Medical Physiology, 3rd Edition

Biology of Spermatogenesis and Semen

Spermatogenesis includes mitotic divisions of spermatogonia, meiotic divisions of spermatocytes to spermatids, and maturation to spermatozoa imageN54-7

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Definitions

Contributed by Emile Boulpaep, Walter Boron

The following are definitions of four related terms:

Spermatogenesis. As noted on page 1100, spermatogenesis includes the mitotic divisions of spermatogonia, the meiotic divisions of spermatogonia (diploid 2N DNA) to haploid spermatids (haploid 1N DNA), and the maturation to spermatozoa. This process includes both spermatocytogenesis and spermiogenesis.

Spermatocytogenesis. This term refers to the meiotic stages of spermatogenesis, in which spermatogonia (diploid 2N DNA) develop into primary spermatocytes (diploid 4N DNA), and then into secondary spermatocytes (haploid 2N DNA), and then finally into spermatids (haploid 1N DNA).

Spermiogenesis. This term refers to the maturation of spermatids (haploid 1N DNA) to mature spermatozoa. Spermiogenesis involves no cell division.

Spermiation. The release of the spermatozoa from the Sertoli cells is termed spermiation.

The primordial germ cells (see pp. 1076–1077) migrate into the gonad during embryogenesis. In males, these cells become the immature stem-cell population of the germinal cell line, or spermatogonia, and reside on the inner basal lamina of the seminiferous tubules (see Fig. 54-1E). Beginning at puberty and continuing thereafter throughout life, these spermatogonia divide mitotically (Fig. 54-7). The spermatogonia have the normal diploid complement of 46 chromosomes (2N): 22 pairs of autosomal chromosomes plus one X and one Y chromosome.

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FIGURE 54-7 Spermatogenesis. imageN54-12

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Spermatogenesis

Contributed by Ervin Jones

Early during embryogenesis, the primordial germ cells migrate to the gonad, where they become type A spermatogonia. Beginning at puberty, the spermatogonia undergo many rounds of mitotic division, with one daughter cell renewing the type A stem-cell population and the other generating type B spermatogonia. After many mitotic divisions, type B spermatogonia (2N DNA) enter the first meiotic division, at which time they are referred to as primary spermatocytes. During prophase, each primary spermatocyte has a full complement of duplicated chromosomes (4N DNA). Each primary spermatocyte divides into two secondary spermatocytes, each with a haploid number of duplicated chromosomes (2N DNA). The secondary spermatocyte enters the second meiotic division, producing two spermatids, each of which has a haploid number of unduplicated chromosomes (1N DNA). Further maturation of the spermatids yields the spermatozoa (mature sperm). Note that one primary spermatocyte yields four spermatozoa.

Spermatogonia can be classified into two types: A and B. Type A spermatogonia form the stem-cell population of male germ cells and divide by mitosis. One of the daughter cells renews the stock of type A spermatogonia and the other becomes a type B spermatogonium that undergoes several additional rounds of mitosis before its progeny initiate meiosis and progress through spermatogenesis (see Fig. 54-7). Type B spermatogonia that enter into the first meiotic division become primary spermatocytes. At prophase of meiosis I, the chromosomes undergo crossing over (see Fig. 53-2B). At this stage, each cell has a duplicated set of 46 chromosomes (4N): 22 pairs of duplicated autosomal chromosomes, a duplicated X chromosome, and a duplicated Y chromosome. After this first meiotic division is completed, the daughter cells become secondary spermatocytes, which have a haploid number of duplicated chromosomes (2N): 22 duplicated autosomal chromosomes and either a duplicated X or a duplicated Y chromosome. Secondary spermatocytes enter the second meiotic division almost immediately. This division results in smaller cells called spermatids, which have a haploid number of unduplicated chromosomes (1N). Spermatids form the inner layer of the epithelium and are found in rather discrete aggregates inasmuch as the cells derived from a single spermatogonium tend to remain together—with cytoplasm linked in a syncytium—and differentiate synchronously.

Spermatids transform into spermatozoa in a process called spermiogenesis,imageN54-7 which involves cytoplasmic reduction and differentiation of the body and the tailpieces. Thus, as maturation progresses, developing male gametes decrease in volume. Conversely, maturation leads to an increase in cell number, with each primary spermatocyte producing four spermatozoa, two with an X chromosome and two with a Y chromosome.

