part of start stage end stage male organism gonad testis ZFA:0000598 TAO:0000598 Male reproductive organ. Adult zebrafish testes are paired organs forming elongated structures, internally organized into tubules. These tubules are formed by seminiferous epithelium containing cysts of spermatogonia and spermatocytes surrounded by somatic support cells, called Sertoli cells. Cysts are arranged around a lumen containing spermatozoa that is connected to the efferent ducts. The interstitial regions between the tubules contain the steroid-producing Leydig cells, connective tissue, and blood vessels. zebrafish_anatomy testes endocrine system larval:days 21-29 adult