

/ N - BREAKDOWN PRODUCTS

FIGURE 3. Diagram to illustrate the process of heterosis in itssimplest form. There is evidence to indicate that hetero-phagosomes obtain hydrolytic enzymes by fusion with sec-ondary lysosomes (e.g., heterolysosomes) as well as withprimary lysosomes (this is not shown in the diagram). Theprocess illustrated here is of micropinocytosis, but the up-take of macromolecules may equally well involve gross pino-cytosis or even the uptake of solid particles, e.g., red bloodcorpuscles and endometrial cell detritus.

FIGURE 4. Diagram to show the disposition of the extraem-bryonic membranes in the rat: (a) at 6'/2 days (post implanta-tion); (b) at 8 days (formation of pro-amniotic cavity); (c) at91/4 days (gastrulation and early formation of extraem-bryonic mesoderm and coelom); (d) at 9'/2 days (division ofegg cylinder into three cavities); (e) at 10 days (developmentof the allantoic bud); (f) at 11 /2 days (early chorio-allantoicplacenta); (g) at 16 days-full term (disappearance ofparietal yolk sac and Reichert's membrane). Inversion of theyolk sac occurs, and on day 16 the parietal wall of the yolksac degenerates with the overlying trophoblast. The visceralyolk sac wall is then exposed to the uterine lumen: (AC) am-niotic cavity; (Al) allantois; (Ch) chorion; (EC) epamnioticcavity; (EEC) extraembryonic coelom; (EE Mes) extraem-bryonic mesoderm; (Ec) ectoplacental cone; (EEct) em-bryonic ectoderm; (EEnd) embryonic endoderm; (EMes) em-bryonic mesoderm; (L) labyrinth; (PAm) proamniotic cavi-ty; (PYS) parietal yolk sac; (R.M) Reichert's membrane; (T)trophoblastic giant cells; (VYS) visceral yolk sac endoderm;(YS) yolk sac cavity.

December 1976 7

the result of fusion of the allantoic bud with thechorion, and a hemochorial relationship is estab-lished (Fig. 4f). Hemotrophic nutrition in the rattherefore only begins to be established at about11 days of gestation when (for example) the fore-limb buds are already in evidence. The yolk sacendoderm, however, continues to be active untilfull term. On day 16 of pregnancy its parietallayer together with the covering trophoblast dis-appears, and the visceral layer is exposed to theuterine cavity (Fig. 4g), where it is in an excellentposition to pinocytize and degrade the uterinesecretions.

Histochemical and biochemical evidence in sup-port of the proposition that the yolk sac ingestsand degrades horseradish peroxidase in vivo isavailable (5) and conclusive evidence that the 17.5day organ digests 125I-labeled bovine serum al-bumin has been obtained by Williams et al. (6).Subsequently Williams et al. (7) have been able tomeasure the rate of uptake (expressed as the so-called endocytic index) and breakdown of 125Ialbumin in vitro (Fig. 5) and Moore et al. (8) haveshown that the so-called endocytic index for thisprotein depends greatly on its degree of denat-uration. Besides acting as an organ of intracellu-lar digestion the rat yolk sac is responsible forthe transfer of some y globulin antibodies frommother to fetus, and there is also evidence thatcertain ions reach the fetus through the yolk sacrather than via the chorio-allantois.The post-nidation rat embryo thus appears to

pass through three phases of placental transfer.

30~~~~~~~~~~

C_

A(.

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*~~~~~ A

2 3 4 5 6 7

Time Ihi

FIGURE 5. Levels of "2'I activity in tissue and culture mediumon incubation of yolk sacs in the presence of '25I Bovineserum albumin: (.) TCA-soluble activity released into theculture medium; (o) TCA-insoluble activity in the yolk sac;(A) TCA-soluble activity in the yolk sac. Data at each timeinterval are from a single yolk sac. From Williams et al. (7).

An initial quiescent phase before gastrulationtakes place during implantation by trophoblasticinvasion; at this stage, decidual formation is pro-nounced, and maternal blood lakes form betweensome of the trophoblastic cells. The second phase,that of yolk sac nutrition, is present at the onsetof gastrulation and is typified by great develop-ment of the visceral layer of the inverted yolksac. The prime importance of that organ appearsto be macromolecular breakdown of material forembryonic nutrition. Finally the chorio-allantoicplacenta appears (at the limb bud stage) andtakes over much of the nutritional function of theyolk sac; it is concerned with hemotrophic ratherthan histiotrophic processes.

