In the Male the Cells From Which Sperm Arise Are Produced Continuously Throughout Life

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Spermatogenesis

Spermatogenesis is an ongoing differentiation process that occurs in the seminiferous epithelium in the testis in males to produce spermatozoa (sperm) and is sustained by a tissue-specific stem cell termed the "spermatogonial stem cell."

From: Encyclopedia of Reproduction (Second Edition) , 2018

Testicular Disorders

Shlomo Melmed MB ChB, MACP , in Williams Textbook of Endocrinology , 2020

Maintenance of Spermatogenesis

In men with prepubertal gonadotropin deficiency (e.g., CHH), once spermatogenesis has been initiated with LH (hCG) and FSH treatment, sperm production may be maintained with LH treatment alone without continued FSH administration. ⁷⁹ However, spermatogenesis is not stimulated by administration of FSH in combination with testosterone (that maintains normal concentrations of serum testosterone but with continued low LH and intratesticular testosterone concentrations) in men with CHH. Spermatogenesis may be reinitiated with LH (hCG) alone in previously gonadotropin-treated men with CHH after a period of gonadotropin deficiency associated with exogenous testosterone replacement therapy. Furthermore, in men with gonadotropin deficiency and azoospermia acquired as an adult (e.g., secondary to a pituitary adenoma), spermatogenesis may be reinitiated and maintained with LH (hCG) treatment alone. ⁷⁹

In normal men with experimental gonadotropin deficiency induced by high-dose testosterone administration, spermatogenesis may be reinitiated and maintained by either LH or hCG alone, despite markedly suppressed FSH concentrations, or by FSH alone, despite severely suppressed LH (and presumably low intratesticular testosterone) concentrations. However, sperm production was not stimulated by either LH or FSH alone to the baseline concentrations that existed before experimental gonadotropin suppression. ⁸⁷ In this model of gonadotropin deficiency, treatment with both LH (hCG) and FSH restored sperm counts fully to baseline values. Finally, in support of the ability of FSH alone to stimulate sperm production, spermatogenesis was maintained despite undetectable serum gonadotropin concentrations in a hypophysectomized man who had an activating FSH receptor mutation. ⁸⁸

Together, these findings suggest that a normal concentration of either FSH or LH is sufficient for maintenance of qualitatively normal sperm production, but both gonadotropins are necessary for quantitatively normal spermatogenesis in male humans.

The effect of gonadotropins on specific stages of spermatogenesis has been studied in normal men with experimental gonadotropin suppression induced by the administration of high-dose progestin and testosterone. In these gonadotropin-deficient men, selective replacement of either FSH or LH (increasing intratesticular testosterone) supported all stages of spermatogenesis, including spermatogonial maturation, meiosis, spermiogenesis, and spermiation, but each agent had predominant actions on specific stages. ⁸⁹ FSH exerted a relatively greater effect on maturation of spermatogonia (conversion of spermatogonia A_p to spermatogonia B), early meiosis, and maintenance of pachytene spermatocytes (conversion of spermatogonia to pachytene spermatocytes). LH (stimulating intratesticular testosterone) had predominant effects on the completion of meiosis (conversion of pachytene spermatocytes to round spermatids) and on spermiation (release of mature spermatozoa). LH and FSH (intratesticular testosterone) exert similar effects on spermiogenesis (conversion of round to elongated spermatids).

Reproduction and Development

R. Renkawitz-Pohl , ... M.A. Schäfer , in Comprehensive Molecular Insect Science, 2005

1.4.1 Introduction

Spermatogenesis is a highly specialized process of cellular differentiation resulting in the formation of functional spermatozoa for successful reproduction. In principle, the process of spermatogenesis is well conserved in all sexually proliferating organisms, although the size and shape of the mature sperm vary considerably among different species. Many details are comparable between mammals and Drosophila making the fly a very good model system to study fertility defects. Drosophila germ cells, like those of mammals, are set aside early in embryonic development and migrate through the primordium of the hindgut into the interior of the embryo where they join the somatic parts of the embryonic gonads (review: Zhao and Garbers, 2002). At the end of the third larval instar and the onset of pupariation, the first germ cells enter meiosis (Figure 1).

