INTRAEMBRYONIC HEMATOPOIETIC-CELL MIGRATION DURING VERTEBRATE DEVELOPMENT

Vertebrate hematopoietic stem cells are derived from ventral mesoderm, which is postulated to migrate to both extra- and intraembryonic posi tions during gastrula and neurula stages. Extraembryonic migration has previously been documented, but the origin and migration of intraembr yonic hematopoietic cells have not been visualized. The zebrafish and most other teleosts do not form yolk sac blood islands during early em bryogenesis, but instead hematopoiesis occurs solely in a dorsal locat ion known as the intermediate cell mass (IM) of Oellacher. In this rep ort, we have isolated cDNAs encoding zebrafish homologs of the hematop oietic transcription factors GATA-1 and GATA-2 and have used these mar kers to determine that the IM is formed from mesodermal cells in a pos terior-lateral position on the yolk syncytial layer of the gastrula yo lk sac. Surprisingly, cells of the IM then migrate anteriorly through most of the body length prior to the onset of active circulation and e xit onto the yolk sac. These findings support a hypothesis in which th e hematopoietic program of vertebrates is established by variations in homologous migration pathways of extra- and intraembryonic progenitor s.