3. Gross anatomy
In the cow, the chorioallantois attaches at about 4 weeks of gestation to the lining of the uterus. This becomes more intimate over specialized areas of the endometrium called
Ž caruncles
approximately 100 are present, evenly distributed throughout the en- .
dometrium . These sites can be seen grossly even in uteri of fetal calves. As the extra embryonic fetal membranes grow and expand within the lumen of the uterus, the flat
surface of the chorioallantois will begin to become irregular over these caruncles. The remodeling of these areas, now recognized grossly as cotyledons, progresses to form
Ž villous projections that interdigitate with recesses in the surface of the caruncle Figs. 3,
. 5 and 6 . The surface of the caruncles develops crypts and the apposing chorioallantois
is modeled to form finger-like villous projections that enter into them, thus, enhancing Ž
. the contact surface area Figs. 4–6 . The combined cotyledonary and caruncular tissues
together are called ‘‘placentomes’’. As gestation progresses, these become domed ovoid Ž
. shaped structures ranging from 10 to 12 cm long and 2–3 cm thick Figs. 4 and 5 .
Significant variation in size normally occurs. Placentomes are arranged in a very orderly Ž
. manner in four rows that run lengthwise along both uterine horns Fig. 7 . They are
largest in the horn in which the fetus develops and tend to decrease in size towards the apex of both horns. The surface area of contact between the cow and fetal calf is greatly
Fig. 6. Bovine placentome with the cotyledonary villi partially separated from caruncular crypts. This photograph taken with the partially separated placentome under water demonstrates the relationship
between the fetal villi and the maternal crypts.
Ž .
Fig. 7. ‘‘Fetal membranes’’ chorioallantois with cotyledons expelled after delivery. This placenta delivered at term demonstrates the regularity of cotyledons and their blood supply from branches
of the umbilical arteries and veins. The chorioallantois is viewed from the allantoic surface. In utero, cotyledonary villi would extend from the opposite side into the endometrial crypts of the caruncle. Four rows
of placentomes two demonstrated line each uterine horn.
Ž .
enhanced by the extensive interdigitation of fetal and maternal tissues Fig. 6 . This is
2
Ž . Ž
estimated to be 130 m Russe and Sinowatz, 1991
equivalent to the entire surface area .
of a large room . The chorioallantois that exists between placentomes opposes the endometrium in flat
to gentle folds and is referred to as either the ‘‘interplacentomal’’ or the ‘‘smooth Ž
. chorioallantois’’ Figs. 5 and 7 . Both the cotyledonary and the intercotyledonary
chorioallantois function to meet the ever-increasing fetal metabolic demands, including the support of rapid tissue growth. Additionally, there is an extensive de novo protein
synthesis within placental tissues and production of an ever-expanding list of hormones and growth factors.
The small end of the chorioallantois that occupies the very tip of each uterine horn usually undergoes degeneration and coagulative necrosis. These ‘‘necrotic placental
tips’’ are very common and their size varies with some approaching 3–5 cm in length. The amnion is attached over much of its surface to the chorioallantois. The inner
surface of the amnion is covered with whitish, firm ‘‘amniotic plaques’’ which are usually 1 cm or less in diameter. When fetuses are stressed in utero, they will defecate
and the amniotic fluids, fetal hair and skin, and the amniotic plaques will appear yellow. Identification of the amniotic plaques for the identification of the amnion is a useful
initial step in orienting placental tissues during gross examination.
Amniotic fluid is produced by several means which include fluxes of fluid across the amnionic epithelium and unkeratinized fetal skin, production by the lungs, and, in later
pregnancy, by urine. During most of gestation, urine produced by the fetus is expelled into the bladder and through the urachus within the umbilical cord into the allantoic
cavity. Near term, however, it has been shown that the composition of amniotic fluid changes to become more like urine. Fetuses older than 240 days gestation urinate
Ž .
through the urethra into the amniotic cavity Reeves et al., 1972 . The amount of amniotic and allantoic fluid is approximately equal in the 3-month gestation bovine
Ž .
fetus, but by term, the volume of allantoic fluid 10–15 l is greater than the volume of Ž
. Ž .
amniotic fluid 2.5–7.5 l Russe and Sinowatz, 1991 .
4. Histology
Kategori