GATA FACTORS AND THE ORIGINS OF ADULT AND EMBRYONIC BLOOD IN XENOPUS - RESPONSES TO RETINOIC ACID

The transcription factors, GATA-1, -2 and -3 play essential roles in t he differentiation of haematopoietic cells. To study the process of bl ood formation during vertebrate development we have used the expressio n of these GATA factors to locate haematopoietic cells in Xenopus embr yos and to act as sensors for the effects of all-trans retinoic acid ( RA), a signalling molecule which influences both anteroposterior patte rning and haematopoietic differentiation. GATA factor expression was d etected in the leading edge of the gastrulating mesoderm, in the ventr al blood island (VBI) and dorsolateral plate (DLP) mesoderms and in a population of cells between the VBI and DLP. The VBI contributes to bo th embryonic and adult blood, whereas the DLP contains precursors of a dult blood only, which have not been identified previously with molecu lar markers. The possibility that the GATA-2-expressing cells between the VBI and DLP were haematopoietic progenitors migrating from the VBI to the DLP was ruled out by transplantation analysis. Differential ef fects of RA on the expression of GATA-1 and GATA-2 suggest that RA has a direct action on haematopoietic differentiation, rather than on the formation of haematopoietic mesoderm.