Other Rodents and the Rabbit

Histricomorph rodents as exemplified by theguinea pig have a placental system very similarto that of the rat (a myomorph rodent). Implanta-tion commences at 6 days of gestation and pre-cedes gastrulation by 7 to 8 days. During thistime penetration of the endometrium occurs, butthe future embryonic cells are relatively quies-cent. The yolk sac is inverted, and its function asan organ of histiotrophic nutrition is similar tothat in the myomorph; it differs only in that theparietal yolk sac endoderm and Reichert's mem-brane never develop and the antimesometrialtrophoblast degenerates before gastrulation thusexposing the visceral yolk sac at an early devel-opmental stage. The hemochorial chorioallantoicplacenta appears at about the same point in em-bryogenesis as it does in the rat (about 20somites). It differs in morphological detail fromthat of the rat (for example it has a well-devel-oped subplacenta) but the significance of the dif-ferences is obscure, and functionally both chorio-allantoic systems are probably broadly similar.

Placentation in the rabbit has many features incommon with rodents, but sufficient dissimilari-ties exist to make it possible that some of theseaccount for observed differences in response toteratogens. Central implantation of a swollenblastocyst coincides with gastrulation. First at-tachment at about 7 days is mesometrial and oc-curs in crescentric areas at the sides of the em-bryonic disc. In this region, the uterine epithelialcells proliferate vigorously to form a symplasmalmass while the uterine glands become highly se-cretory. A nutritional pabulum is thus formed atthe point of trophoblastic penetration which is ab-sorbed within 10 days, while decidual cells con-tinue to develop in the uterine stroma. Endodermand vascularized mesoderm spread out from the

Environmental Health Perspectives

FIGURE 6. Composite diagram showing placentation in therabbit: (A) chorio-allantoic placenta; (Ac) amniotic cavity;(Al) allantois; (Alm) allantoic mesoderm; (BYS) bilaminaromphalopleur; (ChV) choriovitelline placenta; (EEC) extra-embryonic coelom; (UYS) unilaminar omphalopleur; (YS)yolk sac cavity. The region of the chorio-allantoic placenta(A) is depicted at a later stage of development than the yolksac. By this stage the amniotic cavity will have closed by fu-sion (arrows), the yolk sac will have become inverted by in-ward displacement of the embryo (thick arrow), the chorio-vitelline placenta will be completely devascularised and theyolk sac distal to the lines ( 1) will have disappeared.

embryonic area after implantation (the meso-derm extending only about halfway around theblastocyst to the sinus terminalis) and the overly-ing trophoblast thickens and penetrates the en-dometrium. An uninverted trilaminar yolk sacplacenta (choriovitelline placenta) is thus formedin this region which is probably capable of bothhistiotrophic and hemotrophic nutrition. The ex-traembryonic coelom then splits most of the tri-laminar yolk sac into avascular chorion and vascu-lar yolk sac which latter eventually becomes in-verted with growth of the embryo and extra-em-bryonic coelom (Fig. 6). Below the sinus ter-minalis the bilaminar (trophoblast and endodermonly) and unilaminar yolk sac undertakes histio-trophic nutrition as the trophoblast penetrates anew area of uterine epithelium which has under-gone symplasmal transformation. Eventually(about 15 days) the bilaminar yolk sac degen-erates and a true inverted yolk sac nutritionalsystem similar to that found in rodents is estab-lished, the last portion of the choriovitellineplacenta being devascularized at about the bran-chial arch stage of development. Chorio-allantoicplacentation is established mesometrially atabout the 20-somite stage when the forelimb budshave appeared (cf. rat).

In distinction to rodents, there is therefore noquiescent phase of embryonic development be-tween implantation and gastrulation. This may be

reflected in the copious source of histiotroph pro-vided by the uterine symplasma and the develop-ment of a choriovitelline placental system. The in-verted yolk sac system does not begin to be im-portant until just before development of thechorio-allantois.

