Importance of Spermatogenesis

To understand the importance of spermatogenesis let us first define spermatogenesis. It is the formation of haploid spermatozoa from germ cells in the testis' seminiferous tubules. The mitotic division of stem cells near the base membrane of the tubules initiates this process. These cells are termed as the spermatogonial stem cells.

These cells divide into two forms after mitotic division. The type A cells replace stem cells, while the type B cells grow into primary spermatocytes. Meiosis I splits the primary spermatocyte into two secondary spermatocytes, and Meiosis II splits growing secondary spermatocytes into nearly two identical haploid spermatids.

Spermiogenesis is the mechanism by which spermatids become spermatozoa (sperm). They grow into spermatozoa, which are also recognized as sperm cells. Thus, the primary spermatocyte produces two cells, the secondary spermatocytes, and the two secondary spermatocytes contain four spermatozoa and four haploid cells as a result of their subdivision.

Significance of Spermatogenesis

The significance of spermatogenesis is that it releases mature male gametes. These male gametes are termed as sperms but more precisely as spermatozoa, that can fertilize the female gamete, the oocyte, to create a single-celled zygote. The zygote grows into the offspring through conception.

The two gametes each contribute half of the chromosomes (haploid) to create a diploid zygote, in sexual reproduction.

Site of Spermatogenesis in Humans

Spermatogenesis occurs in several systems inside the male reproductive system. The formation of gametes starts in the testes and progresses to the epididymis, whereby they mature and thus are deposited before ejaculation.

The process begins in the testes' seminiferous tubules, where spermatogonial stem cells added to the inner tubule wall differentiate in a centripetal orientation to develop immature sperm, starting at the walls and advancing into the deepest portion, or lumen. The epididymis is where maturation takes place.

The position of the testes or scrotum is crucial because spermatogenesis requires a lower temperature than the normal body temperature of 37 °C (98.6 °F) in order to obtain viable sperm cells. Small temperature variations, such as those caused by an athletic support strap, have been shown in clinical trials to have little effect on sperm viability or the count.

Duration

The whole spermatogenesis process in humans is predicted to take place between 74 days (as per tritium-labeled biopsies) and 120 days (according to DNA clock measurements). It requires 3 months to move using the ductal system. Every day, the testes contain 200 to 300 million spermatozoa. However, just under half of these, or 100 million, turn into viable sperm.

Spermatogenesis Steps

Let us study the spermatogenesis process stages while we describe the process of spermatogenesis. The entire spermatogenesis steps can be divided up into multiple stages, each of which corresponds to a different cell type in humans. The ploidy, copy number, and chromosome/chromatid counts in the following table are for a single cell, normally before DNA synthesis and division (in G1 if applicable). After DNA synthesis, but before division, the primary spermatocyte is arrested.

Spermatocytogenesis: 

• Spermatocytogenesis is the first one amongst the spermatogenesis process stages, and seems to be the male form of gametocytogenesis, in which spermatocytes with half the regular complement of genetic material are produced. 

• A diploid spermatogonium throughout the basal compartment of the seminiferous tubules separates mitotically during spermatocytogenesis, forming two diploid intermediate cells termed primary spermatocytes. 

• Every other primary spermatocyte, therefore, enters the adluminal compartment of the seminiferous tubules, copies its DNA, and passes through meiosis I to create two haploid secondary spermatocytes, which will then differentiate among haploid spermatids.

Spermatidogenesis: 

• The development of spermatids through secondary spermatocytes is known as spermatidogenesis. 

• Early-stage secondary spermatocytes reach meiosis II quickly and differentiate to create haploid spermatids. 

• Secondary spermatocytes are not so commonly seen in histological studies due to the brevity of this point.

Spermiogenesis: 

• While the spermatogenesis process, spermatids tend to appear in a tail by developing microtubules under one of the centrioles, which eventually becomes the basal body. An axoneme is made up of these microtubules. 

• Later, during the centrosome reduction process, the centriole is changed. Since mitochondria are positioned across the axoneme to secure energy supply, the anterior portion of the tail (named the midpiece) thickens. Spermatid DNA is also packed, resulting in a highly compressed form. 

• All through spermatid elongation, the DNA is first filled and bundled with unique nuclear basic proteins, that are then substituted with protamines. The densely compacted chromatin that results is transcriptionally inactive. The acrosome is created as the Golgi apparatus covers the already condensed nucleus.

Influencing Factors

The process of spermatogenesis is extremely sensitive to environmental changes, specifically temperature and hormones. To keep the process going, testosterone must be present in high local concentrations, which is done by testosterone attaching to androgen binding protein in the seminiferous tubules.

In humans and some other animals, the seminiferous epithelium is susceptible to elevated temperatures and can be negatively impacted by temperatures reaching as high as the average temperature of the body.  As a consequence, the testes are found outside the body in the scrotum, a bag of skin. The ideal temperature is 2 degrees Celsius (man) or 8 degrees Celsius (mouse) under body temperature. The cremasteric muscle and the dartos smooth muscle in the scrotum control blood circulation and place the scrotum towards and apart from the body's heat.