As additional generations of type B spermatogonia appear, the cells are displaced toward the lumen of the tubule. Thus, when viewed in cross sections of seminiferous tubules (see Fig. 54-1E), spermatogonia are located adjacent to the basement membrane, whereas the more differentiated spermatids are located nearest the lumen. Groups of spermatogonia at comparable stages of development undergo mitosis simultaneously. However, spermatogenesis is asynchronous along the seminiferous tubule; different areas along the tubule are at different stages of spermatogenesis. In a continuous process, a young man produces sperm at a rate of ~1000 per second.

Each stage of spermatogenesis has a specific duration. The time between formation of type B spermatogonia and differentiation of daughter cells into mature sperm is ~64 days. Each cell type has a characteristic life span: 16 to 18 days for type B spermatogonia, 23 days for primary spermatocytes, 1 day for secondary spermatocytes, and ~23 days for spermatids. The rate of spermatogenesis is constant and cannot be accelerated by hormones such as gonadotropins or androgens. Germ cells must move forward in their differentiation. If the environment is unfavorable and makes it impossible for them to pursue their differentiation at the normal rate, they undergo apoptosis (see p. 1241) and are eliminated in the semen. Healthy 20-year-old men produce sperm at a rate of ~6.5 million sperm per gram of testicular parenchyma per day. The rate falls progressively with age and averages ~3.8 million sperm per gram per day in men 50 to 90 years old. This decrease is probably related to the high rate of apoptosis of germ cells during meiotic prophase. Among fertile men, those aged 51 to 90 years exhibit a significant decrease in the percentage of morphologically normal and motile spermatozoa.

In summary, three processes occur concurrently in the seminiferous epithelium: (1) the production of cells by mitosis, (2) differentiation of spermatogonia to spermatids with a reduction in the number of chromosomes by meiosis, and (3) the conversion of spermatids to mature sperm by spermiogenesis. Thus, spermatogenesis is a regular, ordered, sequential process resulting in the production of mature male gametes.

It is instructive to consider how spermatogenesis in the male differs from oogenesis in the female. The two processes differ in each of the three steps just noted: (1) In the female, the mitotic proliferation of germ cells takes place entirely before birth. In the male, spermatogonia proliferate only after puberty and then throughout life. (2) The meiotic divisions of a primary oocyte in the female produce only one mature ovum with a large amount of cytoplasm and two to three polar bodies (see p. 1073). In the male, the meiotic divisions of a primary spermatocyte produce four mature spermatozoa with a minimal amount of cytoplasm. imageN53-8 (3) In the female, the second meiotic division is completed only on fertilization (see p. 1131) and thus no further development of the cell takes place after the completion of meiosis. In the male, the products of meiosis (the spermatids) undergo substantial further differentiation to produce mature spermatozoa.

The Sertoli cells support spermatogenesis

Sertoli cells (also called sustentacular cells) are generally regarded as support or “nurse” cells for the spermatids (Fig. 54-8). Sertoli cells (1) maintain an environment conducive for spermatogenesis; (2) in response to FSH, secrete substances that promote proliferation of spermatogonia and initiate meiosis; (3) secrete androgen-binding protein, which concentrates testosterone in the proximity of developing gametes; (4) secrete inhibin, which controls pituitary gland production of FSH; (5) phagocytose excess cytoplasm produced by gametes during spermiogenesis; and (6) produce antimüllerian hormone, which represses formation of derivatives of the müllerian duct (see p. 1080).

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FIGURE 54-8 Interaction of Sertoli cells and sperm. This figure is an idealized high-magnification view of a portion of the wall of a seminiferous tubule (see Fig. 54-1C). A single Sertoli cell spans from the basal lamina to the lumen of the seminiferous tubule. Adjacent Sertoli cells are connected by tight junctions and surround developing germ cells. From the basal lamina to the lumen of the tubule, there is a gradual maturation of the germ cells.

The Sertoli cells are large polyhedral cells extending from the basement membrane toward the lumen of the seminiferous tubule. Spermatids are located adjacent to the lumen of the seminiferous tubules during the early stages of spermiogenesis and are surrounded by processes of Sertoli cell cytoplasm. Tight junctions connect the adjacent Sertoli cells, forming a blood-testis barrier—analogous to the blood-brain barrier in both sexes (see pp. 284–287) and to the blood-oocyte/granulosa barrier in the ovarian follicle of females—that presumably provides a protective environment for developing germ cells. In addition, gap junctions between the Sertoli cells and developing spermatozoa may represent a mechanism for transferring material between these two types of cells. Release of the spermatozoa from the Sertoli cell is called spermiation. Spermatids progressively move toward the lumen of the tubule and eventually lose all contact with the Sertoli cell after spermiation (Box 54-2).

