The general aim of this proposal is to provide a clearer understanding of events that occur during early postnatal development of the testis which play a critical role in forming the foundation for spermatogenesis, and hence fertility, in the adult. To this end, the PI will study in detail the way in which neonatal gonocytes mature both in vivo and in vitro, exploring with varied approaches the role of paracrine factors and of cell-cell and cell-matrix interactions in promoting their maturation. Specifically, the PI will explore the relationship between the cell adhesion molecule, NCAM, and adhesion between gonocytes and Sertoli cells, to determine whether changes in NCAM-mediated adhesion underlie critical maturation of gonocyte behavior as these cells move to the basement membrane in vitro and display migratory activity in vitro. In addition, since preliminary data indicate that factors Important in promoting gonocyte development are paracrine and likely to include Stem Cell Factor (SCF), also called "c-kit ligand", and basic Fibroblast Growth Factor (bFGF), the role of each of these peptides in supporting gonocyte motility and mitotic activity, respectively, will also be studied. The PI will identify components of the signal transduction system operating in responsive gonocytes, probe the cellular mechanisms mediating that response, and ask whether interaction between matrix components of the basement membrane or the substrate in vitro have a role in this response. In addition, the general level of transcriptional, or genomic, activity as well as expression of message for SCF, SCF receptor, and bFGF will be characterized in individual gonocytes in vivo and in vitro, with the aim of correlating this information with spatial and temporal relationships of gonocytes in the seminiferous cords and of determining the role of cell- cell and cell-matrix interactions in modifying the functional activity of gonocyte nuclei. Varied methods will be used to approach these questions, including several visual approaches such as in situ nick translation, immunocytochemistry, and autoradiography. Because studies are planned both in vivo, and in vitro, the PI will obtain direct information on the physiological relevance of the findings, which should thus provide new insights into crucial events that precede spermatogenesis and thus contribute to the establishment of fertility in adults.