The FerretThis convenient laboratory animal has a pla-

cental system typical of mustelid carnivores. Cen-tral implantation at 12 days is coincident withgastrulation (cf. rabbit). As in the rabbit, theendometrial epithelium forms symplasmalmasses which are penetrated by tongues oftrophoblast responsible for histiotrophic nutri-tion. This is supplemented by absorption of uter-ine secretions from the so-called mesometrial gut-ter (Fig. 7a). The choriovitelline placentadeveloped at this stage is probably mainly histio-trophic in function because there is no vascu-larised mesoderm in the cores of the invadingtrophoblastic villi. Closure of the amnion isfollowed by extravasation of maternal blood intothe antimesometrial aspect of the implantationchamber where the red blood corpuscles are ab-sorbed by trophoblastic cells in the so-calledhemophagous organ (Fig. 7b) presumably to pro-vide a source of iron for the developing embryo.With growth of the allantois into a rapidly ex-panding extraembryonic coelom the chorioallan-toic placenta develops (Fig. 7b) and eventuallyreplaces the choriovitelline form at about 20 dayswhen the embryo has 35 or more somites. Chorio-allantoic placentation is somewhat different fromthe hemochorial pattern of rodents and rabbits;

A S~~~~E

FIGURE 7. Extraembryonic membranes of the ferret (a) at 19days and (b) at 21 days (AC) amniotic cavity (Al) allantois(EEC) extraembryonic coelom; (EEMes) extraembryonicmesoderm; (Endm) endometrium; (H) hemophagous organ;(Mg) mesometrial gutter; (Mm) mesometrium; (My) myo-metrium; (T) trophoblast; (YS) yolk sac cavity.

December 1976 9

the placental barrier is thick and an endothelio-chorial relationship between the maternal andfetal circulation is established (9, 10). Histio-trophic nutrition persists in the paraplacentalchorion and mesometrial gutter until term.

The Pig

Though this species is somewhat uncommon interatological work, brief reference to the Ungu-late placenta should be made (Fig. 8). The chorio-allantois is epithelio chorial in type but areas ex-ist in which the placental barrier is thin and truehemotrophic nutrition occurs side by side withregions in which histiotrophic digestion of endo-metrial secretions is preponderant. Gastrulationand implantation (in this case intimate contactbetween trophoblast and endometrial epithelium)coincide, and a histiotrophic bilaminar yolk sac isresponsible for early nutrition, to be replaced bya choriovitelline form which is still probablylargely histiotrophic in function. The chorio-allantoic placenta begins to function at the limbbud stage but is maximally effective as a hemo-trophic organ much later than in the formspreviously described.

EECYs Ys. mes. Al.mes.

FIGURE 8. Extraembryonic membranes in the pig: (AC) am-niotic cavity; (Al) allantois; (AlMes) allantoic mesoderm;(ChMes) chorionic mesoderm; (EEC) extraembryoniccoelom; (U) uterine wall (endometrium and myometrium);(Ys) yolk sac cavity; (YsMes) yolk sac mesoderm afterMossman (1).

The Human

The human placental system is so well docu-mented that only a brief summary need be givenhere. Implantation probably begins at about 7'/2days, and a small plaque of trophoblast is formedat the point of penetration. Histiotrophic nutri-tion commences by digestion of a pabulum of en-dometrial tissue and secretion. Within 4-5 daysthe blastocyst becomes completely buried in thestratum compactum, and lacunae, first filled byendometrial secretion and later with maternalblood, develop between the cells of the syncitio-

