Efficiency of spermatogenesis: a comparative approach.
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Efficiency of spermatogenesis is the estimated number of spermatozoa produced per day per gram of testicular parenchyma. Spermatogenesis is the process of cell division and cell differentiation by which spermatozoa are produced in testes. Efficiency of spermatogenesis is influenced by species differences in the numerical density of germ cell nuclei and in the life span of these cells. Activities of spermatogonia, spermatocytes, and spermatids partition spermatogenesis into three major divisions (spermatocytogenesis, meiosis, and spermiogenesis, respectively). Spermatocytogenesis involves mitotic germ cell division to produce stem cells and primary spermatocytes. Meiosis involves duplication of chromosomes, exchange of genetic material, and two cell divisions that reduce the chromosome number and yield four spermatids. In spermiogenesis, spherical spermatids differentiate into mature spermatids which are released in the lumen of seminiferous tubules as spermatozoa. Spermatogenesis and germ cell degeneration can be quantified from numbers of germ cells in various developmental steps throughout spermatogenesis. Germ cell degeneration occurs throughout spermatogenesis; however, the greatest impact occurs during spermatocytogenesis and meiosis. There are species and seasonal influences on the developmental steps in spermatogenesis at which germ cell degeneration occurs. Number of Sertoli cells, amount of smooth endoplasmic reticulum of Leydig cells, and the number of missing generations of germ cells within the spermatogenic stage of the cycle influence efficiency of spermatogenesis. Efficiency of spermatogenesis is influenced to the amount of germ cell degeneration, pubertal development, season of the year, and aging of humans and animals.