A comparison of the histological structure of the placenta in experimental animals

Abstract

The primary function of the placenta is to act as an interface between the dam and fetus. The anatomic structure of the chorioallantoic placenta in eutherian mammals varies between different animal species. The placental types in eutherian mammals are classified from various standpoints based on the gross shape, the histological structure of the materno-fetal interface, the type of materno-fetal interdigitation, etc. Particularly, the histological structure is generally considered one of the most useful and instructive classifications for functionally describing placental type. In this system, three main types are recognized according to the cell layers comprising the interhemal area: (1) epitheliochorial type (horses, pigs and ruminants), (2) endotheliochorial type (carnivores) and (3) hemochorial type (primates, rodents and rabbits). The number of cell layers in the interhemal area is considered to modify the transfer of nutrients between maternal and fetal blood and is one of the important factors with respect to the difference in placental permeability between animal species. Therefore, in reproductive and developmental toxicity studies, careful attention should be paid to the histological structure of the interhemal area when extrapolating information concerning placental transfer characteristics to different animal species.

Keywords: cynomolgus monkey; dog; minipig; placenta; rabbit; rat.

Fig. 1.

Classification by placental gross shape.

Fig. 2.

Classification by relationship between the chorion and uterine wall. BM, basement membrane; Te, trophectoderm; Cy, cytotrophoblast; En, endometrium; FB, fetal blood; FV, fetal vessel; MB, maternal blood; MI, maternal interstitium; MV, maternal vessel; ST, specific trophoblast; Sy, syncytiotrophoblast.

Fig. 3.

Pig (minipig) placenta. Epitheliochorial type placenta. a) Gross appearance on gestation day 100. b) Histological section at low magnification. HE stain, bar=3 mm. c) Interhemal area. HE stain, bar=60 μm. d) Areolus. HE stain, bar=60 μm. e) Endometrial gland. HE stain, bar=60 μm. AC, areolar cavity; Ar, areolus; AT, areolar trophectoderm; EG, endometrial gland; En, endometrium; FV, fetal vessel; IA, interhemal area; MV, maternal vessel; Te, trophectoderm; UM, uterine muscle.

Fig. 4.

Dog placenta. Endotheliochorial type placenta. a) Gross appearance on gestation day 35. b) Histological section at low magnification. HE stain, bar=3 mm. c) Labyrinth zone. HE stain, bar=60 μm. d) Junctional zone. HE stain, bar=60 μm. e) Sponge zone. HE stain, bar=60 μm. f) Marginal hemophagous zone. HE stain, bar=60 μm. FV, fetal vessel; GZ, glandular zone; LZ, labyrinth zone; JZ, junctional zone; MV, maternal vessel; MZ, marginal hemophagous zone; NL, noncellular layer; SZ, sponge zone; Tb, trophoblast; UM, uterine muscle.

Fig. 5.

Rat placenta. Hemotrichorial type placenta. a) Gross appearance on gestation day 15. b) Histological section at low magnification. HE stain, bar=2 mm. c) Labyrinth zone. HE stain, bar=60 μm. d) Basal zone. HE stain, bar=60 μm. e) Metrial gland. HE stain, bar=60 μm. f) Yolk sac. HE stain, bar=60 μm. BZ, basal zone; Cy, cytotrophoblast; De, decidua; EC, epithelial cell; FV, fetal vessel; GC, glycogen cell; LZ, labyrinth zone; MG, metrial glands; MV, maternal vessel; SA, spiral artery; Sp, spongiotrophoblast; Sy, syncytiotrophoblast; TG, trophoblastic giant cells; uNK, uterine natural killer; YS, yolk sac.

Fig. 6.

Rabbit placenta. Hemodichorial type placenta. a) Gross appearance on gestation day 28. b) Histological section at low magnification. HE stain, bar=3 mm. c) Labyrinth zone. HE stain, bar=60 μm. d) Junctional zone. HE stain, bar=60 μm. e) Decidua. HE stain, bar=60 μm. f) Yolk sac. HE stain, bar=60 μm. DN, decidua, zone of necrosis; DS, decidua, zone of separation; EC, epithelial cell; FV, fetal vessel; JZ, junctional zone; LZ, labyrinth zone; MV, maternal vessel; Tb, trophoblast; UM, uterine muscle.

Fig. 7.

Cynomolgus monkey placenta. Hemomonochorial type placenta. a) Gross appearance on gestation day 111. b) Histological section at low magnification. HE stain, bar=3 mm. c) Villus. HE stain, bar=60 μm. d) Chorionic plate. HE stain, bar=60 μm. e) Basal plate and decidua. HE stain, bar=60 μm. AV, anchoring villus; BP, basal plate; CP, chorionic plate; Cy, cytotrophoblast; De, decidua; FV, fetal vessel; IS, intervillous space; MC, mesenchymal cell; Sy, syncytiotrophoblast; Vi, villus.