CELL-CELL INTERACTIONS IN THE TESTIS OF TELEOSTS AND ELASMOBRANCHS

In this paper we present the state of knowledge on cell-cell interacti ons in the testis of two groups of anamniote vertebrates-teleosts and elasmobranchs-which include most fish. In these fish, the structural o rganization of the testis differs fundamentally from that which charac terizes amniotes in which the germinal tissue is located in tubules op en at both ends and consists of a permanent population of Sertoli cell s associated with successive stages of germ cell development. In fish, the spermatogenic unit of testis is the spermatocyst, which correspon ds to one germ cell or to a clone of isogenetic germ cells, enclosed b y one or several Sertoli cells, which form the wall of the cyst. In fi sh testis, the Sertoli cells do not represent a permanent population o f cells. Although both are of the cystic type, the teleost and elasmob ranch testes are differently organized. In elasmobranchs, primary sper matogonia and Sertoli cells lie initially free within the interstitial tissue, before becoming sequestered by a basement membrane; the testi s is then composed of a mass of spermatocysts which contain many Serto li cells, each being associated with a clone of germ cells. In contras t, in teleosts, the cysts are confined to large elongated structures l imited by a basement membrane. These structures are either lobules ori ginating under the albuginea or tubules which, in contrast to those of mammals, are anastomosed. In the lobules, the spermatocysts start to develop at the blind end of the lobules and migrate towards the effere nt system, whereas in the tubules, the spermatocysts are located again st the basement membrane, all along the tubules and do not migrate. In elasmobranchs, unlike teleosts, Leydig cells are either absent from t he interstitial tissue or rare and undifferentiated and their role in steroid production is at best marginal. While many studies have focuse d on topographical and functional interactions between the diverse cel l types present in mammalian testis, only a few studies have brought p articular attention to these aspects in fish. In fish, like in mammals , testicular cell-cell interactions are based on structural elements a nd chemical factors. Occasionally, various adhering junctions have bee n observed, essentially in teleosts, between Sertoli cells, between Se rtoli cells and germ cells, between germ cells themselves, and interst itial cells. Furthermore, in some teleost species, using horseradish p eroxidase or lanthanum salts, the presence of tight junctions between Sertoli cells has been correlated to the occurrence of a Sertoli barri er. In these species, the barrier develops after meiosis so that only haploid germ cells are shielded from the vascular system. In fish, rec ent development of techniques which enable the preparation and in vitr o culture of enriched populations of testicular cells and of spermatoc ysts, has allowed investigations on functional aspects of cell-cell in teractions. In particular, data have been obtained, in the trout, on t he control of spermatogonia proliferation by Sertoli cell-conditioned media and, in the dogfish, on the steroidogenic activity of Sertoli ce lls, in relation to the differentiation stage of the associated germ c ells. Furthermore information exists, in the trout, showing that intra tubular macrophages may participate in the re-initiation of spermatogo nial proliferation. In conclusion, the cytoarchitecture of fish testis , as compared to that of mammals, presents original features which pro vide unique opportunities to develop fruitful studies for a better und erstanding of the complex control mechanisms underlying testicular fun ction in vertebrates. (C) 1995 Wiley-Liss, Inc.