The union of male and female gametes creates offspring. The production of these vital reproductive cells occurs in the testis and ovary during the processes of spermatogenesis and oogenesis, respectively.[1] The primary male reproductive organs, the testes, are located inside the scrotum and function to produce sperm cells as well as the primary male hormone, testosterone. As mentioned above, spermatogenesis is the process by which sperm cell production occurs; the germ cells give rise to the haploid spermatozoa. Sperm production takes place inside the seminiferous tubules, which is a convoluted cluster of tubes located inside the testes. Testosterone production occurs in cells surrounding the seminiferous tubules, called Leydig cells. After being formed, sperm cells travel outside of the tubules into the epididymis, where they mature and prepare for ejaculation.

The complex process of spermatogenesis occurs in three steps. The first step involves mitotic cell division that allows the early cell stage, spermatogonia, to multiply. The second step requires meiosis, in which the diploid cells form haploid cells. A division occurs until a round spermatid formation occurs. The final stage of spermatogenesis includes the production of spermatozoa, mature and motile sperm cells, from round spermatids, through a process called spermiogenesis.[2] 

Diminished fertility or infertility may result from a decrease in spermatozoa number, alteration in shape, and inefficient in motility.[3]

The three steps represent the foundation of spermatogenesis. Functional abnormalities may occur in any one of them, which can cause the entire process to fail. These abnormalities can lead to defective or reduced sperm production. In more severe conditions, a complete absence of spermatozoa can result, leading to infertility. Therefore, we must expand our knowledge of spermatogenesis as a whole so that we can provide essential information regarding the regulatory mechanisms. The testicular environment is complex; therefore, the study of spermatogenesis can be quite tricky in most species. To achieve this understanding, experimental studies completed in rodents and primates are the cornerstone of this crucial knowledge.[2]

The necessity for the production of an infinite number of gametes increases the specific requirements needed for spermatogenesis to occur: The reproductive lifecycle of the male requires a large amount of stem cell production. A wide array of particular progenitor cells is necessary for the production of enough gametes to establish fertilization. Certain genes specific to spermiogenesis are required for the differentiation of sperm as well as acquiring sperm mobility. The continuous pool of spermatozoa becomes readily available by an advanced level of control and organization. One of the principal functions in male reproduction is carried out by the spermatogonia stem cells. Spermatogonia need to stem cells to maintain their numbers by self-renewal and form the necessary progenitor cells required to proceed with the process of spermatogenesis. Germ cells need constant nutrition to be able to differentiate into mature sperm that are capable of fertilization. Unavailability of these factors can cause spermatogenesis to fail ultimately and lead to sperm cell production.[4][5]

Spermatogenesis is far less efficient in terms of quality management. The loss of germ cells occurs quite often. Also, the ejaculate can have extremely high numbers of malformed spermatozoa. Apoptosis or degeneration allows for the loss of about 75% of the developed germ cells. Only 25% of the germ cells reach the ejaculate, and research reveals that about 50% are malformed. Therefore, the spermatogenic potential that is accessible for reproduction is about 12%.[3]

Recent reports have noticed a decline in the spermatozoa concentrations in the ejaculates of healthy males. This decline has occurred over the last decades, and specific factors affecting embryonal development seems to be the cause. These factors include prenatal influences such as hormones, drugs, radiation, metabolites in the drinking water, and nourishment of the mother. Moreover, the spermatogenetic process of the testis is affected by increased temperatures. These negative influences lead to a reduction in spermatogenesis, which manifests as a reduction in the number of mature spermatids or the formation of malformed spermatids.

Additionally, these influences may cause the process of meiosis to be disturbed. An arrest of spermatogenesis may occur after the creation of primary spermatocytes, and apoptosis of spermatogonia can occur. Rescue of spermatogenesis may arise if the spermatogonia survive. If they do not survive, spermatogenesis comes to a halt, and seminiferous tubules will appear as shadows.[3] 

Testicular biopsies evaluate the disturbances of spermatogenesis through histological sections. A suitable technique is known to be semithin sectioning of material embedded in epoxy resin. The excellent preservation of the cells much helps the evaluation of the specific details of the cells.[3]

The human testes, exhibiting an ellipsoid shape, are two reproductive organs that reside in a sac called the scrotum. Each testis is about 2.5 x 4 cm in diameter, surrounded by an active capsule of connective tissue called tunica albuginea. The parenchyma of the testis is divided roughly into 370 conical lobules by thin septula testis. Within the lobules are the seminiferous tubules as well as inter-tubular tissue. This tissue contains groups of Leydig cells and further cellular elements.[3]

The seminiferous tubules consist of entwined loops whose ends unfold into the open space of the rete testis. Rete testis is a network of tubules, found in the hilum of the testicle, and function to carry collected sperm to the efferent ducts of the epididymis as the seminiferous tubules secrete it. The germinal epithelium constitutes the seminiferous tubule. The invagination of Sertoli cells houses many cells. These cells include germ cells that exist in different developmental stages such as spermatogonia, primary and secondary spermatocytes as well as spermatids. The tight junctions of cellular membranes connect Sertoli cells. The specialized zones of tight junctions form the blood-testis barrier. Germ cells pass the hurdle during maturation, where they get protection from the diffusion of external substances. Sertoli cells are known to be the “biological clock” of the testis. When visualized in histological sections, they manifest rising amounts of lipid droplets with time. Therefore, the increasing numbers of lipids correlate to the advancing age of the individual.[3] 

