The synthesis of both products is regulated by endocrine hormones produced in the hypothalamus and pituitary, as well as locally within the testis. Testosterone is indispensable for sperm production, however both testosterone and Follicle Stimulating Hormone FSH are needed for optimal testicular development and maximal sperm production. Sperm are produced via the extraordinarily complex and dynamic process of spermatogenesis that requires co-operation between multiple testicular cell types.
While it has long been known that testosterone and FSH regulate spermatogenesis, years of research has shed light on many of the intricate mechanisms by which spermatogonial stem cells develop into highly specialized, motile spermatozoa. Spermatogenesis involves the concerted interactions of endocrine hormones, but also many paracrine and growth factors, tightly co-ordinated gene and protein expression programs as well as epigenetic modifiers of the genome and different non-coding RNA species.
This chapter provides a comprehensive overview of the fascinating process of spermatogenesis and of its regulation, and emphasises the endocrine regulation of testicular somatic cells and germ cells. The chapter also provides a summary of the clinically significant aspects of the endocrine regulation of spermatogenesis. For complete coverage of all related areas of Endocrinology, please see our online FREE web-book, www. The secretion of hypothalamic gonadotropin-releasing hormone GnRH stimulates production of luteinizing hormone LH and follicle stimulating hormone FSH by the pituitary.
LH is transported in the blood stream to the testes, where it stimulates Leydig cells to produce testosterone: The testes, in turn, feedback on the hypothalamus and the pituitary via testosterone and inhibin secretion, in a negative feedback loop to limit GnRH and gonodotropin production.
Both androgens and FSH act on receptors within the supporting somatic cells, the Sertoli cells, to stimulate various functions needed for optimal sperm production. Spermatogenesis is the process by which immature male germ cells divide, undergo meiosis and differentiate into highly specialized haploid spermatozoa. Optimal spermatogenesis requires the action of both testosterone via androgen receptors and FSH. Spermatogenesis takes place within the seminiferous tubules of the testis.
These tubules form long convoluted loops that pass into the mediastinum of the testis and join an anastomosing network of tubules called the rete testis. Spermatozoa exit the testes via the rete and enter the efferent ductules prior to their passage through, and final maturation in, the epididymis. The seminiferous tubules are comprised of the seminiferous epithelium: Surrounding the seminiferous epithelium is a layer of basement membrane and layers of modified myofibroblastic cells termed peritubular myoid cells.
Between the tubules is the interstitial space that contains blood and lymphatic vessels, immune cells including macrophages and lymphocytes, and the steroidogenic Leydig cells. Male germ cell development relies absolutely on the structural and nutritional support of the somatic Sertoli cells. Sertoli cells are large columnar cells, with their base residing on basement membrane on the outside of the seminiferous tubules, and their apical processes surrounding germ cells as they develop into spermatozoa.
Androgens and estrogens and FSH act on receptors within Sertoli cells: Sertoli cells regulate the internal environment of the seminiferous tubule by secreting paracrine factors and expressing cell surface receptors needed for germ cell development. Sertoli cells form intercellular tight junctions at their base: The seminiferous tubules are also an immune-privileged environment.
The number of Sertoli cells determines the ultimate spermatogenic output of the testes. In humans, Sertoli cells proliferate during the fetal and early neonatal period and again prior to puberty.
At puberty, Sertoli cells cease proliferation and attain a mature, terminally differentiated phenotype that is able to support spermatogenesis.
Disturbances to Sertoli cell proliferation during these times can result in smaller testes with lower sperm production. Conversely, disturbances to the cessation of proliferation can result in larger testes with more Sertoli cells and a greater sperm output. It seems likely that the failure of many men with congenital hypogonadotropic hypogonadism HH to achieve normal testicular size and sperm output, when treated by gonadotropic stimulation, may result from deficient Sertoli cell proliferation during fetal and prepubertal life.
The action of both androgens and FSH on Sertoli cells is necessary for the ability of Sertoli cells to support full spermatogenesis. In addition, the expression of many genes and paracrine factors within Sertoli cells is necessary for spermatogenesis.
