Late gestation pregnancy and placentation
Keywords: bovine, calving, partus, parturition, placenta, caruncles, placentomes, cotyledons, cervixThese images show some of the characteristics of a pregnant uterus removed from a Simmental cow that died suddenly during the early stages of calving. The cause of death was not determined.
As shown below, the calf was located in the right uterine horn. In that regard, it well to remind readers that the uterine horn occupied by the fetus is of little significance during cesarean sections. This is because the so called "non-pregnant" horn contains such a small volume of fetal fluid that it is deflected towards the midline by the horn that contains the calf. Therefore (in a normal pregnancy) albeit left or right, the "pregnant" horn is readily accessible from a normal left flank approach.
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In the image above, arrow A indicates that if one were to raise the right uterine horn and inspect the mesometrium (arrow B) the uterine artery would be visible. From about 4 months of gestation, the volume of blood flow in this artery excedes the ability of its thin wall to constrain blood flow in a linear fashion. In bovine pregnancies, this results in turbulent blood flow, reminiscent of air flowing through a "wind sock". In this video, the author illustrates turbulent air flow that is akin to blood flow vibration felt during palpation. This phenomenon is known as fremitus (< Latin. "To murmur"). Fremitus appears later in the vessel supplying the "non-pregnant" horn than that supplying the "pregnant" horn. In both vessels however, it ceases shortly after calving. Therefore it can still be present after calving and in the strictest sense, cannot be regarded as an infallible indicator of pregnancy.
Interestingly, fremitus is not detectable in pregnant mares.
As shown below, the corpus luteum (CL) of pregnancy (ringed in green) was still present in this specimen, shortly before calving would have occurred. This is normal.
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Although the CL of pregnancy is still present at term, most cows remain pregnant even if total ovariectomies are performed within the last 30 days of gestation. Indeed, some cows becomes independent of CL progesterone secretion as early as 6 months of gestation. After ovariectomy however, pregnancies do not end normally. Although pregnancies are maintained to term, cervical dilation, dystocia and retained fetal membranes are common. This is because the corpus luteum is a also a source of relaxin, prostaglandins E1 & E2 and even oxytocin. It is therefore, essential for normal parturition.
Soon (12 to 16 hours) after substantial uterine contractions begin, the cervix relaxes rapidly. This relaxation is largely under the effect of PGE2 but as mentioned earlier, other hormones are also involved.
The initial phase of cervical dilation is largely passive i.e. independent of the dividing force of the fetal head and forelimbs. However, the effect of the fetus can not be excluded completely because the uterus is pushing it towards the cervix, even during the early stages of cervical dilation. Within 8 to 10 hours in cows and somewhat longer in heifers, the cervical canal dilates to a diameter of 12 to 20 centimeters i.e.
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In the specimen discussed here, passive dilation was incomplete yet almost sufficient to accommodate a human hand (about 12 cm wide, including the thumb).
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Normally, in pre-term pregnant cows, even in autolysed specimens, this would be impossible. Therefore had this cow lived, the second (expulsive) stage of calving would have been imminent.
The image below confirms the fact that the fetus had not yet entered the birth canal i.e. active dilation of the cervix and second stage parturition had not yet begun.
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The following image shows the fetal membranes and the calf, removed from the uterus. The normal number of placentomes in bovine pregnancies varies between 75 and 120, the largest measuring 12 to 14 cm in length. Also, as reviewed elsewhere, amnionic fluid volume varies between 2 and 8 liters and allantoic fluid, between 4 and 15 liters. This pregnancy appeared to be normal in those respects.
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Placentation in ruminants consists of numerous complexes. Hence the term "multiplex placentation" as apposed to "diffuse, discoidal, zonary" etc. The two parts of each complex are of course, a caruncle on the maternal side and a cotyledon on the fetal side. Together, they form a placentome. A placentome is shown here:
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As illustrated elsewhere in LORI, the site and number of caruncles in a cow has already been established when the cow herself was a fetus!
Note how the fetal villi are sliding out of the maternal crypts, a process that is impossible if placental maturation (flattening of maternal epithelium and loosening of fetal-maternal contact) has not occurred during the last few days of gestation. Incomplete placental maturation is a cause of retained placenta. Placental maturation is impaired if the fetus is born prematurely or the diet of the cow is deficient in selenium and vitamin A, nutrients that are essential for epithelial function. This explains in part why retained placenta is common when selenium and vitamin A are deficient in the diet of pregnant cattle.
