Morphological and histomorphological structures of testes and ovaries in early developmental stages of the silkworm, Bombyx mori

Abstract

Gonads develop as testes in males or ovaries in females. In the silkworm, B. mori, little is known about testes and ovaries in the embryonic stages and early larval stages. In this study, morphological and histomorphological observations were performed of ovaries and testes from the late embryonic stage to the first instar larval stage. Results obtained with lack of accurate information on the sex of the examined individuals may be misleading, and thus phenotypic observations of gonads were also performed by utilizing sex-limited strains that enables easy discrimination of female embryos from male ones based on the egg colors. In the testes, four testicular follicles were clearly observed at the first instar larval stage, and boundary layers were formed between the testicular follicles. At the late embryonic stage, the testes consisted of four testicular follicles, while the boundary layers were still obscure. In the ovaries, four ovarioles were easily recognizable at the first instar larval stage, and boundary layers were formed between the ovarioles. However, in the late embryonic stage, it was quite difficult to identify four ovarioles. Morphological characteristics were almost similar between the testes and ovaries in the early developmental stages. The present study demonstrates that the most reliable difference between the testes and the ovaries in the early developmental stages is the attaching point of the duct. Formation and development of the duct may be sensitive to the sex-determining signal and may display sexual dimorphism in early embryonic stages.