The Placenta

Gross anatomy of the fetal membranes

The first description of the camelidae placenta was given in the middle of the 18th century.⁽⁸³⁾ This study was the first to demonstrate that the dromedary placenta is not cotyledonary but rather diffuse, as in the mare. This observation was confirmed later by several authors for all camelidae.^{(11, 45, 58, 80, 110)} The placenta in the llama was described as hemochorial⁽⁸²⁾ but this was later rejected by others who described the placenta as epitheliochorial.⁽⁵⁷⁾

The placenta of camel idae presents a bright red external surface and a smooth, white or yellowish inner surface (Figure 5.61). It is expulsed as a sac-like structure with a greater and lesser curvature measuring 140 to 200 and 300 to 350 cm in the dromedary and the Bactrian camel, and 195 to 280 and 95 to 210 cm in the llama. The maximum circumference for the three species respectively is 60 to 90 cm (dromedary and Bactrian) and 50 to 80 cm (llama).^{(39, 60)}
 

The umbilical cord is long and twisted. Its length and diameter at term are 50 to 110 cm and 3 to 5 cm respectively in the dromedary and Bactrian camel, and 30 to 50 cm and 2 to 3 cm in the llama. ^{(39, 60)} The umbilical cord contains two arteries and two veins which send ramifications over the inner chorionic surface and an allantoic duct (urachus) between them. In the fetus, the umbilical cord extends to the amniotic membrane at a site on the lesser curvature of the placenta where the amnion is in apposition to the allantois.^{(39, 60)}

At full term, the llama amnion is loosely adhered to either the chorion or the allantois. The amniotic sac extends to the top of the pregnant horn and occupies all the space not occupied by the allantoic sac. The amnion may be peeled off easily from the chorion whereas the attachment to the allantois is more secure. The volume of amniotic fluid is quite low. At parturition, dehiscence of the placenta takes place through the amniochorion. The amnion remains with the chorion.⁽³⁹⁾

The red velvety outer (uterine) surface of the chorion contains numerous villous processess (papillae) which vary in size and density. The papillae are columnar, cuboidal, or conical with a narrow base. Patches of papillae are most dense on the greater curvature and toward the center and less dense at the tips of the placenta. The areas in proximity to the large vessels of the allantochorion are devoid of patches.^{(39, 60)}

The amnion is delivered with the newborn. It is partly adhered to the chorion (chorioamnion) and also forms the allantoamnion which is lightly vascularized near the umbilical region.⁽⁶⁰⁾ In the llama, the amnion is not intimately adhered to the chorion at less than 90 days of gestation.⁽³⁸⁾

The allantois occupies the whole right horn and extends along the lesser curvature of the left. The amnion is completely confined to the left horn. The umbilical cord from the fetus is attached to the allantoamnion. The placental vessels radiate from the umbilical cord over the allantois to the allantochorion.⁽³⁹⁾

One or two masses of proteinaceous deposits called hyppomanes are sometimes found in the allantoic cavity.^{(60, 80)} The hippomane consists of a nucleus of cellular debris surrounded by concentric deposits of mucein, calcium, and phosphorus. This structure has no known function.⁽³⁹⁾

A distinct feature of the fetal membranes in the camelidae is the presence of an additional 1- to 2-mm thick opaque membrane covering the fetus and attached to the mucocutaneous junctions at the lips, nostrils, anus, vulva, prepuce, the foot pad, coronet of the nail, and at the umbilicus (Figure 5. 54).^{(26, 39, 56, 62, 112)} This membrane appears to be produced by the basal layer of the epidermis and has different names according to authors: 4th membrane, epidermal membrane, or epithelion. The membrane is probably formed as the epidermal cells multiply and push toward the surface and becomes keratinized with advancing pregnancy. In the dromedary, the membrane appears early during pregnancy (CRL 24 cm). The epidermal membrane is thicker and more adherent in premature fetuses.

Histology of the fetal membranes

The chorionic villosities appear under low power microscopy as folded villous tufts with a central mesenchymal core covered by a thin layer of trophoblast. Large masses of syncytiotrophoblast are present. The intra-epithelial tissue presents a rich vascular network of capillaries which join larger vessels in the mesenchymal core and form sub-epithelial plexuses at regular intervals. The intra-epithelial capillaries are absent from the areas of columnar cytotrophoblast.^{(42, 60, 111)}

The umbilical cord comprises two veins, two arteries, and the remnant of the allantoic duct as well as many small vessels. The umbilical veins are larger than the arteries and present a muscular wall interlaced by elastic tissue and a lining of low cuboidal endothelium. The alantoic duct is lined by tall columnar cells which rest on a basal layer of cuboidal cells. The umbilical co.^{(42, 60, 111)}

The amnion is lined by a single layer of flattened epithelial cells which form low comified plaques. The epidermal membrane consists of a deep layer of stratified squamous epithelial cells lying next to the fetus and an outer keratinized layer, with indistinct cell outlines and no nuclei. There are no elastic fibers in the membrane, and histochemical techniques indicate that it is rich in disulfide and sulfhydryl groups.⁽⁶²⁾

Histological features of the fetal membranes in the Bactrian camel and llama are similar to those of the dromedary. The histological arrangement of the placenta in camelidae, especially the presence of a trophoblastic layer in the chorion, confirms the epitheliochorial nature of the placenta in these species.^{(42, 60)}

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