EXPERIMENTAL DISENGAGEMENT OF MORPHOGENETIC PROCESSES AND ITS EFFECTS ON PRIMORDIAL GERM CELL COLONIZATION OF THE GONADAL RIDGES IN RANA PIPIENS

[摘要] During normal embryogenesis in Rana pipiens the primordial germ cells (PGCs) undergo a displacement from deep within the ventral endoderm of the Stage 17 tailbud embryo to the dorsal endodermal crest of the hatched Stage 20 larva; eventually they exit the endoderm and populate the paired gonadal primordia. This study examines the migratory properties of PGCs and the mechanisms of their displacement by temporally uncoupling PGC migration from the morphogenetic events that normally accompany it. The methodology consisted of reciprocally transplanting germ cell-containing endoderm regions between Stage 17 embryos and Stage 20 larvae. The hosts were UV-irradiated to prevent their contribution to the gonadal germ cell population. Migration from the donor graft was determined by direct gonadal germ cell counts at host Stage 25. Normal Stage 20 PGCs transferred to 2 1/2 day younger Stage 17 hosts were able to colonize the gonadal ridges of the heterochronic graft larvae. The number of gonocytes present was approximately 20% of the unoperated control number. These data and the histological study of the heterochronic larvae (Subtelny and Penkala, 1984), demonstrate that PGCs can be experimentally retained within the endoderm for at least 2 1/2 days and are still able to migrate to the gonadal ridges. Reciprocal transfers of Stage 17 PGCs into Stage 20 hosts resulted in colonization of the host gonadal ridges 2 1/2 days earlier than would normally have occurred. The proportion of PGCs occupying the host gonadal ridges was significant, but restricted. Nevertheless, the results show that (1) intraendodermal PGCs can colonize gonadal ridges at least 2 1/2 days earlier than they normally would and (2) they do so in the absence of the main events of embryogenesis that occur during intraendodermal migration (Stages 17 to 20). Under conditions of temporally disengaged morphogenesis, the PGCs in either form of heterochronic graft emerge from the endoderm during a restricted time of development of the host, corresponding to Stage 22+/23-. This emergence is associated with the movements of mesodermal cells of the host lateral plates involved in dorsal mesentery formation. These findings support the hypothesis that morphogenesis plays a major role in the migration of PGCs from the endoderm to the gonadal ridges. (Abstract shortened with permission of author.)

[发布机构] Rice University