371 CELL LINEAGE SEGREGATION IN THE DEVELOPMENT OF THE PERIPHERAL NERVOUS SYSTEM. N.M. Le Douarin. Inst i tut d'Embryologie du CNRS et du Coll6ge de France, 49his, Avenue de la Belle-Gabrielle 94130 NOGENT-sur-MARNE (France)

The neural crest is a transitory embryonic structure from which a great variety of cell types, including those which participate in the formation of the peripheral nervous system (PNS) arise during ontogeny. The quail-chick marker system has been used to follow the migration and differentiation of crest cells in the avian embryo in ovo and to evaluate their differentiation potentialit ies at various times of development and in different microenvironments. Although fate map studies showed that the neural crest is regionalized in distinct areas with respect to its contribution to the PNS ganglia, other types of grafting experiments demonstrated that the potentialit ies to yield v i r tual ly all the PNS cell types are present along the whole neural axis. A decisive role had therefore to be attributed to the microenvironment in which the crest cells migrate and differentiate in selecting among these developmental capacities. This view was fu l ly confirmed by in vi tro cultures of neural crest cells in which m a n i p u l a t ~ c u l t u r e conditions lead to the expression of different sets of phenotypes. Further stu@ies showed that, at the stage of migration cell lineage segregation is already in progress within the crest cell population which is composed by distinct cell types whose developmental capacities are limited to a definite set of phenotypes.

373 MOLECULAR STRATEGIES FOR CONTROL OF DIF- FERENTIATIVE PROCESSES. William J. Rut- ter, Hormone Research Institute, Univer- s-ity of California, San Francisco.

Differentiative mechanisms involve a change in the cellular program of gene expression as a result of some trigger, e.g., cell-cell interactions. Our stu- dies on the mechanism of selective ex- pression of pancreatic genes have led us to propose that selective gene expression is due to a set of trans-acting regula- tory proteins (differentiators) that con- trol expression of physiologically func- tional genes by interacting with cis-act- ing regions (specific enhancers, promo- ters) in the gene itself. Thus the ex- pression of the differentiator genes determines the differentiative state.

Cell-cell interactions may be medi- ated through receptors. The structure of receptors for insulin and EGF sug- gests a mechanism for signal transduction to control gene expression.

Presumably differentiator genes and functional genes are regulated by differ-

ent mechanisms.

372 CYTODIFFERENTIATION AND ORGANO- GENESIS IN THE METANEPHRIC KIDNEY. Lauri Sax6n. Department of Patho- logy, Unlversity of Helsinki, SF- 00290 Helsinki, Finland.

Early differentiation of the metanephric nephron involves a transformation of mesenchymal cells into epithelial elements which assembly into nephric vesi- cles and tubules. The process has been studied at two levels to dis- tinguish between cytodifferentia- tion and organogenesis: induced mesenchymal cells were followed in monolayer outgrowths, and their epithelialization was compared with the events in an organotypic culture of the same tissue. Early epithelial transformation was char acterized in both systems by changes in the cytoskeleton and in the synthesis of certain matrix proteins. It is concluded that at the cellular level these events are not dependent on the three- dimensional assembly of the cells, whereas their polarization and ag- gregation occur only in the organotypic conditions.

152S