Spermatogenesis is a continuous process during adult life and, thus, the adult testes contain all stages from stem cells to mature sperm (Figure 1). As in mammals, the germ cells develop in close contact with somatic cells, in this case the cyst cells, which are of mesodermal origin. At the very tip of the testis tube, the so-called hub is formed by somatic support cells (asterisk in Figure 1) to which the germline stem cells (GSCs) and the cyst cell progenitors (somatic stem cells, SSCs) are physically connected. In close contact to the hub, both stem cell types divide asymmetrically depending on the JAK-STAT signaling pathway (see Section 1.4.3.1 for details). One daughter cell remains connected to the hub and maintains stem cell characteristics. The other daughter cell becomes disconnected from the hub and enters the differentiation process. This germ cell, now called spermatogonium, is surrounded by two cyst cells, thus forming a cyst in which the germ cell undergoes four mitotic divisions, meiosis, and sperm morphogenesis until individualization (see Figure 3 for an overview).

The ultrastructure and cytology of Drosophila melanogaster spermatogenesis has been extensively reviewed by Fuller (1993). This complex differentiation process from round cells to the highly specialized structure of spermatozoa is controlled by a large number of genes (up to 1500) affecting spermatogenesis, which is reflected in the large number of male sterile mutants (reviews: Lindsley and Tokuyasu, 1980; Hackstein et al., 2000). The focus here is on the recent advances in understanding the molecular basis for the dramatic changes in cell morphology during germ cell development starting with the asymmetric division of stem cells into a stem cell and a spermatogonium as the entry point to germ cell differentiation. Control of mitotic proliferation and entry into meiosis are discussed and various aspects of sperm morphogenesis highlighted, such as the formation of the axoneme and the mitochondrial derivative, the Nebenkern. Then the importance of cell interactions during the process is discussed (Figure 4), and finally transcriptional and translational control mechanisms during meiotic prophase, and their relevance for sperm morphogenesis (Figure 3).

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Physiology and Disorders of Puberty

Shlomo Melmed MB ChB, MACP , in Williams Textbook of Endocrinology , 2020

Spermatogenesis

The first histologic evidence of spermatogenesis appears between ages 11 and 15 years ( Fig. 26.13; also seeFigs. 26.6 and 26.9). Spermaturia may be the first sign of pubertal development, but the presence of sperm in urine is intermittent and therefore not a reliable indicator in all boys. Spermaturia is more prevalent in early puberty than in late puberty, suggesting that there may be a continuous flow of sperm through the urethra in early puberty but that ejaculation is necessary for sperm to appear in the urine in late puberty. Spermaturia in the first-morning urine specimen occurs at a mean chronologic age of 13.3 years and at a mean pubic hair stage 2 to 3 in one study (or 16 years in another study), but may be found in normal boys with bilateral testicular volumes of only 3 mL and no signs of puberty. ¹³⁹ Normospermia (i.e., normal sperm concentration, morphologic appearance, and motility) is not present until a bone age of 17 years. The first conscious ejaculation occurs at a mean chronologic age of 13.5 years in normal boys and at a mean bone age of 13.5 years in boys with delayed puberty. ¹⁴⁰ The age of the first ejaculation, spermarche, decreased in China between 1995 and 2010, and a higher BMI led to an earlier age of spermarche; this pattern mirrors to a degree the secular trend of menarche in girls. ^141,142

The potential for fertility is reached before an adult phenotype is attained, before adult plasma testosterone concentrations are reached, and before PHV occurs.

Volume II

David M. de Kretser , ... Moira O'Bryan , in Endocrinology: Adult and Pediatric (Seventh Edition), 2016

Seminiferous Tubules

Spermatogenesis takes place within the seminiferous tubules, which, in humans, are ~200 μm in diameter and have a total length of ~600 meters occupying ~60% of the testis volume ( Fig. 136-1).

