Testis  - Development and Stem Cells

Function of the Testis:

Testis is a part of the male reproductive system. The adult testis has two main functions: production of mature, haploid spermatozoa (sperm cells) and secretion of steroid hormones, the most important of which is testosterone.

Embryonic Development of the Testis:

Testes differentiation begins at earlier stage than ovaries. Sry, the testis-determining gene, expression is detected by E10.5 in mouse gonads, and is followed by testis cord formation, the earliest morphological sign of testis differentiation, within approximately 36 hours (around E12). Between E12.0-12.5, Sry expression in gonadal somatic cells initiates the differentiation of Sertoli cells, a key testicular cell type that is essential in subsequent differentiation of the testis. Sertoli cells aggregate around the germ cells and reorganize the gonad into two compartments: the tubular testis cord, composed of Sertoli and germ cells, and the interstitial space between the cords. Peritubular myoid cells surround Sertoli cells and cooperate with them in depositing the basal lamina at the periphery of tubule structures. Other interstitial cell types include steroid-secreting Leydig cells, uncharacterized fibroblasts and the typical vasculature of the XY gonad. 

Spermatogenesis, the development of mature spermatozoa from diploid spermatogonial stem cells, takes about 75 days in man and 35 days in mice. In most mammals, this process is continuous throughout the adult life.

XY germ cells arrest in mitosis and divide again only postnatally as spermatogonia. Spermatogonia, that form the basal layer of the germinal epithelium, comprised of several types: (1) spermatogonial stem cells (SSCs; also called A single (As) spermatogonia), which are a single cell located at the basement  membrane of the seminiferous tubules. Spermatogonia either undergo self-renewal divisions, that generate new SSCs to ensure maintenance of the stem cell pool, or divide into differentiating (2) spermatogonia type A. Spermatogenesis begins by division, after which the daughter cells type A spermatogonia remain bound together by thin bridges of cytoplasm. After further mitotic division,  (3) type B spermatogonia arise, and then undergo the last mitotic division to give rise to primary spermatocytes, which undergo the first meiotic division (M-I) that generates secondary spermatocytes. Thereafter, the second meiotic division (M-II) follows, giving rise to haploid, round spermatids that elongate and are released into the lumen of the semineferous tubules as spermatozoa (sperm cells). During the course of spermatogenesis, the germ cells gradually move towards the lumen of the seminiferous tubules as they mature.