Spermatogenesis relies on the ability of Leydig cells to produce testosterone under the influence of LH. Fetal Leydig cells appear following gonadal sex differentiation gestational weeks in humans and, under the stimulation of placental human chorionic gonadotropin hCG , results in the production of testosterone during gestation. In humans, fetal cells decrease in number towards term and are lost from the interstitium at about twelve months of age.
The adult population of Leydig cells in the human arises from the division and differentiation of mesenchymal precursor cells under the influence of LH at puberty. Factors secreted by Sertoli cells and peritubular myoid cells are also necessary for Leydig cell development and steroidogenesis. Optimal Leydig cell steroidogenesis also relies on a normal complement of macrophages within the testicular interstitium as well as on the presence of androgen receptors in peritubular myoid cells, presumably because these cells secrete factors necessary for Leydig cell development and function.
The process of spermatogenesis begins in the fetal testis, when the Sertoli cell population is specified in the embryonic testis under the influence of male sex determining factors, such as SRY and SOX9.
Newly-specified Sertoli cells enclose and form seminiferous cord structures and direct primordial germ cells to commit to the male pathway of gene expression. Fetal Sertoli cells proliferate and drive seminiferous cord elongation; this process is also dependent on factors secreted by Leydig cells.
In the neonatal testis, primordial germ cells undergo further maturation and migrate to the basement of the seminiferous tubules where they provide a pool of precursor germ cells for postnatal spermatogenesis.
Spermatogonia are the most immature germ cell type. This heterogeneous population includes spermatogonial stem cells, which self-renew throughout life to provide a pool of stem cells available for spermatogenesis, as well as proliferating cells that differentiate and become committed to entry into meiosis.
Spermatogonial development is hormonally independent and as such they are present even in the absence of GnRH. Spermatogonia eventually differentiate into spermatocytes that proceed through the process of meiosis that begins with DNA synthesis resulting in a tetraploid gamete. At the end of prophase, the meiotic cells proceed through two rapid and successive reductive divisions to yield haploid spermatids. The completion of meiosis depends absolutely on androgen action in Sertoli cells; in the absence of androgen, no haploid spermatids will be produced.
Newly formed haploid round spermatids differentiate, with no further division, into the highly specialized spermatozoan during the process of spermiogenesis.
This remodelling of the DNA involves the cessation of gene transcription up to 2 weeks prior to the final maturation of the sperm; therefore spermiogenesis involves the translational delay of many mRNA species which must then be translated at precise times throughout their final development. Spermatogenesis ends with the process of spermiation. Both the survival of spermatids during spermiogenesis and their release at the end of spermiation is dependent on optimal levels of androgen and FSH.
Spermatogenesis is a long process, taking up to 64 days in the human, and its inherent complexity demands precise timing and spatial organization. Within the seminiferous tubules, Sertoli cells and surrounding germ cells in various phases of development are highly organized into a series of cell associations, known as stages. These stages result from the fact that a particular spermatogonial cell type, when it appears in the epithelium, is always associated with a specific stage of meiosis and spermatid development.
This cycle along the length of tubule is obvious in rodents, however in humans several cycles are intertwined in a helical pattern; thus a human seminiferous tubule viewed in cross section will contain up to three stages.
These pulses of sperm release allow the testes to continually produce millions of sperm, with the average normospermic man able to produce approximately sperm per heartbeat. The precise timing and co-ordination of spermatogenesis is achieved by many factors. Emerging evidence suggests that retinoic acid, metabolized within the testis from circulating retinol a product of vitamin A is a major driver of spermatogenesis. A precise pulse of retinoic acid action is delivered to a particular stage of the spermatogenic cycle; this pulse is achieved by the constrained expression of enzymes involved in retinoic acid synthesis, degradation and storage, as well as the local expression of retinoic acid receptors.
This pulse of retinoic acid acts directly on spermatogonia to stimulate their entry into the pathway committed to meiosis. It also acts directly on Sertoli cells to regulate its cyclic functions. This clock appears to be set by retinoic acid, however the timing of the clock can be influenced by the germ cells themselves. The timing of spermatogenesis also relies on an extraordinarily complex program of gene transcription and protein translation.
