Anatomy of finfish and shellfish

Unit 9 - Endocrine system

9.2.5. Gonads
In fishes, the maturation and possibly also the elaboration of gametes requires the presence of sex harmones in addition to secondary sex characteristics such as coloration,breeding tubercells and the maturation of gonopodium depends upon the pressure of endocrine substances from the sex glands.
Testis
Testis is made up of lobular units.in teleost lobular unit may be short and long tubules and internally divided space with their apex at the center of the organ and their broader ends directed towards the periphery.In most of fishes however the com-plex network of elongated and intergreatly divided lobules, and lobularspaces are bound together forming a compact organ. Septum and covering contain much elastic but no muscle.the lobules open into a spermatic duct which may be long and tortuous with a lining of secretory epitheliumor may be straight and simple.
Interstitial tissue and serotoli cells.Seasonal variation and the amount of interstitial and connective tissue cells of gonads have been described in details.in this species the development of interstitial tissue probably the source of testicular androgen considers with the appearance of secondary sexual characters and complex breeding behavior. Sertoli cells have been described in both Elasmobranch and teleost and show cyclical changes in relation to spermatogenesis.this is noted sperms are often discharged at the stage.

Ovary
The ovary is made up of numerous ovarian follicles embedded in connective tissue.the arrangement of ovarian follicles within the supporting tissue varies greatly in different groups.The ovary in most teleost fishes is a hollow sac-like organ into which extend numerous ovigerous folds lined by germinal epithelium. The germ cells, the endodermally derived oogonia, multiply mitotically and get transformed into nonyolky primary oocytes whose, nuclei are arrested at the prophase of the first meiotic division until maturation. These processes can take place even in the absence of the pituitary (Barr, 1968; Hoar, 1969). Primary oocytes, covered generally by two layers of follicle cells, an outer thecal layer and an inner granulosa layer, undergo vitellogenesis when yolk is deposited in the. ooplasm. During maturation, the first polar body is given cut and the second meiotic division is arrested at metaphase. The eggs are spawned by the fish at this stage and the second polar body is released only after fertilization. In some fishes, ovulation and- spawning occur almost at the same time, whereas in ethers (rainbow trout and milkfish) ovulated oocytes are retained in the ovarian or peritoneal cavity and spawning takes place much, later.