The terminal ends of the seminiferous tubules in the mediastinum empty via straight tubular extensions termed tubuli recti. Depending on the species, individual seminiferous tubules may be highly convoluted (e.g., human), or they may form numerous relatively linear segments linked by cranial and caudal hairpin turns (e.g., rodent testes). ³

Within the epithelium of the seminiferous tubules, germ cells undergo spermatogenesis, which commences with the spermatogonia that lie adjacent to the basement membrane of the tubules and divide by mitosis. Spermatogonia, as well as renewing themselves, give rise to cells that lose contact with the basement membrane and commence the process of meiosis, now called primary spermatocytes. Having completed the first meiotic division, these cells give rise to daughter cells called secondary spermatocytes, which divide to complete meiosis to form round spermatids.

The round spermatids do not divide but undergo a complex metamorphosis, called spermiogenesis, to become spermatozoa that are released into the lumen of the seminiferous tubule by a process called spermiation.

Interspersed between the germ cells within the seminiferous epithelium are the supporting cells, called Sertoli cells, which extend from the basement membrane of the tubule to the lumen like a tree with its trunk abutting on the basement membrane and its branches being interspersed between the germ cells. The physical relationship between the nondividing Sertoli cells in the adult testis and the various types of dividing and differentiating germ cells is difficult to appreciate by light microscopy since the Sertoli cell cytoplasmic extensions between the germ cells are thin. The complexities of this association have, however, been thoroughly described in ultrastructural studies (see reviews elsewhere ^4,5 ) to reveal a dynamic convoluted architecture. As germ cells progress through spermatogenesis, they are progressively moved apically through the seminiferous epithelium separated by processes of Sertoli cell cytoplasm that create pockets, or recesses between the Sertoli cells. The most mature germ cells, the spermatozoa, are ultimately released into the lumen of the seminiferous tubule (Figs. 136-2, 136-3, and 136-4).

Where adjacent Sertoli cells interface with each other above the basal spermatogonia, a specialized tight cell junction is formed preventing intercellular transport of substances, thus creating basal and adluminal compartments of the seminiferous tubules. These tight junctions effectively form the blood-testis barrier, which can open to enable spermatogonia to lose their connection with the basement membrane and enter the adluminal compartment.

The sperm and luminal fluid are moved by irregular contractions of the peritubular myoid cells that lie on the external surface of the tubules through the mediastinum into the rete testis. The rete testis is a maze of anastomosing spaces within the mediastinum and drains into the epididymis. The morphology of the rete is species-specific, ⁶ but can generally be divided into three principal zones. The septal rete is composed of straight tubules that empty into the mediastinal rete, a network of anastomosing channels. These, in turn, drain into the extratesticular rete, which is characterized by wider spaces in continuity with the 6 to 12 fine efferent ductules leading to the head of the epididymis.

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Male Reproductive Physiology

Alan W. Partin MD, PhD , in Campbell-Walsh-Wein Urology , 2021

Spermatogenesis

Spermatogenesis is a remarkably complex and specialized process of DNA reduction and germ cell metamorphosis. Older studies have estimated that the entire process in humans requires approximately 64 days ( Clermont, 1972). However, an in vivo kinetic study in healthy men revealed thatthe total time to produce an ejaculated sperm ranges from 42 to 76 days, suggesting that the duration of spermatogenesis can vary widely among individuals (Misell et al., 2006;Fig. 64.13).Spermatogenesis involves (1) a proliferative phase as spermatogonia divide to replace their number (self-renewal) or differentiate into daughter cells that become mature gametes; (2)a meiotic phase when germ cells undergo a reduction division, resulting in haploid (half the normal DNA complement) spermatids; and (3)a spermiogenesis phase in which spermatids undergo a profound metamorphosis to become mature spermatozoa. (For excellent reviews, seeSteinberger [1976] andde Kretser and Kerr [1988].)

Acycle of spermatogenesis involves the division of primitive spermatogonial stem cells into subsequent germ cells. Several cycles of spermatogenesis coexist within the germinal epithelium, and they are described morphologically asstages. If spermatogenesis is viewed from a single fixed point within a seminiferous tubule, six recognizable cellular associations or stages are predictably observed in humans (Heller and Clermont, 1964) (seeFig. 64.11). In addition, there is also a specific organization of spermatogenic cycles within the tubular space, termedspermatogenic waves. The best evidence suggests that human spermatogenesis exists in a spiral or helical cellular arrangement that ensures sperm production is a continuous and not a pulsatile process (Schulze, 1989;Fig. 64.14).

