Structure and Development of the Testis of Bullfrog, Rana catesbeiana, and Their Changes during Seasonal Variation

Abstract

The testes of fully mature bullfrogs, Rana catesbeiana, were studied by light microscope. The germ cells in the developing testis can be classified into 12 stages based on nuclear characteristics. Primary(type A) and secondary(type B) spermatogonia are the earliest germ cells, with the former showing large and completely euchromatic nuclei with prominent nucleoli and the latter with small blocks of heterochromatin distributed along the nuclear envelope. Spermatocytes consist of five stages; namely, leptotene, zygotene, pachytene, diplotene and diakinesis metaphase spermatocytes. Succeeding stages show increasing condensation of chromatin: from the coarse fibers, that are evenly distributed throughout the nucleus in leptotene stage to the highly condensed blocks of heterochromatin in pachytene and diplotene stages. Nucleoli are not detected in any stages. Secondary spermatocytes have blocks of completely condensed heterochromatin attaching to the nuclear envelopes. There are three stages of spermatids: the early stage shows coarse chromatin granules occurs evenly over the nucleus, the middle stage has increased chromatin condensation over the entire oval nucleus while the nuclear size decreases. The late stage exhibits completely condensed chromatin in an elongated nucleus, and its cytoplasm becomes highly vacuolized and starts to degenerate. In fully mature spermatozoa, the nucleus becomes highly elongated and chromatin completely condensed. During development of the testis, sex cords in putative testis appear in two-month-old frogs. Seminiferous tubules containing primary spermatogonia appear in the definitive testis of four-month-old frogs while spermatocytes are present in five-month-old frogs. Spermiogenesis and full production of spermatozoa could be detected from the seventh month onwards. The frogs become fully mature about sixteen months when their testes undergo cyclical change. During breeding period (April-September), there are abundant spermatozoa, round spermatids in seminiferous tubules, while during non-breeding period (October-March), such cells are much fewer in number and most remaining cells are spermatogonia and primary spermatocytes.