Equine placentation overview
Keywords: placenta, diffuse, equine
This image shows the diffuse placentation typical of horses. The fetus is about two months old. Its placenta detached very easy from the endometrium in this fresh specimen.
Image size: 1800 x1350px
As shown in the inset, a close up image of a placenta at term, micro cotyledons (red arrows) eventually form an intimate attachment to the endometrium. These micro cotyledons are similar in structure to those of cattle but microscopic in size. The bond between the placenta and the endometrium becomes firmly established by about six or seven months of gestation. Only after this time, is the placenta likely to be retained in the case of abortion.
In the image below, the placenta has been opened to show the relationship between the fetus and the placental membranes.
Image size: 1182 x 660px
Most of the image is self-explanatory but some comment may be beneficial to the viewer.
The equine placenta has both intra- and extra-amnionic portions of the umbilical cord. This differs from ruminants, where there in only an intra-amnionic cord because the ruminant amnion is adhered to the inside of the chorion along much its length. In ruminants therefore, urine immediately escapes from the distal end of the intra-amnionic cord into a "hollow belt" that connects the sections of the allantois that lie on either pole of the amnion. In equids by contrast, the amnion (strictly speaking, the amnio-allantois) is completely free from the chorion (the allantochorion). Therefore, the equine amnion can move and rotate freely within the allantoic cavity. This makes it possible for an equine fetus to change its presentation from cranial to caudal and vice versa until pregnancy is advanced, perhaps up to nine months or later. By contrast, the presentation of a bovine fetus has become fixed by approximately five months of gestation.
A distinct disadvantage of having an amnion that is free of the chorion is that it is possible for a foal to be born with an intact amnion covering its head. This can result in suffocation and perinatal death. In ruminants, that is practically impossible because the amnion is strongly adherent to the chorion and cannot follow the fetus after it has been born.
Note that the large vessels seen on the amnion here are temporary; they will have regressed and will be barely visible at the time of foaling.
Incidentally, the prepuce of the fetus can be seen just caudal to the attachment of the umbilical cord. It can easily be can be seen on ultrasound examination and is valuable in diagnosing the gender of a fetus.
Additional details on equine placental anatomy can be found elsewhere in LORI
