Embryological Basis for Germ Cell Chimerism in Mammals

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Abstract
That blood cell chimerism of dizygotic twins is accompanied by germ cell chimerism was suggested by previous demonstrations of the presence of female metaphase figures in the testis of males born twin to genetic females in cattle (Ohno et al., 1962) and the marmoset (Benirschke and Brownhill, 1963). The present findings in cattle on fetal triplets only 12 mm in crown-rump length furnishes the embryological basis for the observed phenomenon of germ cell chimerism. The liver of each triplet contained both male and female metaphase figures; cell chimerism was already established in all three. A histochemical test on alternate serial transverse sections revealed that alkaline-phosphatase-positive primordial germ cells were still in the middle of migration from the yolk sac to the developing gonadal ridges. A number of alkaline-phosphatase-positive cells were found within blood vessels, particularly within the venous sinusoids of the liver. They were interpreted to be primordial germ cells which had wandered into the blood stream.

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