Spermatogenesis in Atlantic cod (Gadus morhua): a novel model of cystic germ cell development.

Precocious male puberty significantly compromises sustainability aspects of aquaculture in a number of finfish species. As part of a program aiming to understand and eventually control testis maturation in farmed Atlantic cod, we studied the first reproductive cycle. The gonadosomatic index shows a 41-fold increase from immature (August) to mature (March) stages, reaching almost 10% of the total body weight. The paired cod testes are composed of several lobes arranged around a central collecting duct. In each individual lobe, spermatogenesis occurs in a marked gradient of development, with undifferentiated spermatogonia in the periphery of the lobe and the most advanced germ cells in the vicinity of the collecting duct, suggesting a tight spatiotemporal organization of spermatogenesis in the testis lobes of this species. Spermatogonial proliferation starts in August and continues for about 6 mo. Meiosis and spermiogenesis are first observed in October and are completed in all cysts by February, when a 2-mo-long spawning season starts. Spermatogonia go through 11 mitotic divisions before differentiating to primary spermatocytes. Apoptosis is rare, but when observed it occurs mainly during the last spermatogonial generations. Our observations suggest a model in which a maturational wave progresses through each growing lobe that is first driven by appositional growth from the lobe's periphery, reflecting spermatogonial proliferation and cyst formation which, when ceasing, is terminated by completing spermiogenesis and spermiation that progress toward the lobe's periphery.