Comparative mammalian placenta slides
The placenta is more variable in its form between species than any other organ (Wooding and Burton, 2008). This variability may reflect the fact that the placenta is very recent in terms of evolutionary history, relative to, for example, the kidney or liver, and, as such, it is still evolving. Expansion of the four superorders of mammals, the Xenarthra, Afrotheria, Euarchontoglires and Laurasiatheria occurred in different geographical regions on different landmasses, and thus may have been exposed to contrasting environmental selective pressures, such as climatic stability, nutrient supply, predation and pathogens(Capellini et al., 2015). These pressures influence the number of offspring in a pregnancy, the degree of maturation of the offspring at birth, and the gestation length of a species. Consequently the rate at which nutrients need to be delivered across the placenta, varies between species, which in turn impacts on the histology of the maternal-fetal interface (Capellini et al., 2011).
An alternative, but not necessarily mutually incompatible hypothesis, is that the rapid evolution is facilitated by the high activity of endogenous retroviruses within the trophoblast (Chuong, 2013). Compared to somatic tissues, the trophectoderm and primitive endoderm lineages are globally hypomethylated, generating a permissive epigenetic environment in which retroviral sequences act uniquely as gene enhancer elements (Chuong et al., 2013). Insertion of different retroviruses at different points of the eutherian mammalian radiation could thus account for the diversity of placental forms.
The point to remember is that all placental types are equally effective and efficient in supporting the development of live offspring in the natural habitat, but that data relating to transport pathways, placental metabolism and endocrinology should only be extrapolated between different species with extreme caution.
Here, we present images from different species available to us, and have annotated these to describe the main features. For a more comprehensive coverage you may wish to visit the site of the late Kurt Benirschke, (http://placentation.ucsd.edu) which features many exotic species obtained through his work with San Diego Zoo.
Licensing: CTR Slide Collections by Centre for Trophoblast Research is licensed under CC BY-NC-SA 4.0   
Select the species to view high resolution image:
| 1. | Baboon
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The deposition of fibrinoid starts wherever the syncytiotrophoblast is damaged, as syncytiotrophoblast acts as the endothelium of the intervillous space. The syncytium has NOS enzymes and the nitric oxide generates generally prevents platelet adhesion and activation. If the deposit becomes too extensive it envelopes the villus which then degenerates, as in the example above. Fibrinoid is deposited wherever the syncytiotrophoblast is deficient. It acts as a scaffold over which syncytiotrophoblast may differentiate, but that is often not sustained. Villous cytotrophoblast cells derived from the original villus adopt the extravillous phenotype (two examples are seen just to the left) and may contribute to the deposition of the matrix. The remainder of the villous tissue gradually degenerates. The syncytiotrophoblast forms the epithelial covering of the villous tree. It is responsible for active transfer, hormone synthesis, and immune protection. It is in a post-mitotic state, and is generated by fusion of the underlying progenitor cytotrophoblast cells. The villous cytotrophoblast cells lie sanwiched between the syncytiotrophoblast and the trophoblastic basement membrane. They are distinguishable by their large size and the pale staining of their cytoplasm compared to that of the syncytiotrophoblast. This is a villous haemochorial placenta, very similar in structure to that of the human. This is a villous haemochorial placenta, very similar in structure to that of the human. There is an almost complete multilayered cytotrophoblastic shell derived from the anchoring villi. Note there is no invasion of the extravillous trophoblast in to the decidua in this species. |
| 2. | Cow
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Endometrial glands open onto the surface of the uterus in between the caruncles. The overlying trophoblast is thickened and forms an areola, which is the site of histotrophic nutrition. The cotyledons of the cow placenta are larger than those in the sheep. The maternal side, the caruncles, are more convex so that the fetal membranes are draped over them rather than inserting into a central depression as in the sheep. The fetal villi grow into the crypts of the maternal caruncle establishing a large surface area for exchange. The interface is synepitheliochorial with no invasion of the maternal side but fusion of fetal binucleate cells with the uterine epithelium. The fetal villus (pale staining) interdigitates with the epithelium of the maternal crypt (darker staining). The binucleate cells produce placental lactogens which are transported across the placenta by migration and fusion with the maternal uterine epithelium. This space separating the maternal and fetal epithelia is an artefact caused by shrinkage during tissue processing. |
| 3. | Cat
