Expression of c-kit encoded at the W locus of mice in developing embryonic germ cells and presumptive melanoblasts.

The W locus of mice encodes the c-kit tyrosine kinase receptor. In embryos homozygous for severe W mutations, the number of germ cells does not increase after 8 days of development, melanocytes do not appear, and production of erythrocytes and mast cells is deficient. To gain some insight into the role of the c-kit receptor, we have used in situ hybridization to explore the time period of expression of c-kit transcripts in early germ cells and melanoblasts. At 6 1/2 days of development, expression was not seen in the embryonic cylinder, but did appear in parietal endoderm. Germ cells displayed a low level of c-kit transcripts from their first appearance in the 7 1/2 -day embryo, continuing through early proliferation and migration to the gonad. During migration, surrounding tissues also expressed c-kit. Expression increased in gonia and then ceased as they became nonproliferative. Expression in presumptive melanoblasts was first seen in the cervical region of 10-day embryos and continued as they spread over the surface of the body, entered the epidermis, and differentiated in hair follicles after birth. The effects of mutations of c-kit on germ cells and melanoblasts can be interpreted as an absence of a proliferative signal shortly after their segregation from other cell types. This signal may be required throughout the proliferative phase of early germ cells [and also in postnatal stages of germ cell development (Manova et al. (1990). Development 110, 1057-1069]. In melanoblasts, c-kit may play a role during both proliferation and differentiation.