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Abstract
Summary
The present work was carried out on 116 specimens of Oreochromis niloticus, the fishes were weighted to the nearest grams and measured to the nearest millimeters. The gonads of fishes were processed for histological study, in addition to some morphometric parameters of both ovaries and testes were accounted.
The external features of females didn’t show any variation in neither during the breeding (April to September) nor the non-breeding season (October to March), however, the males showed bright black coloration on the dorsal and lateral parts of their body and red colour on the ventral part of their body and head during the breeding season.
1-The ovaries:
Ovaries were paired elongated, cylindrical structure of approximate equal size, located in the posterior body cavity, ventral to the swim bladder and attached to the dorsal body wall by mesovarium. During the non-breeding season, ovaries were small, yellowish red in colour and occupied small portion of the body cavity. The mean ovarian length was 3.63 + 0.11 cm, the mean ovarian diameter was 3.61 + 0.31 mm and the mean weight of the ovaries during the non- breeding season was 8.28 + 0.51 gm. During the breeding season, ovaries were extremely long & wide, yellowish in colour and vascularized. They occupied almost the entire body cavity. The mean ovarian length was 4.01 + 0.20 cm, the mean ovarian diameter was 8.2 + 0.36 mm and the mean weight of the ovaries during the breeding season was 12.58 + 0.60 gm. Large numbers of spherical bright orange oocytes were easily visible.
Ovary of O. niloticus was covered by a thick capsule during the non-breeding season, but became thin and vascular during the breeding period.
Six arbitrary stages of oogenesis process had been established among the ovarian follicles; oogonia (stage1), chromatin nucleolus stage (stage 2), perinucleolar stage (stage 3), yolk vesicle stage (stage 4), yolk globule stage (stage 5) and mature stage (stage 6). Yolk nucleus appeared in perinucleolar stage while yolk vesicles appeared peripherally in the yolk vesicle oocytes.
The mature oocytes characterized by their large diameter, the yolk granules accumulated to form large yolk globules and the large yolk vesicles were peripherally located. The nucleus migrated to the animal pole and then broken down at the end of vitellogenesis. The wall of mature follicle was consisted of a thick zona radiata surrounded by granulosa cells and a thecal layer, the later divided into outer and inner thecal layers. After spawning season, there was an increase in atretic follicles.
During the non-breeding season, the ovaries were filled with previtellogenic oocytes in perinucleolus stage. Oocytes in the oogonium and chromatin nucleolus stages were abundant; the oogonia reached 10 + 1.5 / unit area and the chromatin nucleolus reached 9.8 + 1.4 / unit area during the non-breeding season. While during the breeding season, the ovaries were in a condition of active vitellogenesis and mature oocytes increased both in number (4.0 + 0.5 / unit area) and in diameter (806.0 + 11.0 µm).
2- Testis:
Testes were paired long narrow structure of approximate equal size, located in the posterior body cavity ventral to the swim bladder, attached to the dorsal body wall by mesorchium. During the non-breeding season, the testes were small, thread- like and dull white in colour with slight vascularization. The mean testicular length was 3.96 + 0.11 cm, the mean testicular diameter was 3.13 + 0.23 mm and the mean weight of the testes during the non- breeding season was 6.33 + 0.80 gm. During the breeding season, testes were pinkish in colour, increased in weight and the blood vessels were prominent. The mean testicular length was 4.49 + 0.15 cm mean testicular diameter was 4.46 + 0.20 mm and the mean weight of the testes during the breeding season was 10.79 + 0.80 gm.
Testes of O. niloticus were of radial type, a system of seminiferous tubules passed from the dorsal and lateral wall of the testis to the central lumen that lead to efferent ducts which opened into the sperm duct or vas deferens which lead to the urogenital pore.
The testis covered with a capsule consisted of few collagenous, elastic fibers and some smooth muscle cells. The capsule increased in thickness during the non-breeding season.
Testicular parenchyma consisted of coiled and branched seminiferous tubules and interstitial tissue. The seminiferous tubules lined with spermatogenic and Sertoli cells. Seminiferous tubules made up of spermatocysts, which consisted of the cytoplasmic projections of Sertoli cells. Within each cyst, the germ cells were at the same stage of development. During spermatogenesis, primary spermatogonia proliferated to form clusters of secondary spermatogonia which divided mitotically to form primary spermatocytes. The later undergo meiotic divisions to form secondary spermatocytes that passed with second meiotic divisions giving many spermatids which transformed into spermatozoa in the lumen of the seminiferous tubules.
Sertoli cells were pyriform cells with slightly eosinophilic cytoplasm and one basal nucleus.
There were numerous polygonal interstitial cells arranged in groups within the connective tissue between the seminiferous tubules.
During the non- breeding season, seminiferous tubules were dominated by early stages of spermatogenesis. The mean volume percentage of seminiferous tubules was 70.19 + 1.0 %. The diameter of the seminiferous tubules reduced, and its mean diameter reached 102.94 + 1.83 µm and the mean number of Sertoli cells was 6.4 + 0.5 / cross section of seminiferous tubules and 1.79 + 0.16 / spermatocyst. The volume percentage of interstitial tissue increased during the non-breeding season, and its mean value reached 29.8 + 1.72 %.
During the breeding season, seminiferous tubules were dominated by later stages of spermatogenesis and early stage of spermiogenesis. Also during this period, the lumen of efferent ducts was distended and filled with spermatozoa. The volume percentage of seminiferous tubules increased, and its mean value reached 78.43 + 0.81 %. The diameter of the seminiferous tubules was significantly increased during the breeding season and its mean diameter reached 124.78 + 2.32 µm. There was a significant increase in the number of Sertoli cells, its mean number reached 10.6 + 0.67 / cross section of seminiferous tubules and 3.01 + 0.14 / spermatocyst. The volume percentage of interstitial tissue decreased during the non-breeding season, and its mean value reached 21.54 + 0.81 %.