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At either margin of the zonary placenta is the haemophagous region. Here maternal blood is released and is phagocytosed by yhe trophoblast as a means of transporting iron across the placenta. The breakdown products of the haemoglobin give the zone a characteristic colouration, which is brownish in cats and green in dogs. The fetal membranes are thrown into an elaborate series of folds that interdigitate with those from the mother. Branches of the umbilical arteries and vein can be seen in the membranes. Profiles of endometrial glands are prominent in the deeper part of the endometrium. This is the maternal side of the cat placenta. The cat has a zonary placenta that encircles the chorionic sac. At the histological level it is a lamellar endotheliochorial placenta. A maternal capillary embedded within the extracellular matrix of a maternal lamella. There is a bi-layer of trophoblast separating the two circulations, with an inner layer of unicellular cytotrophoblast cells and an outer layer of syncytiotrophoblast. A fetal capillary embedded within the pale staining mesenchymal core of a fetal lamella. A row of cytotrophoblast cells lying on a basement membrane that separates them from the stromal core of the fetal lamella. |
| 4. | Ferret
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The ferret has a huge large sac-like haemophagous region where maternal blood is extravasated and phagocytosed by the lining trophoblast cells. This is a means for transport of maternal iron to the fetus in the endotheliochorial placenta. This material is interposed between the maternal endothelium and the trophoblast layer. The endothelium of the maternal capillaries is by contrast relatively thick. The endothelium of the fetal capillaries is very thin, and the vessels invaginate into the syncytial layer as gestation advances, reducing the maternal-fetal diffusion distance. The ferret placenta comprises two discs which come into close contact at the mesometrial border of the uterus, as shown here. The remains of the endometrial glands are dilated with secretions, and are in contact with the tips of the chorionic villi at their apices. The tips of the chorionic villi come into contact with the remains of the endometrial glands and the endometrium. The ferret placenta is endotheliochorial, meaning that the uterine epithelium has been eroded and the trophoblast abuts the endometrial connective tissue in which the maternal blood vessels are embedded. |
| 5. | Horse
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The maternal side of the microcotyledons comprises non-glandular endometrial stroma that forms a number of crypts that enclose the fetal villi. The horse has a microcotyledonary epitheliochorial placenta. Each microcotyledon is only 1-2 mm in diameter and hence hard to distinguish with the naked eye. The endometrial glands open onto the uterine surface between the microcotyldons, providing histotrophic nutrition that is taken up by the overlying trophoblast. The fetal side of the microcotyledon comprises a branching villous tree that fits into the maternal crypts. There is no invasion at the interface, but a simple microvillous intergitiation of the two epithelia. |
| 6. | Pig
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An area where the fetal and maternal epithelia are still in contact. In many areas shrinkage during tissue processing as caused the two epithelia to separate. This is an epitheliochorial placenta with no invasion of the maternal tissues. The fetal capillaries indent into the trophoblast layer as gestation advances, reducing the diffusion distance between the maternal and fetal circulations. Hence, the number of tissue layers separating the two circulations is not necessarily a guide to gas transfer efficiency. The placental tissues form a series of short folds which interdigitate with folds from the maternal sideThey cover the whole of the placental sac but are not visible to the naked eye. Hence the pig placenta is described as diffuse. The endometrium contains numerous glands. These cluster and open together in aerolae. Here the trophoblast is separated away from the maternal tissues, becomes more columnar in shape and phagocytoses the secretions from the glands. At these sites the maternal and fetal tissues separate and the cavity between the two is filled by gland secretions. These are sites of histotrophic exchange. The secretions from the endometrial glands separate the maternal and fetal tissues. The trophoblast epithelium is columnar at an areola, and takes up the histotroph from the maternal glands. This is a means of transfer of iron to the fetus, bound to lactoferrin. |
| 7. | Sheep
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The fetal villi interdigitate with the maternal crypts. In vivo the maternal and fetal epithelia would be in contact, but tissue shrinkage following fixation and embedding has caused the two to separate. At the tips of the maternal crypts there is localised breakdown of the maternal tissues causing the release of free maternal erythrocytes. The erythrocytes are phagocytosed by the fetal trophoblast and broken down as a means of transporting iron across the epitheliochorial placenta. |
| 8. | Giraffe
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The Giraffidae are considered to form a separate family within the ruminant suborder. Their major characteristics: polycotyledonary epitheliochorial structure. Giraffe and okapi show characteristic ruminant trophoblast binucleate cells (BNC) which migrate and fuse with individual uterine epithelial cells as in the cow. However, there are many fewer BNC, of limited distribution, when compared with other ruminants so far investigated. |
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