Development of ovarian tissue and female germ cells in the protandrous black porgy, Acanthopagrus schlegeli (Perciformes, Sparidae)

The morphology of developing bisexual gonads and female germ cells in the protandrous black porgy, Acanthopagrus schlegeli (Sparidae), is described using light and electron microscopy. The central (ovarian) cavity and sex differentiation were observed in the differentiating gonad at 5 mo of age. Testicular tissue was the main tissue in the gonad with a small portion of ovarian tissue limited to the area around the central cavity and main blood vessel. Oogonia which were grouped as cell nests in the germinal epithelium and oocytes appeared during the period from the differentiated gonad to the 1st spawning period. Ovarian lamellae, containing oogonia and primary oocytes, developed towards the central cavity after the 1st spawning season. The testicular tissue regressed but ovarian tissue developed during the non-spawning period in 1(+)- and 2(+)-yr-old fish. Female germ cells were classified into oogonia, chromatin-nucleolus oocytes, perinucleolar oocytes, cortical alveolar oocytes, vitellogenic oocytes, and maturing oocytes. Only primary oocytes, the most advanced type of oocyte, could be found in the bisexual ovary before the occurrence of the sex change. Testicular tissue regressed together with the development of vitellogenic oocytes during the sex change in some 3-yr-old fish. A blood vessel reached each oocyte. There were light and dark electron-dense areas in the germinal epithelium connected by cell junctions. Oogonia and oocytes had many mitochondria and highly electron-dense structures such as cement and nuage. Granulosa and theca cells surrounded the oocytes. Organelles such as mitochondria, endoplasmic reticula, and Golgi bodies were found in the highly electron-dense area of granulosa cells at the vitellogenic oocyte stage. Microvilli extended from oocytes through the zona radiata to granulosa cells. The developmental profile of ovarian lamellae in ovarian tissue and changes in the testicular/ovarian tissue in the bisexual gonad were further summarized. These histological data provide important information for further studies on the molecular mechanism of sex differentiation and sex change in the protandrous black porgy.