63.5 Saturday, Jan. 5 Differentiation and development of steroid-producing cells during ovarian differentiation in tilapia M.N.*, M; Univ. of the Ryukyus email@example.com
Sex hormones produced from steroid-producing cells (SPCs) play important role in sexual phenomena such as sex differentiation, gonadal development, maturation, sexual behaviour etc. Differentiation and development of SPCs and folliculogenesis during ovarian differentiation in Nile tilapia Oreochromis niloticus were ultrastructurally and immunohistochemically. SPCs with ultrastructural features were first observed in the area near the blood vessels in the gonads of fish at 20-25 day after hatching (dah) around the time of ovarian differentiation. Ultrastructural results showed that differentiation and development of SPCs from undifferentiated to maturation occurred in the area near blood vessels, indicating that it would be the original site of SPCs. The process of folliculogenesis was ultrastructurally observed. SPCs enclosed by fibroblastic cells invaded the interstitial areas among oocytes and some reached the surfaces of oocytes. The upper portions of these elongations opened and began to enclose the outer surfaces of developed oocytes to become thecal layer. Later, newly migrated SPCs reach the thecal layer to become thecal cells. These results indicate that steroid-producing thecal cells originate from the SPCs in the area near blood vessels. After thecal layer formation, an immunopositive reaction against P450arom AB, but not against P450scc or 3β-HSD ABs, appeared first in the granulosa cells enclosing the vitellogenic oocytes at 100 dah. At this time, estrogen production in serum levels rapidly increased. At 70–80 dah, IPC clusters invaded the interstices among oocytes at the perinucleolar stage from the area near the blood vessels. IPCs increased in number in the interstices among the previtellogenic oocytes, and some clusters began to enclose the outer thecal layer of the previtellogenic oocytes at 90 dahThus, folliculogen- esis could be essential for active production of estrogen in the ovary.