Endocrinology of the male reproductive system

SPERMATOGENESIS

Spermatogenesis represents the process by which precursors termed spermatogonia undergo a complex series of divisions to give rise to spermatozoa (4,5). This process takes place within the seminiferous epithelium which is a complex structure composed of germ cells and radially oriented supporting cells called Sertoli cells (Figure 5). The latter cells extend from the basement membrane of the seminiferous tubules to reach the lumen. The cytoplasmic profiles of the Sertoli cells are extremely complex as this cell extends a series of processes that surround the adjacent germ cells in an arboreal pattern (5-7).
Spermatogenesis can be divided into three major phases (i) proliferation and differentiation of spermatogonia, (ii) meiosis, (iii) spermiogenesis which represents a complex metamorphosis involved in the transformation of round spermatids arising from the final division of meiosis into the complex structure of the spermatozoon.
Spermatogonial Renewal and Differentiation
These cells represent a population that divide by mitosis providing both a renewing stem cell population as well as spermatogonia that are committed to enter the meiotic process. The identification of different types of spermatogonia is complex due to a lack of definitive markers that can identify specific stages. To date, classification of these cells has depended on the features of their nuclei and, in particular, their chromatin patterns (5). The identification of the latter, which represents a crucial step in classification, is often obscured by poor fixation especially if the tissue is fixed in formalin. The ideal fixatives for testicular tissue are Bouin's or Cleland's solution. In general two main classes of spermatogonia can be identified in all mammals: Type A exhibiting fine pale staining nuclear chromatin and Type B with coarse chromatin collections found close to the nuclear membrane(8). In many mammals, the Type A spermatogonia can be divided into several