IDENTIFICATION AND TEMPOROSPATIAL DISTRIBUTION OF BOVINE PRIMORDIAL GERM-CELLS PRIOR TO GONADAL SEXUAL-DIFFERENTIATION

The temporospatial distribution of bovine primordial germ cells was st udied in 34 embryos (18 to 39 days). For a reliable identification of bovine primordial germ cells in varying localizations and at different developmental stages the alkaline phosphatase reaction combined with the use of selected lectins was applied. The first potential primordia l germ cells were identified in an 18-day-old trilaminar embryo in the caudal wall of the proximal yolk sac at a distance of less than 100 m u m from the germ disc. These cells are alkaline phosphatase-positive, but do not yet react with lectins. From 18 through 23 days, morphogen etic folding converts the flat trilaminar disc into a cylindrical embr yonic body. During this folding process primordial germ cells located in the proximal yolk sac area are incorporated into the embryo when th is portion of the yolk sac becomes the hind- and mid-gut. Consequently , in 23- to 25-day-old embryos putative primordial germ cells (alkalin e phosphatase- and lectin-positive) are situated predominantly in the axial body region at the level of the mesonephros. When the gonadal ri dge develops in this region (about day 27) it contains a certain numbe r of primordial germ cells present from the very beginning. Thus, the assumptions of a long-range chemoattraction of primordial germ cells b y the gonadal ridge, of active immigration from an extraembryonic site , or of a passive transportation via the blood stream are not necessar y to explain the initial settlement of bovine primordial germ cells in the gonadal ridge. Within the gonadal ridge (days 27-31) and later in the still sexually indifferent gonadal fold (32-39 days) the primordi al germ cells are unevenly distributed. Extragonadal potential primord ial germ cells (alkaline phosphatase-positive, but with reduced or no lectin staining) are regularly present in large numbers in bovine embr yos with indifferent gonads. Such cells occur predominantly in the par aaortal tissue and in the liver, but also in the branchial arches. The different locations of extragonadal primordial germ cells are discuss ed in the light of recent evidence that germ cells and haematopoietic cells share common ancestors.