Iris de Nie

11 GENERAL INTRODUCTION AND THESIS OUTLINE 1 Figure 3. Tanner staging in birth-assigned males. Adjusted from Engebretsen et al 7 , with permission from BMJ Publishing Group Ltd. Spermatogenesis and semen quality From Tanner stage 3 onwards, intratesticular testosterone concentrations are generally high enough for spermatogenesis. Spermatogenesis is the process by which germ cells differentiate into spermatozoa (Figure 4). Spermatogonial stem cells, also called spermatogonia, are located in the outer wall of the seminiferous tubules in the testicles, and their mitotic division produces two types of cells. Type A spermatogonia replenish the spermatogonial pool and Type B spermatogonia differentiate into primary spermatocytes. These primary spermatocytes then undergo meiotic division I and differentiate into secondary spermatocytes, which in their turn go through meiotic division II and become spermatids. Subsequently, the final stage of spermatogenesis-called spermiogenesis- takes place which sees the maturation and elongation of spermatids into mature spermatozoa. Sertoli cells are located between the developing germ cells in the seminiferous tubules, and their function is to nourish these cells by secreting different substances that enhance spermatogenesis. The process of differentiation from spematogonia into spermatozoa generally takes 10 weeks, and starts in the outer wall and moves deeper into the tubule onto the inner wall (Figure 4). 8 As a result, mature spermatozoa are released in the lumen of the seminiferous tubules and are then transported to the epididymis in testicular fluid, with the aid of peristaltic contraction. Spermatozoa acquire their motility in the epididymis, and their ability to bind oocytes from an enzyme called fertilization promoting peptide, which is produced in the prostate.