3.3 Spermatogenesis

• Introduction
• Structure of the germinal epithelium
• Developmental stages of spermatogenesis
• The temporal course of spermatogenesis
• Spermatocytogenesis
• Local course of spermatogenesis - the spermatogenesis wave
• Spermiogenesis (spermatohistogenesis) and structure of the sperm cell
• Leydig's interstitial cells and hormonal regulation

Introduction

Spermatogenesis is initiated in the male testis with the beginning of puberty. This comprises the entire development of the spermatogonia (former primordial germ cells) up to sperm cells. The gonadal cords that are solid up till then in the juvenile testis develop a lumen with the start of puberty. They then gradually transform themselves into spermatic canals that eventually reach a length of roughly 50-60 cm. They are termed convoluted seminiferous tubules (Tubuli seminiferi contorti) and are so numerous and thin that in an adult male testicle their collective length can be 300 to 350 meters. They are coated by a germinal epithelium that exhibits two differing cell populations: some are sustentacular cells (= Sertoli's cells) and the great majority are the germ cells in various stages of division and differentiation.

In certain forms of impotence and for an assisted fertilization the testicle is "biopsied": from a piece of testicular parenchyma the contorted seminiferous tubules are extracted with forceps and unwound.
Fig. 8
Testicular biopsy: Pulling with the forceps makes the testicular canals visible.
Fig. 9
The tubules are clearly visible using a binocular magnifying glass.
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Fig. 10
Histological preparation of a section through a convoluted seminiferous tubule in an adult. Outside its basal lamina a layer of myofibroblasts and fibrocytes surround the tubule. The germinal epithelium lies on the tubule wall. One can recognize the spermatogonia sitting on the basal lamina.
The nuclei of the Sertoli's sustentacular cells have a rarified chromatin and the nuclei with clear nucleolus that are often oriented perpendicular to the basal lamina. The overall picture, though, is dominated by the cells occupied with spermatogenesis.

The development of the germ cells begins with the spermatogonia at the periphery of the seminal canal and advances towards the lumen over spermatocytes I (primary spermatocytes), spermatocytes II (secondary spermatocytes), spermatids and finally to mature sperm cells.