Other functions attributable to Sertoli cells include providing nutrition for the germ cells, delivering mature spermatids to the tubular lumen, regulating spermatogenesis by producing endocrine and paracrine substances, maintaining the epithelia of the efferent ductal system by secreting ABP (androgen binding protein) as well as interacting with the inter-tubular Leydig cells. The lamina propria of seminiferous tubules, also known as the peritubular tissue, constitutes the basement membrane of the basal lamina and functions to surround the cellular elements. It consists of connective tissue intermingled around five layers of myofibroblasts. These myofibroblasts play a vital role in the peristaltic contraction of seminiferous tubules to allow for the transport of immotile spermatozoa to the hilum of the testis, the rete testis. Peritubular tissue usually is about 8 micrometers in thickness. Alteration of spermatogenesis can increase the width to 12 micrometers by connective tissue.[3]

Leydig cells found to be most prominent in the inter-tubular space, form clusters that surround the capillaries. They produce and secrete one of the essential male reproductive hormones, testosterone. Testosterone is vital for male reproduction as it activates the hypophyseal testicular axis. It is crucial for the development of male genital organs as well as masculinization and formation of the secondary sex characteristics. Testosterone plays a significant role in initiating, processing, and maintaining spermatogenesis. Hormone production does not correlate with the number of Leydig cells. Immunohistochemical investigations prove that the creation of testosterone only takes place in a few Leydig cells despite their increased number. An increased number of cells occurs in hyperplasia or tumors.[3]

Spermatogenesis involves the following three complex integrated processes initiated at the onset of puberty:

Meiosis

Male fertility requires the formation of millions of gametes. Production of functional gametes or sex cells requires a single cell to undergo cell division to reduce the number of chromosomes by half. This process is known as meiosis. Meiosis occurs twice, thereby creating four daughter haploid cells, also known as spermatids. For every diploid spermatocyte, meiotic divisions produce four haploid spermatids.[1][5]

Mitotic cell division: diploid spermatogenic stem cells differentiate into two sets of diploid primary spermatocytes, also known as tetraploid cells. The largest germ cells found in the germinal epithelium are the primary spermatocytes, and they contain the largest cell nuclei.[3]

Meiotic cell division I

The division begins with the leptotene stage of prophase. The stage takes place in the germinal epithelium, more specifically the basal compartment. Spermatocytes enter the ad luminal compartment after reaching the Sertoli cell barrier. Here, further prophase stages continue, and these include zygotene, pachytene, and diplotene stage. During these stages, DNA reduplication, and chromosomes condensation, homologous chromosomes pairing, as crossing over will occur. Each set of diploid primary spermatocyte differentiates into two haploid secondary spermatocytes, where the total number of chromosomes gets reduced to half.[3][6]

Meiotic cell division II

Each haploid secondary spermatocyte differentiates into two haploid spermatids, therefore, resulting in four haploid cells. This process occurs quickly, and no DNA replication takes place.[3]

Spermiogenesis

This process is the final phase of spermatogenesis. Spermatids mature and form spermatozoa, fully differentiated sperm cells. This stage ends when the mature cells leave the germinal epithelium. These free cells, at this point, are called spermatozoa. Spermatozoa are known to have a unique shape, which is essential in their movement to the female gamete. The condensed nucleus and the presence of an acrosome needed to establish contact with the female gamete provide them with their unique shape. They exhibit extensive motility as they contain a connected flagellum. As they transform into mature sperm, spermatids undergo a wide array of morphological changes.[3][5] It takes about 30 to 40 days for spermatogenic stem cells to generate spermatozoa.[4] Spermatogenesis is a process that occurs in the seminiferous tubules, and many internal and external factors regulate it. Intrinsic regulation requires the production and release of testosterone, neuroendocrine substances, and growth factors, secreted by Leydig cells. They communicate with nearby Leydig cells, blood vessels, peritubular tissue of the seminiferous tubules as well as Sertoli cells. They maintain the trophic factors of these cells and take part in the regulation of peristalsis that occurs in the seminiferous tubules. These growth factors and neuroendocrine substances influence the contractility of myofibroblasts to ensure the proper transport of spermatozoa. Intrinsic factors also play a vital role in regulating the flow of blood in the intertubular microvasculature. This process is quite intricate and is mainly investigational in laboratory animals. In humans, it is still unclear.[3]

Extrinsic regulation of spermatogenesis requires stimuli from the hypothalamus and hypophysis. The hypophysis awaits a signal from the hypothalamus to begin the release of LH (luteinizing hormone). This signal is the pulsatile secretion of the hormone GnRH (gonadotropin-releasing hormone). The release of LH stimulates the Leydig cells to produce testosterone. Testosterone has a significant effect on spermatogenesis as well as other functions throughout the body. Sertoli cells are stimulated by FSH (follicle-stimulating hormone), which is an important signal that allows for the maturation of germ cells. Sertoli cells secrete inhibin that is involved in the feedback mechanism.[3]