Chimerism in Horses
Observations made on chorial membranes from about 50 cases of dizygotic twin pregnancy in the mare have shown that nearly all cases are bicornual and that invagination of the chorial sacs develops in about 75%. Tissular fusion was observed from the fifth month on in about 40% of cases. Macroscopic vascular anastomosis between both foetal placentae was shown in 5 out of 27 cases. Blood groups in the parents and foals of 10 cases of twin-pregnancy have been determined, using 18. reference-antibodies. Identical blood groups were found in both foals from 4 cases of twin gestation, indicating chimerism and blood-anastomosis between both chorions; blood chimerism was probable in 2 other cases in which one foal only could be examined. Macroscopic fusion between both chorial sacs may occur without the establishment of vascular anastomosis, and blood-chimerism has been shown in some cases where both chorial sacs seemed to be free, thus indicating the occurrence of rather microscopical blood anastomosis. Different blood groups occurred in three cases of twin-pregnancy. Thus there was altogether a surprisingly high incidence of vascular anastomosis between the chorial sacs in cases of bicornual twin-gestation.
In two cases of still-born and in three cases of living bisexual twins of one to two years of age, the genital apparatus of the female co-twin was found to be normal. These observations point to a much lower incidence of freemartin-deformation than in cattle.
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