Testis Stem Cell Migration, Renewal, and Proliferation

Testis Stem Cell Migration

During early prenatal development,primordial germ cells migrate to the gonadal ridge and associate with Sertoli cells to form primitive testicular cords (Witschi, 1948). These primitive germline stem cells are termedgonocytes after the gonad differentiates into a testis by forming seminiferous cords. They are calledspermatogonia after migration to the periphery of the tubule (Gondos and Hobel, 1971).These early migrating germ cells have properties similar to embryonic stem cells and are likely the source of adult germ cell tumors (Ezeh et al., 2005). The failure of germ cells to migrate into the primitive testicle is also thought to be a cause ofextragonadal germ cells tumors and adult infertility resulting fromazoospermia with Sertoli cell–only testicular histology (Nikolic et al., 2016).

Human Male Spermatogenesis

Laurence A. Cole , in Biology of Life, 2016

Endocrine Control of Spermatogenesis

Spermatogenesis begins at puberty, when testosterone levels rise. Testosterone is critical to spermatogenesis. In the lack of testosterone, spermatogenesis only proceeds as far as the prophase 1-leptotene stage of meiosis ( Fig. 18.3). Hypophysectomy or removal of the pituitary gland leads to an absence of luteinizing hormone (LH). With the absence of LH, Leydig cells stop producing testosterone and spermatogenesis comes to a halt. In this respect, LH is as critical to spermatogenesis as testosterone.

The role of follicle stimulating hormone (FSH) in men is less sure. FSH promotes growth of testosterone receptors on Sertoli cells and seminiferous tubules, this is important. Data with rodents advises that FSH binding Sertoli cells increases the number of spermatogonia or resting spermatocytes formed prior to meiosis. Basically, LH and testosterone are critical to spermatogenesis in men, but the role of FSH is secondary and less critical.

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Anatomy and physiology of the reproductive system

Irina Szmelskyj DipAc MSc MBAcC , ... Alan O. Szmelskyj Do MSc AdvDipClinHyp FRSPH , in Acupuncture for IVF and Assisted Reproduction, 2015

Spermatogenesis and spermiogenesis

Spermatogenesis is the process of spermatozoa (sperm) formation. ¹² Spermatogenesis starts at puberty, when the Leydig cells in the testes start to produce androgens under the influence of the Follicle-Stimulating Hormone (FSH) and the Luteinizing Hormone (LH), which are in turn controlled by the Gonadotrophin-Releasing Hormone (GnRH) produced by the hypothalamus. ³ In the absence of LH and FSH, androgen levels drop, and spermatogenesis stops. ¹²

Spermatogenesis begins with spermatogonia (the diploid (2n) immature sperm cells derived from embryonic germ cells) dividing by mitosis. ³ During their prolonged meiotic phase, the spermatocytes are sensitive to damage. ¹³ Some of the spermatogonia develop into primary spermatocytes.

At puberty, there is an increase in testosterone levels; this initiates meiosis I. During this stage, a primary spermatocyte generates two secondary spermatocytes, which then undergo meiosis II. Two haploid spermatids (haploid cells) are generated by each secondary spermatocyte, resulting in a total of four spermatids. Spermiogenesis is the final stage of spermatogenesis, and, during this phase, spermatids mature into spermatozoa (sperm cells) (Figure 2.5). ³

The spermiogenesis phase is completed with maturation of a spermatozoon. ¹² Spermatogenesis takes 65–75 days ³ and takes place simultaneously at different times in different regions of the testis for an even production and availability of mature sperm.

