Leukocytes trapped in the fetal membranes of newborn foals
Upon the delivery of a foal’s pelvis at the end of stage 2 of labor, the foal’s hind legs remain inside the mare’s vagina, presumably to prevent any tension on or the rupture of the still intact umbilical cord. Current thinking on the purpose of maintaining an intact umbilical cord for several minutes after birth is that fetal blood that is still in the vascular bed of the fetal membranes has to be given some time to drain back into the neonate [1,2]. However, immediately after delivery of the foal a strong pulse is palpable in the arteries of its umbilical cord, implying that blood is not only draining from the membranes to the fetus, but many be pumped from the fetus into the membranes. As part of a larger study on the extent and possible significance of umbilical blood flow during the immediate post-delivery period we measured blood gas concentrations in umbilical venous and arterial blood. In order to be better able to interpret our blood gas findings, routine hematological evaluations were performed on the umbilical blood samples of three of the foals and in all three we discovered that more white blood cells were entering the fetal membranes than were leaving though the vein. This study tested the hypothesis that white blood cell counts in umbilical venous blood are lower than white blood cell counts in arterial umbilical blood of newborn foals. We also hypothesized that the extent of this apparent entrapment of fetal white cells in the allantochorion plays a role in placental separation and, hence, the interval between the delivery of the foal and the expulsion of its afterbirth.
Carlos M. Gradil, D.H. Volkmann, R.O. Gilbert, and R. Linke. "Leukocytes trapped in the fetal membranes of newborn foals" Theriogenology 58 (2002): 787-790.
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