Spermatogenesis, is the origin and development of sperm within the male reproductive organs, the testes. The testes are composed of numerous thin, tightly coiled tubules known as the seminiferous tubules; sperm are produced within the walls of the tubules. Within the walls of the tubules, too, there are many randomly scattered cells, called Sertoli cells, that function to support and nourish immature sperm by providing them with nutrients and blood products.
As the young germ cells grow, Sertoli cells help transport them from the outer surface of the seminiferous tubule to the central canal of the tubule. The testes continually produce sperm, but not all areas of the seminiferous tubules produce sperm at the same time. An immature germ cell takes up to 74 days to reach final maturation and during this growth process, there are intermittent resting phases.
The immature cells (called spermatogonia) are all derived from cells called stem cells in the outer wall of the seminiferous tubules. Stem cells are composed almost entirely of nuclear material. (The nucleus of the cell is the portion that contains the chromosomes.) Stem cells begin their process by multiplying in the cell duplication process known as mitosis. Half of the new cells in this initial culture become future sperm cells and the other half remain as stem cells, so there is a constant supply of additional germ cells.
Spermatogonia destined to become mature sperm are known as primary sperm. These move from the outer portion of the seminiferous tubule to a more central location and coalesce around the Sertoli cells. The primary sperm cells then develop somewhat by increasing the amount of cytoplasm (substances outside the nucleus) and structures called organelles within the cytoplasm.
After a resting phase, the primary cells divide into a form called secondary sperm. During this cell division, a breakdown of nuclear material occurs. In the nucleus of primary sperm, there are 46 chromosomes; in each of the secondary spermatozoa, there are only 23 chromosomes, as there are in the egg. When the egg and sperm combine and their chromosomes unite, the characteristics of both individuals mix and the new organism begins to grow.
The secondary sperm must still mature before it can fertilize an egg; maturation involves certain changes in the shape and form of the sperm cell. The nuclear material becomes more condensed and oval in shape; this area develops as the sperm head. The head is partially covered by a cap, called an acrosome, which is important in helping sperm enter the egg. Attached to the opposite end of the head is the tailpiece.
The tail is derived from the cytoplasm of the secondary sperm. In mature sperm, it consists of a long, thin bundle of filaments that propel the sperm by their undulating motion. Once the sperm have matured, they are transported through the long seminiferous tubules and stored in the epididymis of the testicles until they are ready to leave the male body.
Characteristics of normal spermatogenesis based on histological sections
– Diameter of the seminiferous tubule 180 μm minimum
– Presence of spermatogonia type A pale, type A dark, type B
– Presence of primary and secondary spermatocytes
– Differentiation of spermatids
– Spermiation zones
– Score count of at least 8 (see section – “Score count for the evaluation of spermatogenesis”).
– lumen of seminiferous tubule
– Normal distribution of lipids in the cytoplasm of Sertoli cells
– Presence of spermatogenesis stages
– Formation of germ cell clones
– Thickness of the lamina propria of the seminiferous tubule of 8 μm or less
– Structure and normal distribution of Leydig cells
Kinetics of spermatogenesis
Spermatogenesis begins during puberty and continues throughout life and into old age due to the inexhaustible reservoir of stem cells. A large number of germ cells develop and are released from the seminiferous tubules. The process of spermatogenesis is highly organized: the spermatogonia divide continuously, in part spermatogonia remain and in part give rise to spermatogenesis. Originating from the division of the spermatogonia, the groups of cells migrate from the basal to the adluminal position of the germinal epithelium.
Differently developed cell groups are found in a section of a seminiferous tubule and contribute to the typical appearance of the germinal epithelium. Six of these typical aspects were described in the human testis as “stages of spermatogenesis”. In any given region of the germinal epithelium, the same typical appearances of groups of germ cells appear every 16 days. This period of time is called the “seminiferous epithelial cycle”.
The development of a type A spermatogonium to mature spermatids requires 4.6 cycles, e.g. 74 days Mature spermatids released from the germinal epithelium as spermatozoa are transported through the epididymal duct system for an additional 12 days. Therefore, a minimum of 86 days should be calculated for a complete spermatogenetic cycle from spermatogonium to mature sperm.
Alterations of spermatogenesis
The proliferation and differentiation of male germ cells and the intratesticular and extratesticular mechanisms of regulation of spermatogenesis can be altered at all levels. This may occur as a result of environmental influences or may be due to diseases that directly or indirectly affect spermatogenesis. In addition, different nutritional and therapeutic substances, drugs, hormones and their metabolites, different toxic substances or X-rays can reduce or destroy spermatogenesis. Finally, also a fairly simple nose since the increase in temperature reduces the spermatogenetic activity of the testicles.
Under these negative influences, testicles respond relatively monotonically by reducing spermatogenesis. This can be expressed in the reduced number of mature spermatids, in spermatid malformation, lack of spermiation, impaired meiosis, arrest of spermatogenesis at the primary spermatocyte stage, reduced multiplication or apoptosis of spermatogonia. If the spermatogonia survive, then spermatogenesis can be rescued.
Otherwise, spermatogenesis ceases, and the shadows of the seminiferous tubules remain. Alterations in spermatogenesis are evaluated in histological sections of testicular biopsies. The most suitable technique is the semi-thin cut of the material embedded in epoxy resin. In a semi-thin section, all the details of the testicle cells can be optimally evaluated due to their excellent preservation.