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Sperm Physiology and Assessment of Spermatogenesis Kinetics In Vivo

Sandro C. Esteves MD, PhD , Ricardo Miyaoska MD, PhD , in Handbook of Fertility, 2015

Conclusions

Spermatogenesis is a highly organized and complex sequence of differentiation events that yields genetically distinct male gametes for fertilization. Sperm production is a continuous process, initiated at puberty and continuing throughout life, which occurs in the seminiferous tubules within an immune privileged site. Spermatozoa released from the seminiferous tubules into the epididymis undergo post-testicular maturation. Before fertilization can occur, spermatozoa must undergo further biochemical changes via capacitation and acrosome reaction, both of which occur after ejaculation. Recent knowledge originated from a novel direct measurement of human spermatogenesis kinetics in vivo indicates that the entire sperm production process is shorter than previously believed. Based on this new method involving a stable isotope labeling with enriched heavy water and analysis of DNA isotopic enrichment in ejaculated sperm by gas chromatography/mass spectrometry, it has been also suggested that there is a large individual biological variability in the duration of spermatogenesis. This method may become a novel tool for characterizing the relationship between spermatogenesis and semen quality in male infertility, including the measurement of the effects of gonadotoxic exposure as well as medical and surgical interventions.

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Hormonal Control of Reproductionin the Male

H. Maurice Goodman , in Basic Medical Endocrinology (Fourth Edition), 2009

Leydig Cells and Seminiferous Tubules

The two principal functions of the testis, sperm production and steroid hormone synthesis, are carried out in morphologically distinct compartments. Sperm are formed and develop within seminiferous tubules, which comprise the bulk of testicular mass. Testosterone is produced by the interstitial cells of Leydig, which lie in clusters between the seminiferous tubules (Figure 12.1). The entire testis is encased in an inelastic fibrous capsule consisting of three layers of dense connective tissue and some smooth muscle.

Blood reaches the testes primarily through paired spermatic arteries and first is cooled by heat exchange with returning venous blood in the pampiniform plexus. This complex tangle of blood vessels is formed by highly tortuous and convoluted venules that surround and come in close apposition to the spermatic artery before converging to form the spermatic vein. This arrangement provides a large surface area for warm arterial blood to transfer heat to cooler venous blood across thin vascular walls. Rewarmed venous blood returns to the systemic circulation primarily through the internal spermatic veins.

Leydig cells are embedded in loose connective tissue that fills the spaces between semi-niferous tubules. They are large polyhedral cells with an extensive smooth endoplasmic reticulum characteristic of steroid-secreting cells. Although extensive at birth, Leydig cells virtually disappear after the first six months of postnatal life, only to reappear more than a decade later with the onset of puberty. In the adult, Leydig cells comprise 10 to 20% of testicular mass.

Seminiferous tubules are highly convoluted loops that range from about 120 to 300 µm in diameter and from 30 to 70 cm in length. They are arranged in lobules bounded by fibrous connective tissue. Each testis has hundreds of such tubules that are connected at both ends to the rete testis (Figure 12.2). It has been estimated that, if laid end to end, the seminiferous tubules of the human testis would extend more than 500 meters. The seminiferous epithelium that lines the tubules consists of three cell types: spermatogonia, which are stem cells; spermatocytes which are in the process of becoming sperm; and Sertoli cells, which nurture developing sperm and secrete a variety of products into the blood and the lumina of seminiferous tubules. Seminiferous tubules are surrounded by a several layers of peritubular myoid-epithelial cells, which are contractile and help propel the nonmotile sperm through the tubules toward the rete testis.

Spermatogenesis goes on continuously from puberty to senescence along the entire length of the seminiferous tubules. Though a continuous process, spermatogenesis can be divided into three discrete phases:

1: Mitotic divisions, which maintain a stem cell population of spermatogonia and provide the cells destined to become mature sperm.
2: Meiotic divisions, which reduce the chromosome number and produce a cluster of haploid spermatids.
3: Transformation of spermatids into mature spermatozoa (spermiogenesis), a process involving the loss of most of the cytoplasm and the development of flagella (Figure 12.3).

Figure 12.3. The formation of mammalian germ cells. Each primary spermatogonium ultimately gives rise to 64 sperm cells. Cytokinesis is incomplete in all but the earliest spermatogonial divisions, resulting in expanding clones of germ cells that remain joined by intercellular bridges. Maturing spermatids are closely associated with and enfolded by the Sertoli cells.

