A NEW APPROACH TO THE STUDY OF HEMATOPOIETIC DEVELOPMENT IN THE YOLK-SAC AND EMBRYOID BODIES

To understand the mechanisms that control the differentiation of uncom mitted mesoderm precursors into haematopoietic stem cells (HSCs) and t he activation of haematopoiesis, we conducted a study to identify gene s expressed at the earliest stages of both in vivo and in vitro haemat opoietic development. Our strategy was to utilize Differential Display by means of the Polymerase Chain Reaction (DD-PCR) to compare pattern s of gene expression between mRNA populations representing different l evels of haematopoietic activity obtained from the mouse embryo, embry oid bodies (EBs) and mouse cell lines, We report the molecular cloning of two groups of genes expressed in the yolk sac: a group of genes ex pressed in the day-8.5 yolk sac at higher levels than in the day-8.5 e mbryo proper and up-regulated during EB development, and another group of day-8.5 yolk sac genes not expressed in the day-8.5 embryo proper or in EBs. Specifically, we describe the molecular cloning of the firs t nucleobase permease gene to be found in vertebrates, yolk sac permea se-like molecule I (Yspl1), The Yspl1 gene has the unique property of encoding both intracellular, transmembrane and extracellular protein f orms, revealing novel aspects of nucleotide metabolism that may be rel evant during mammalian development.