Give a concise account of spermatogenesis.

Outline:

·        Location of spermatogenesis

·        Role of Sertoli cells

·        Production of male gametes

·        Spermiogenesis

·        Final maturation

·        Hormonal influences

Essay:

            The male testis is the site of the production of the male gametes, spermatozoa. The testis is covered by a dense fibrous tissue called tunica albuginea which gives rise to numerous collagenous septa, dividing the testis into about 250 testicular lobules. Within each lobule are one to four highly convoluted tubes, the seminiferous tubules, in which the spermatozoa are produced. Each lobule is bounded beneath a basement membrane, beneath which are the spermatogonia and Sertoli cells. As the spermatogonia divide and develop successively into spermatocytes and spermatids, a column of germ cells is formed that reaches from the basement membrane to the lumen of the tubule, culminating in the spermatozoa. Sertoli cells are bound to one another by junctional complexes containing extensive tight junctions. The junctional complex is located towards the basal layer of the spermatogenic epithelium so as to divide the tubule into basal and adluminal compartments. Sertoli cells provide structural and metabolic support for the developing spermatogenic cells.

            The primitive germ cells of the male, the spermatogonia, are present only small numbers in the male gonads before sexual maturity. After puberty, the spermatogonia multiply continuously by mitosis to provide a supply of cells which then undergo meiosis to form male gametes. Each spermatogonium can give rise to 64 spermatozoa. The first two mitotic divisions of a spermatogonum give rise to four cells – a single resting cell that will eventually serve as the ancestor of a later generation of spermatozoa and three active cells. The active cells divide by further mitoses to yield type B spermatogonia, which then give rise to a number of primary spermatocytes. These cells enter the prophase of meiosis, in which they remain for about 20 days. This process occurs within the basal compartment of the seminiferous tubules. The primary spermatocytes undergo meiosis to give secondary spermatocytes which divides again in the adluminal compartment, forming spermatids. Each spermatid contains 22 autosomes, and either an X or a Y sex chromosome. The spermatids lie near the lumen of the seminiferous tubule and are attached to the abutting Sertoli cells by specialized junctions.

            Spermiogenesis is the process by which spermatids, the gametes produced by meiotic division, are transformed into motile mature spermatozoa. In this process, spermatids undergo nuclear condensation, shrinking of cytoplasm, formation of an acrosome, and development of a tail to emerge as flagellated spermatozoa. The Golgi apparatus forms a large vesicle, the acrosomal vesicle, which accumulates carbohydrates and hydrolytic enzymes. The acrosomal vesicle becomes applied to one pole of the progressively elongating nucleus to form a acrosomal head cap. As the flagellum elongates, excess cytoplasm is phagocytosed by the enveloping Sertoli cell prior to release of the spermatid into the lumen. The mitochondria become arranged in a helical manner around the fibrils which surround the first part of the flagellum. The spermatozoa are then extruded into the lumen of the tubule by a process called spermiation, during which most of the cytoplasm of the spermatozoa is ejected as the residual body and remains embedded in the Sertoli cell cytoplasm.

            After spermiation, the spermatozoa reach the epididymis, which they traverse in a period of 2 to 4 weeks. During this time, the spermatozoa undergo further maturation, gaining motility and losing all their cytoplasm. Spermatozoa are initially transported into the epididymis by seminiferous tubular fluid currents generated by the peritubular myoid cells. Proteins provided by epididymal and seminiferous tubular fluid bind to the membranes of sperm and enhance their motility and fertilizing ability.

            The gonadotropins and testosterone are essential for spermatogenesis. The pulsatile release of GnRH and resultant arrival of LH and FSH at their target cells stimulates spermatogenesis. FSH stimulates Sertoli cells, whose functions are in turn required for initial germ cell mitotic and early meiotic activity. LH stimulates Leydig cells to secrete testosterone, which diffuse across the basement membrane and can enter the Sertoli cells, which contain androgen receptors. The high concentration of testosterone is essential for completion of the later stages of spermatogenesis.

            In humans, it takes an average of 74 days to form a mature sperm from a primitive germ cell by the process of spermatogenesis.