Bovine placental anatomy
Keywords: amnion, chorion allantois, bovine, fetus, sexing, tubercle, ultrasound, slip, pregnancy diagnosis
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A bovine fetus at about 50 days of gestation (based on crown-rump length). Note the early formation of the cotyledons, these form opposite the caruncles in the uterus, structures that have been present since the birth of the dam. Together, the caruncles and cotyledons will form "placentomes"; multiple complexes of placental attachment; hence the morphological name of this type of placetation i.e. "muliplex". There is attachment of the fetus at the placentomes at this stage of gestation but it is not firm; this fetal-placental unit could easily be removed from uterus.
During pregnancy diagnosis by transrectal palpation at this stage of gestation, it is relatively easy for a practiced operator to lightly grasp the wall of the uterus and the fetal membranes and to let the membranes slip away, feeling its crisp escape from beneath ones fingers. This is known as "slipping membranes".
With regard to the basics of placentation, note that the allantois (the inner sac at the extremities of the fetal-placental unit in the top image) is not yet attached to the chorion. Later in gestation, these two membranes will become inseparable and will be known as the allantochorion or less commonly, the chorioallantois. The central, bean-shaped structure is the amnion.
A close up view of the amnion shows how easily the chorion, allantois and amnion can be separated from one another at this stage of gestation. A pair of forceps has been placed under the allantois as it emerges from the amnion (or "...as it emerges from the end of the intra-amnionic umbilical cord"). Proximal to the fetus and within the amnion, the urachus leaves the bladder and runs down the intra-amnionic umbilical cord, directing urine into the allantois. Unlike horses, ruminants do not have both intra- and extra-amnionic segments of the cord; only intra-amnionic cords.
If one was to separate the allantois from the surface of the amnion (easy to do at this stage of gestation) one would see that the allantois actually crosses the amnion on one side only; like a bridge or belt. This is shown clearly in the lowest of the three images.
| A note on the allanto-amnion and its significance: On the far side of this fetus (and every bovine fetus in this orientation) the amnion is attached to the chorion. Therefore there is no allantochorion on that side of the amnion, only an "allanto-amnion". The allanto-amnion is seldom, if ever, referred to in scientific literature but it does occur and forms a distinct entity of bovine placentation. At term, the attachment of the amnion to the allantois (the allanto-amnion) is firm. The allantois in turn, is firmly attached to the endometrium. This means that the bovine amnion cannot leave the uterus without tearing while the fetus exerts pressure on its caudal-most extremity. Therefore a bovine fetus cannot suffocate in its fetal membranes after birth. Unfortunately the same is not true of horses, where the allantois surrounds the amnion in its entirety, isolating it completely from the chorion. (the allantois covers the outer surface of the amnion and the inner surface of the chorion). Therefore the equine amnion floats freely within the placenta, like a balloon, tethered to the allantochorion by the extra amnionic umbilical cord This allows the equine amnion to leave the uterus intact, covering the face of the equine fetus, causing suffocation. As mentioned, this is impossible in cattle. |
Note the pale genital tubercle in this fetus, just under the tail. This is a female. In a male, the genital tubercle would be situated just caudal to the umbilical cord. The situation of the genital tubercles allows fetal sexing by transrectal ultrasound in both cattle and horses at 55 to 60 days of gestation.
Another bovine feto-placenta unit, in this case, exactly 54 days old (according to the breeding date):
Again, the typical arrangement of the placentomes can be appreciated but when the image is enlarged, the fetal villi can be seen clearly. In other words, the establishment of epithelio-chorial placentation is well underway although the feto-placental unit can easily be separated from the uterus by pulling on it gently.
Older nomenclature held that most bovine placentation was epitheliochorial but presently is is referred to as synepitheliochorial because large multinucleate cells from the chorion invade the epithelium of the maternal placenta (the caruncles) so the chorion ostensibly becomes part of the maternal epithelium ("syn-" < Greek together).
When the chorion was stripped away from the amnion, the allantoic belt that crosses the amnion could be seen clearly. That anatomy is evident below. In this image, the trumpet-like section of the allantois that originates from the urachus, cannot be seen because the fetus has been rolled over to reveal the top of the amnion.
Again, note the prominence of the genital tubercle; another female. Its prominence at this time clearly demonstrates why many fetuses are successfully sexed using transrectal ultrasound at about 55 to 65 days of gestation. In that regard, see this ultrasound image.
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In this image, the chorion has been transected along its longitudinal axis, thereby opening the allantois and exposing the amnion (allantoamnion/amnioallantois). As mentioned earlier, there is no allantois between the amnion and chorion over a substantial part of the surface area of the amnion. Therefore, the amnion is fused to the chorion in that area. This allows one to suspend the amnion from the chorion as shown here.
Again, the hook-shaped genital tubercle is very obvious at this stage of gestation, allowing gender determination by transrectal ultrasonography. In the mature female, the genital tubercle will form the clitoris.
