| From June to September, 1988 and from October, 2001 to May, 2002, gonadal sex differentiation, changes on the structure of oocytes, formation of yolk granules, development of follicle cells as well as formation of the micropylar cell and the egg membrane in Mystus macroptems were studied by means of histological and histochemical methods. The research results are as follows:Three days after hatching (dan), a primordial germ cell (PGC) was discovered at the mesoblast below the segmental duct. PGC is distinct from the neighboring cells for its own specific characters: size of 15 X 17.5urn, nuclear diameter of 7.Sum, distinct nuclear membrane, one nucleolus, faint staining nuclear matrix, transparent cytoplasm as well as protuberances on cytomembrane.In 5 dah fish, two distinct genital ridges appeared as the outgrowth of basophilic cells from the peritoneal mesoblasts. The genital ridge and several PGCs migrated into it form the primitive gonad which stick up toward the celom and suspend on the membrane of celom. The primitive gonad will differentiate into testis or ovary gradually in the later stage. Somatic cells in genital ridge were derived from the peritoneal wall showing no recognizable signal of dual origion as amphibians before or during the sex-differentiation period. Primordial germ cells are not derived from the epithelia of genital ridge but from those cells which belong to the early embryo and migrate into the genital ridge. PGCs were seen as beads on a chain on the longitudinal section of early stage of gonad.The genital ridges begins to laterally elongate to form the distal and proximal sides of the gonadal folds in 12 dah fish, and the two lateral folds are almost fused in 19 dph fish. Several PGCs could be seen on a transverse section. These maybe the early signals for ovary differentiation.The oogenesis process in Mystus macroptems can be divided into seven phase: primordial germ cell, oogonium, oocyte with single layered follicle cells, oocyte with yolk vacuole, oocyte with full of yolk granules, mature ovum and retrogressive oocyte. The PGCs which has newly migrated into genital ridges are egg-like with a high nuclear-cytoplasmic ratio in terms of its diameter of 7-13nmX 7.5-1 Sum and diameter of nucleolus of 5.2-6.25|amX6.0-7um. When stained with H.E, acidophillic cytoplasm show light red compared with a faint stain in nuclear matrix which often contain 1-2 big basophilic nucleolus. Then PGCs grow and differentiate into spermatogonia or oogonia. Oogonia are ovate or elliptic with a high nuclear-cytoplasmic ratio and itsnuclear matrix disperse around the inner layer of the nuclear membrane. The diameter of cell and nucleus ranges from 1 l-18um and 6-10um respectively. The oogonia, with clear nuclear membrane and 1-2 nucleolus, show faint staining when stained with various staining methods. Oogonia were seen in the genital ridge irregularly, or stacked around one area. They maintain an active state (in active mitosis or developing into primary oocyte) through all the time except in the reproductive season. During the phase of oocyte with single layered follicle cells, oocyte increase in size and become more basophillic, staining is thicker with various staining reagent compared with oogonia and an "outer ring" appeare around the outer area of the cytoplasm. Later on, yolk nucleus manifest clearly. Characteristics of oocyte with yolk vacuole were as follows: size of the body increases; follicle cells develop into 2-3 layer and the inner layer develop into cuboidal granule cells; vitelline envelope appear and thicken; yolk vacuole emerge in the ooplasm; Wave like-nuclear membrane become evident; components of nucleolus were released.Also, the characteristics during the phase of oocyte with full of yolk granules are as follows: vitelline envelope growth halt and become thinner; granule cells become columniation-shaped with its cytoplasm full of granule particles; oocyte is teemed with yolk granule which shaped round, elliptical or randomly with a diameter ranging from 1.2 urn to 4. |
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