The fully formed spermatozoa are then released into the tubular lumina (spermiation). These events occur along thelength of the seminiferous tubules in a definite temporal and spatial pattern. A spermatogenic cycle includes all the transformations from spermatogonium to spermatozoan and requires about 64 days. As the cycle progresses, germ cells move from the basal portion of the germinal epithelium toward the lumen. Successive cycles begin before the previous one has been completed, so that different stages of the cycle are seen at any given point along a tubule at different depths of the epithelium. Spermatogenic cycles are synchronized in adjacent groups of cells, but the cycles are slightly advanced in similar groups of cells located immediately upstream, so that cells at any given stage of the spermatogenic cycle are spaced at regular intervals along the length of the tubules.

This complex series of events ensures that mature spermatozoa are produced continuously. About 2 million spermatogonia, each giving rise to 64 sperm cells, begin this process in each testis every day. More than 200 million spermatozoa are thus produced daily, or about 6 3 10¹⁴ in the six or more decades of reproductive life.

Sertoli cells are remarkable polyfunctional cells whose activities are intimately related to many aspects of the formation and maturation of spermatozoa. They extend through the entire thickness of the germinal epithelium from basement membrane to lumen and in the adult take on exceedingly irregular shapes determined by the changing conformations of the 10 to 12 developing sperm cells embedded in their cytoplasm (Figure 12.4). Differentiating sperm cells are isolated from the bloodstream and interstitial fluid, and must rely on Sertoli cells for their sustenance. Adjacent Sertoli cells arch above the clusters of spermatogonia that nestle between them at the level of the basement membrane. A series of tight junctions binds each Sertoli cell to the six adjacent Sertoli cells and limits passage of physiologically relevant molecules into or out of seminiferous tubules.

This so-called blood–testis barrier actually has selective permeability that allows rapid entry of testosterone, for example, but virtually completely excludes cholesterol. The physiological significance of the blood–testis barrier has not been established, but it is probably of some importance that spermatogonia are located on the blood side of the barrier, whereas developing spermatids are restricted to the luminal side. In addition to harboring and nurturing developing sperm, Sertoli cells secrete the watery fluid that transports spermatozoa through the seminiferous tubules and into the epididymis, where 99% of the fluid is reabsorbed. Sertoli cells also take up and degrade the residual bodies of cytoplasm shed by the developing spermatocytes.

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Roles of Small Ubiquitin-Related Modifiers in Male Reproductive Function

Margarita Vigodner , in International Review of Cell and Molecular Biology, 2011

Abstract

Spermatogenesis consists of the mitotic division of spermatogonia, meiosis of spermatocytes, and postmeiotic differentiation of spermatids, processes tightly controlled by hormones and growth factors secreted by testicular somatic cells. The events during spermatogenesis are precisely regulated by the sequential appearance of different proteins and their posttranslational modifications. Sumoylation (covalent modification by small ubiquitin-like modifiers; SUMO proteins) has emerged as an important regulatory mechanism in different cell types, and data obtained from studies on germ cells imply that SUMO proteins are involved in multiple aspects of spermatogenesis. Although progress has been made in the initial characterization of sumoylated proteins during spermatogenesis, the targets of sumoylation, their corresponding pathways in the testis, are mostly unknown. In this chapter, I review what we know about sumoylation in somatic cells, summarize the expression patterns, suggest possible functions of SUMO proteins in testicular cells, and discuss some difficulties and perspectives on the studies of sumoylation during spermatogenesis.

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Source: https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/spermatogenesis

Hayes Beamer

In the Male the Cells From Which Sperm Arise Are Produced Continuously Throughout Life

Spermatogenesis

Testicular Disorders

Maintenance of Spermatogenesis

Reproduction and Development

1.4.1 Introduction

Physiology and Disorders of Puberty

Spermatogenesis

Volume II

Seminiferous Tubules

Male Reproductive Physiology

Spermatogenesis

Testis Stem Cell Migration, Renewal, and Proliferation

Testis Stem Cell Migration

Human Male Spermatogenesis

Endocrine Control of Spermatogenesis

Anatomy and physiology of the reproductive system

Spermatogenesis and spermiogenesis

Sperm Physiology and Assessment of Spermatogenesis Kinetics In Vivo

Conclusions

Hormonal Control of Reproductionin the Male

Leydig Cells and Seminiferous Tubules

Roles of Small Ubiquitin-Related Modifiers in Male Reproductive Function

Abstract

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