The placenta provides an interface for metabolic exchange between the dam and the foetus.
It is a transitional endocrine organ that produces hormones responsible for:
- maintenance of pregnancy
- stimulation of the maternal mammary gland
- promotion of foetal growth.
Ruminants have a cotyledonary placenta, which is defined as a placental unit of trophoblastic origin consisting of abundant blood vessels and connective tissue.
In sheep, there are between 90 and 100 cotyledons distributed across the surface, and in cattle 70-120 have been observed.
The placentome consists of a foetal cotyledon contributed by the chorion and a maternal caruncle originating from the endometrium. These placentomes start forming about day 16 in the sheep and day 25 in the cow. In the sheep the placentomes form a concave structure, while in the cow they are convex. During gestation, these placentomes will greatly increase in diameter, nearing 5-6cm in the cow by the end of pregnancy.
Dog and cat placenta
Dogs and cats have a zonary placenta which includes a prominent region of exchange that forms a broad zone around the chorion near the middle of the conceptus.
There are also highly pigmented rings at either end of the central zone, called marginal haematomas. They are thought to be important in iron transport from the dam to the foetus.
The mare placenta is diffuse but has many specialised zones called microcotyledones. They are only visible microscopically and are distributed over the entire placental surface.
The placenta contains unique transitory structures called endometrial cups. They range from a few milimetres to several centimetres in diameter. There are usually between 5 and 10 endometrial cups distributed over the surface of the placenta. They are of both trophoblastic and endometrial origin. These structures produce equine chorionic gonadotrophin (eCG) and develop between days 36 and 60 of pregnancy. From day 60 they begin to be sloughed and are no longer functional. The debris from the sloughing of the cup may form allantochorionic pouches, up to 2cm in length, which can be present in the placenta at term.
The hippomane is another normal structure of pregnancy: it is liver-like in texture and thought to arise as an out-pouching of the allantois which eventually forms a pedicle and separates to float free in the allantoic cavity. Other suggestions include that it is a collection of debris, thought to be deposits from the foetal urine and cells.
The cervical star is an area of fibrosis where the placenta did not attach to the uterus during pregnancy and is the area that the foal breaks through at birth.
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Senger, P. (2003) Pathways to Pregnancy and Parturition Current Conceptions Inc
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