The Role of Activin in Spermatogenesis in Fish

Abstract

Spermatogenesis is an extended process of differentiation and maturation of germ cells resulting in haploid spermatozoa. Although it is generally accepted that the principal stimuli for vertebrate spermatogenesis are pituitary gonadotropins and androgens, the specific role played by individual hormones has not been clarified (1–4). Progress in this field has been hampered by the complex organization of the testis of higher vertebrates, in which seminiferous tubules contain several successive generations of germ cells. In contrast, certain teleosts, such as eel and salmonid fishes, are advantageous for studying the regulation of spermatogenesis as a result of synchronous development of germ cells in the testis (5, 6). Within the lobules, spermatogonia undergo numerous mitotic divisions and produce cysts, each containing several spermatogonia. During maturation, all the germ cells within each cyst are at approximately the same stage of development. As spermatogenesis and then spermiogenesis proceed, the cysts expand and eventually rupture, liberating spermatozoa into the lobular lumen continuous with the sperm duct. This chapter summarizes the current view on the regulation of spermatogenesis in eel testes, with particular emphasis on the role of activin B in the initiation of spermatogenesis. Most of the data presented in this chapter are still unpublished.