Studies of Sl / Sld + + + / + mouse aggregation chimaeras II . Effect of the steel locus on spermatogenesis

Mutant mice of Sl/Sf genotype are deficient in melanocytes, erythrocytes, mast cells and germ cells. Deficiency of melanocytes, erythrocytes and mast cells is not attributable to an intrinsic defect in their precursor cells but to a defect in the tissue environment that is necessary for migration, proliferation and/or differentiation. We investigated the mechanism of germ cell deficiency in male SI/SI* mice by producing aggregation chimaeras from Sl/SF and + / + embryos. Chimaeric mice with apparent white stripes were obtained. Two of four such chimaeras were fertile and the phenotypes of resulting progenies showed that some SI/Sf germ cells had differentiated into functioning sperms in the testis of the chimaeras. In cross sections of the testes of chimaeras, both differentiated and nondifferentiated tubules were observed. However, the proportions of type A spermatogonia to Sertoli cells in both types of tubules were comparable to the values observed in differentiated tubules of normal + / + mice. We reconstructed the whole length of four tubules from serial sections. Differentiated and nondifferentiated segments alternated in a single tubule. The shortest differentiated segment contained about 180 Sertoli cells and the shortest nondifferentiated segment about 150 Sertoli cells. These results suggest that Sertoli cells of either Sl/Sf or + / + genotype make discrete patches and that differentiation of type A spermatogonia does not occur in patches of Sl/St* Sertoli cells.