Histological studies on the yolk granule formation in the egg character mutant, vit, of Bombyx mori

The formation of protein yolk spheres during oogenesis in the egg character mutation, vit, of Bombyx mori, was studied by light microscopy and transmission electron microscopy of sections of the resin-embedded follicles. The vit ovary exhibited the normal polytrophic meroistic form, in which the follicle has an oocyte, nurse cells and follicular epithelial cells. Also the accumulations of lipid droplets, glycogen granules and small yolk spheres took place normally in the vit oocytes during oogenesis. However, abnormality of the mutant appeared because large protein yolk spheres did not form. Instead, many small spheres were accumulated, which were not observed in the normal. Since the vit egg has been reported to have the egg specific protein but lack the two major yolk proteins, vitellin and the 30 kDa proteins, we infer that the vit mutation affects the incorporation of the precursors for these proteins from hemolymph to oocyte.