Udder – The udder is located in the ventral midline between the thighs. It is a modified sweat gland that contains sebaceous glands and has no hair. A non-lactating udder is small with flattened teats and is hardly noticeable in a virgin mare. The udder is made up of the mamma (body) and 2 papilla (teat) as well as a lactiferous duct system. There are 4 mammary glands that produce on average 10 L of milk per day during lactation in response to hormone changes. The mammary tissue consists of alveoli held by connective ducts. Each quarter of the udder has its own lactiferous sinus which drains into a teat sinus, then into 2 streak canals/teat. The extremity of the teat is closed to prevent leakage by a tight sphincter muscle. The sinus also acts as a storage for milk between feedings.
The udder is supported by medial suspensory ligament which runs along the midline. The glands are given extra support by lateral suspensory ligaments.
Placenta – the placenta of a horse is defined as diffuse & epithelialchorial. The placenta is made up of 2 membranes, chorioallantois & the amnion . The placenta doesn’t fully mature until about day 150 of gestation. The chorioallantois membrane is the outer layer of the placenta that is in direct contact with the endometrium. The outside surface, the chorion, has a velvety red appearance due to its vascular supply and finger like projections that invaginate into the endometrial wall, while the inner surface, the allantois, has a white glistening appearance. Between the chorion & the allantoic membranes is the allantoic cavity where allantoic fluid is found.
The amnion is the opaque membrane that directly surrounds the foal. The foal floats within the amniotic cavity within amniotic fluid. The amnion is not vascularized.
The cervical star of the placenta opposes the cervix and it where the foal breaks through the chorioallantoic membrane during parturition. This is the only location where the placenta doesn’t contact the uterus.
When a foal is born the placenta is turned inside out as the foal breaks through the cervical star pulling the inverted placenta with it by way of the umbilical cord.
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