trophoblast. A sluggish circulation develops atabout 13 days of gestation and immediately thechorionic vesicle enlarges with the formation ofan extraembryonic coelom (Fig. 9). Within 3days gastrulation begins, and although nutritionis still histiotrophic it occurs largely from thematernal blood in the trophoblastic lacunae. Lessthan a week after the beginning of gastrulationan active circulation is established in the cho-rionic villi and embryonic nutrition becomes, atleast to a considerable extent, hemotrophic. Thecharacteristic features of human placentationtherefore are a relatively quiescent phase of em-bryonic development during which interstitialimplantation occurs and nutrition is histiotrophic;the chorionic vesicle begins to grow rapidly assoon as a maternal circulation begins to developin the trophoblastic lacunae, and this is rapidlyfollowed by gastrulation. Early embryonic devel-opment is characterized by precocious formationof the circulatory system with the result that per-fusion of the chorio-allantois establishes hemo-trophic nutrition (in a hemochorial placenta) atabout 5 somites, i.e., much more quickly than inother mammals. Placental differentiation afterformation of tertiary villi is concerned with re-gional specialization and does not change thebasic mode of placental function, except insofaras there is a gradual thinning of the placental bar-rier as gestation proceeds. Some degree of histio-trophic nutrition however (mediated, for exam-ple, by the cytotrophoblastic shell) probably ex-ists throughout gestation and certainly well intothe second half of pregnancy.

Other Higher PrimatesOld World monkeys such as the macaque have

superficial implantation with a rather less in-

Connecting (body) stalkPFochordal plate Primary cho rionic villus t

Wallofamnoc o t Extraembryoniccemcaboity (plar ofpervadi gt i tior splanchnopleure

einaemblyonick-Fo c, Mfa an Lo (11somatople-ure) /E.* EC traembryonc

Choron (F:art of _.,,,_extraembryonic , J/ \\somatopleure) Yolk. sac ca,v,ty

> pr~~~~~~~~~~~~~~imary chorionic villus

Uterine epithelium

FIGURE 9. Diagram of the human chorionic vesicle at the stageof formation of the extraembryonic coelom. The maternalblood lacunae pervading the syncitiotrophoblast are shownin black. From Beek, Moffat, and Lloyd (11).

Environmental Health Perspectives

vasive trophoblast than that found in man. Thisbecomes attached to a well marked epithelialplaque of endometrium which is absent in thehuman and no doubt is a potent source of histio-troph. As in man vascular maternal lacunae formbetween the trophoblastic cells and in the maca-que chorionic villi grow into the lacunae. Withintwo days gastrulation occurs and once again thecardiovascular system develops precociously sothat hemotrophic nutrition (i.e., the develop-ment of tertiary placental villi) is established atabout 5 somites-only a little after similar eventsoccur in man.

In New World monkeys such as the marmoset,superficial implantation is afforded by an exten-sive proliferation of trophoblast involving a largearea of the blastocyst and a reticulated syncitio-trophoblast penetrates the uterus. A large endo-metrial epithelial plaque and much glandular se-cretion show that histiotrophic nutrition is farbetter developed in these early stages than inrhesus or man. Eventually, cytotrophoblastic villiwith a mesodermal core penetrate into the reticu-lated syncitiotrophoblast and replace the latterby a placenta that looks very like that of the OldWorld monkey. However, the whole has taken farlonger to develop than the Old World monkeyplacenta and the marmoset is clearly dependentupon histiotrophic nutrition for a far longerperiod than is the macaque.

Both Old and New World monkeys possess de-cidual cells which are smaller than those in thehuman. Furthermore, the gross form of the pla-centa is often double discoidal, but this is ofdoubtful consequence in functional terms.

ConclusionsThis presentation has described the placental

system in some laboratory animals in order to seewhether they make appropriate models for com-parison with man. Clearly the simian primatesmost nearly approximate to the ideal but impor-tant differences between them and man still ex-ist. Even in Old World monkeys, but particularlyin the New World species, histiotrophic nutritionis more important in early development than it isin the human. This is quite likely to affect the wayin which drugs and other potential teratogens arehandled at these critical developmental stages.

In the rat, which is the cheapest and most wide-ly used test species, a true hemochorial chorio-allantoic placenta is not established until aboutthe 20-25 somite (anterior limb bud) stage. Be-fore that, histiotroph forms the only source of

nutrition. A further complication is that duringthe greater part of organogenesis histiotrophicnutrition occurs by a breakdown of a transudateof maternal serum in the endoderm of the in-verted yolk sac placenta. The capacity of the lat-ter to endocytize macromolecules has been shownto be affected by circulating teratogens (such astrypan blue) which cease to be effective as soonas the chorio-allantoic placenta is established(12).Though inverted yolk sac nutrition begins

rather later in lagomorphs, its characteristics ap-pear to be similar to that of rodents. In car-nivores, as typified by the ferret, histiotrophicnutrition in one form or another seems to be themajor source of nutrition until day 19-20 of gesta-tion, when about 35 somites are present in theembryo and the limb buds are already well devel-oped. The situation is also complicated by the factthat the chorio-allantoic placenta is endothelio-chorial in form and the placental barrier isthicker than in all other mammals. This contrasts

