Expression of the c-kit gene is critical for migration of neonatal rat gonocytes in vitro

Rat gonocytes migrate to the basement membrane during the first postnatal week, a change in position crucial for their survival. These cells express the c-kif gene from the day of birth through Day 5 in vivo and develop the ability to migrate in Sertoli cell-gonocyte cocultures. In this study, we asked whether c-kif expression and synthesis of Kit protein are required for pseudopod production by gonocytes in vitro. To determine whether gonocyte migration in vitro is invariably accompanied by c-kif expression, we quantified percentages of gonocytes expressing c-kif with increasing time in vitro and correlated these data with pseudopod development by individual cells. We also determined the effect of exposure to Kit antibodies on gonocyte migration in vitro, and, conversely, asked whether addition of exogenous stem cell factor (SCF), the Kit ligand, stimulates pseudopod development. We found that 1) increasing numbers of gonocytes express c-kif with increasing time in vitro; 2) once these cells begin migrating in vitro, the appearance of a pseudopod on a gonocyte is absolutely correlated with kif expression by that cell; 3) incubating cocultures with Kit antibodies significantly reduces the number of cells with pseudopods, without any detectable decrease in numbers of gonocytes; and 4) addition of exogenous SCF to cocultures prepared on Day 5 results in a transient but significant increase in the percentage of gonocytes with pseudopods even though we found that Sertoli cells in the cultures produce endogenous SCF. Thus, our findings provide evidence to support a role for c-kif expression by neonatal gonocytes and, presumably, SCF expression by neonatal Sertoli cells in stimulating migration of these germ cells in vitro.