Alternative splicing of mRNA is highly prevalent in the testis, and generates many germ cell-specific transcripts that are important for the ordered procession of germ cell development.
Indeed, studies on male germ cells have revealed much of what is known about the biology and function of non-coding RNAs. These non-coding RNAs have many and varied roles and are particularly required for the transcriptional program executed during meiosis and spermiogenesis. The male germ cell transmits both genetic and epigenetic information to the offspring.
Epigenetic modifications of the genome are heritable; epigenetic processes such as DNA methylation and histone modifications regulate chromatin structure and modulate gene transcription and silencing.
The male germ cell undergoes major epigenetic programming in the fetal testis, during the genome wide de-methylation and re-methylation to establish the germline-specific epigenetic pattern that is eventually transmitted to the offspring. The sperm epigenome is then further remodelled during postnatal spermatogenesis by various mechanisms. It is clear from the above summary that spermatogenesis relies on many intrinsic and extrinsic factors. However spermatogenesis is absolutely dependent on androgen-secretion by the Leydig cells; androgens stimulate and maintain germ cell development throughout life.
Testicular testosterone levels are very high, by virtue of its local production, however they are considerably higher than those required for the initiation and maintenance of spermatogenesis. Androgen action on receptors within Leydig cells, peritubular myoid cells and Sertoli cells is essential for normal steroidogenesis and spermatogenesis. While testosterone is essential for spermatogenesis, it is also important to note that exogenous testosterone administration resulting in even slightly supraphysiological serum levels suppresses gonadotropin secretion via negative feedback effects on the hypothalamus and pituitary, leading to the cessation of sperm production.
In contrast to androgens, spermatogenesis can proceed in the absence of FSH; however, testes are smaller and sperm output is reduced. While FSH is thus not essential for spermatogenesis, it is generally considered that optimal spermatogenesis requires the combined actions of both androgen and FSH, with both hormones having independent, co-operative and synergistic effects to promote maximal sperm output. These factors are an important consideration in the stimulation of spermatogenesis in the setting of HH.
As androgens are essential for the initiation of sperm production, the induction of spermatogenesis in HH acquired after puberty is achieved by the administration of hCG as an LH substitute. Prolonged therapy is required to produce sperm in the ejaculate, given that human spermatogenesis takes more than 2 months to produce sperm from spermatogonia.
Treatment with hCG alone may be sufficient for the induction of spermatogenesis in men with larger testes due to potential residual FSH action, however, for many men, and particularly for those with congenital HH, the co-administration of FSH is needed for maximal stimulation of sperm output. In summary, the testes, under the influence of gonadotropins, produce testosterone and sperm. These processes require the co-ordinated actions of multiple cell types and the secretion of paracrine factors.
Spermatogenesis is a long and complex process that relies on multiple somatic cells as well as on the co-ordinated expression of genes, proteins and non-coding RNAs. This structure forms a closed cavity representing the remnants of the processus vaginalis into which the testis descends during fetal development Figure 1. Along its posterior border, the testis is loosely linked to the epididymis which at its lower pole gives rise to the vas deferens 1.
Figure 1 The relationships of the tunica vaginalis to the testis and epididymis is illustrated from the lateral view and two cross sections at the level of the head and mid-body of the epididymis. The large arrows indicate the sinus of the epididymis posteriorly. Reproduced with permission from de Kretser et. The testis is covered by a thick fibrous connective tissue capsule called the tunica albuginea.
From this structure, thin imperfect septa run in a posterior direction to join a fibrous thickening of the posterior part of the tunica albuginea called the mediastinum of the testis. The testis is thus incompletely divided into a series of lobules. Within these lobules, the seminiferous tubules form loops, the terminal ends of which extend as straight tubular extensions, called tubuli recti, which pass into the mediastinum of the testis and join an anastomosing network of tubules called the rete testis.
From the rete testis, in the human, a series of six to twelve fine efferent ducts join to form the duct of the epididymis. This duct, approximately m long in the human, is extensively coiled and forms the structure of the epididymis that can be divided into the head, body and tail of the epididymis 1. At its distal pole, the tail of the epididymis gives rise to the vas deferens Figure 2.