100-

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80-

70-

60

50-

40

30

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FIGURE 10. Externally visible abnormalities in fetuses follow-ing injection of trypan blue (50 mg/kg irto ferrets and 100mg/kg into rats) at (1) the primitive streak and (4) 23-33somite stages of development in the rat and the ferret. Thenumber of implantation sites is shown at the top of eachblock and the number of mothers used above this. Saline-treated controls are marked S. Both species respond totrypan blue when histiotrophic nutrition is the only formavailable (stage 1). At stage 4, the rat has a functionalhemochorial placenta, while in the ferret morphologicalevidence indicates that nutrition is still mainly histiotrophic.

December 1976

95 2

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1-100"IO"1-10

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11

with the attenuated barrier in the hemochorialplacentae present not only in simian primates butalso in rodents and rabbits. In this connection, itis interesting to note that the response of ferretsto the teratogenic action of trypan blue is qualita-tively and quantitatively completely differentfrom that of rats (Fig. 10).

Experimentalists who work with ungulatesmust recognize a prolonged period during whichthe embryo is entirely dependent upon histio-troph. In the pig the chorio-allantois probablyfunctions as a hemotrophic organ only at thestage of digital ray formation, and even thenhistiotroph remains vitally important, probablyuntil term. It is interesting, nevertheless, thatalthough the ungulate placenta is epithelio-chorial, the placental barrier in certain regions isthinner than that found in carnivores.

In summary, no experimentally available pla-cental model is identical to man. Therefore it isonly by an intelligent understanding of any sys-tem used that a proper appreciation of the appli-cability of results obtained to the human situationcan be made.

REFERENCES1. Mossman, H. W. Comparative morphogenesis of the foetalmembranes and accessory uterine structures. Contrib.Embryol. Carneg. Inst. Wash. 26:129 (1937)

2. Dancis, J., and Schneider, H. Physiology: transfer and bar-rier function In: The Placenta. P. Gruenwald, Ed., Medicaland Technical Publishing Co., Lancaster, England, 1975.

3. Grosser, 0. Fruhentwicklung, Eihautbildung und Placen-tation des Menschen und der Saugetiere. Munchen, 1927.

4. Brambell, F. W. R. The Transmission of Passive Immunityfrom Mother to Young. American Elsevier, New York,1970.

5. Beck, F., Lloyd, J. B., and Griffiths, A. A histochemical andbiochemical study of some aspects of placental function inthe rat using maternal injection of horseradish peroxidase.J. Anat. 101: 461 (1967).

6. Williams, K. E., et al. Digestion of an exogenous protein byrat yolk sac cultured in vitro. Biochem. J. 125: 303(1971).

7. Williams, K. E., et al. Quantitative studies of pinocytosis.II. Kinetics of protein uptake and digestion by rat yolk saccultured in vitro. J. Cell. Biol. 64: 123 (1975).

8. Moore, A. T., Williams, K. E., and Lloyd, J. B. The effect ofchemical treatment of "25I iodinated bovine serumalbumin on its rate of pinocytosis by 17.5 day rat yolk saccultured in vitro. Biochem. Soc. Trans. 2: 648 (1974).

9. Amoroso E. C. Placentation. In: Marshall's Physiology ofReproduction, Vol. II. A. S. Parkes, Ed., Longmans Green& Co., London, 1952.

10. Gulamhusein, A. P., and Beck, F. Development and struc-ture of the extraembryonic membranes of the ferret. Alight microsopic and ultrastructural study. J. Anat.120: 349 (1975).

11. Beck, F., Moffat, D. B., and Lloyd, J. B. Human Embryol-ogy and Genetics, Blackwells, London, 1973.

12. Wilson, J. G., Beaudoin, A. R., and Free, H. J. Studies onthe mechanism of teratogenic action of trypan blue. Anat.Rec. 133: 115 (1959).

12 Environmental Health Perspectives