HEMATOPOIETIC-INITIATING CELLS

Primordial germ cells are cells which can be detected first in the epiblast of the day 3 blastocyst and later as alkaline phosphatase positive cells in the extraembryonic mesoderm at the base of the allantois prior to day 7 p. c. At this time there are probably less than ten such cells in the embryo cylinder. After migrating back into the embryo, primordial germ cells migrate along the wall of the hind gut, through the para-aortic splanchnopleura, and reach the genital ridges on day 10.5 p. c. Their growth factor requirements, together with their coincident presence not only at the time definitive hemopoiesis begins, but probably also when primitive hemopoiesis is initiated, lends support for the hypothesis that the cells called primordial germ cells could actually be hemopoietic-initiating cells.