الفهرس 
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Abstract
Nile tilapia is one of the most economically important freshwater fish species in Egypt, which characterized by fast growth rate, convenient size and early sexual maturity. Continuous production of spermatozoa throughout life in adult tilapia depends on spermatogonial stem cells (SSCs), which a subpopulation of Type A spermatogonia. The identification and isolation of Type A spermatogonia are crucial to the development of high-quality transplantation technique as well as to understand the regulation of spermatogonia in vivo. However, specific molecular markers for type A spermatogonia have not yet been studied sufficiently in tilapia. Therefore, we investigated type A spermatogonia in tilapia testis using histological and ultrastructural characterization, in addition to, the expression of the stem cell marker, CD44 via histochemical analysis and flow cytometry. Histological and ultrastructural analysis of type A spermatogonia showed an irregular nuclear membrane, presence of nauges and poor cristae of mitochondria. The expression of CD44 marker using flow cytometric and immunohistochemical analysis was strongly restricted to type A spermatogonia in tilapia testes. In conclusion, we suggested that CD44 is a good marker for type A spermatogonia in fish which, in the future, could be used to characterize and isolate type A spermatogonia in the Nile tilapia.