Box 54-2

Sertoli Cell–Only Syndrome

Clinicians have described a group of normally virilized men whose testes are small bilaterally and whose ejaculates contain no sperm cells (azoospermia). In these men, the seminiferous tubules are lined by Sertoli cells but show a complete absence of germ cells. Sertoli cell–only syndrome (or germinal cell aplasia) accounts for 10% to 30% of male infertility secondary to azoospermia and can either be caused by a single-gene defect or be acquired (e.g., as a result of orchitis, alcoholism, or toxic agents). Leydig cell function is usually preserved. Plasma testosterone and LH levels are usually normal, whereas FSH levels are often, but not always, elevated. It is not entirely clear why FSH levels are elevated in these men. This elevation may result from the absence of germ cells or from suboptimal secretion of inhibin by the Sertoli cells, inasmuch as inhibin is a powerful inhibitor of FSH secretion at the level of the anterior pituitary gland. Segments of Sertoli cell–only tubules may be observed in conditions such as orchitis or after exposure to agents that are toxic to the gonads. However, these individuals generally have functional spermatogenesis in the other seminiferous tubules.

Sperm maturation occurs in the epididymis

The seminiferous tubules open into a network of tubules, the rete testis, that serves as a reservoir for sperm. The rete testis is connected to the epididymis via the efferent ductules (see p. 1078), which are located near the superior pole of the testicle. The epididymis is a highly convoluted single long duct, 4 to 5 m in total length, on the posterior aspect of the testis. The epididymis can be divided anatomically into three regions: the head (the segment closest to the testis), the body, and the tail (see Fig. 54-1C).

Spermatozoa are essentially immotile on completion of spermiogenesis. Thus, transfer of spermatozoa from the seminiferous tubule to the rete testes is passive. Secretions flow from the testes through the epididymis with assistance by ciliary action of the luminal epithelium and contraction of the smooth-muscle elements of the efferent duct wall. Thus, sperm transport through this ductal system is also primarily passive. After leaving the testes, sperm take 12 to 26 days to travel through the epididymis and appear in the ejaculate. The epididymal transit time for men between the ages of 20 and 80 years does not differ significantly.

Sperm are stored in the epididymis, where they undergo a process of maturation before they are capable of progressive motility and fertilization (Table 54-2). Spermatozoa released at ejaculation are fully motile and capable of fertilization, whereas spermatozoa obtained directly from the testis are functionally immature insofar as they cannot penetrate an ovum. However, these immature spermatozoa can fertilize if they are injected into an ovum. During maturation in the epididymis, spermatozoa undergo changes in motility, metabolism, and morphology. imageN54-8 Spermatozoa derived from the head (caput) of the epididymis (see Fig. 54-1C) are often unable to fertilize ova, whereas larger proportions of spermatozoa captured from the body (corpus) are fertile. Spermatozoa obtained from the tail (cauda) of the epididymis or from the vas deferens are almost always capable of fertilization.

TABLE 54-2

Aspects of Sperm Maturation in the Epididymis

Progressive increase in forward motility

Increased ability to fertilize

Maturation of acrosome

Molecular reorganization of the plasma membrane:

Lipids (stabilization of plasma membrane)

Proteins (shedding as well as acquisition of new proteins)

Ability to bind to zona pellucida

Acquisition of receptors for proteins of the zona pellucida

Increased disulfide bonds between cysteine residues in sperm nucleoproteins

Topographic regionalization of glycosidic residues

Accumulation of mannosylated residues on the periacrosomal plasma membrane

Decreased cytoplasm and cell volume

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Sperm Maturation

Contributed by Ervin Jones

Most of the changes that occur to the sperm within the epididymis have to do with the acquisition of motility. Sperm isolated from the caput of the epididymis exhibit inconsistent patterns of motility, ranging from immotility to highly random movements of the flagellum. As sperm pass through the epididymis, rapid forward progression occurs and random tail flexing is reduced; these changes are first seen in a few spermatozoa obtained from the corpus and comprise the predominant pattern in those obtained from the cauda of the vas deferens. Motility is a prerequisite for fertility; it may be surmised that the acquisition of progressive forward motility is largely a function of maturation of the spermatozoa.

Biochemical changes also occur as sperm pass through the epididymis. It has been suggested that membrane-bound enzymes play a role in modifying the surface of spermatozoa during epididymal transit. Total protein concentrations decrease by more than half in spermatozoa but remain unchanged in epididymal fluid during epididymal transit. Further, it appears that the majority of the testicle-derived proteins are removed from the spermatozoal surface during epididymal maturation, to be replaced by new low-molecular-weight peptides. These new peptides appear almost simultaneously with the development of forward-progressive motility.

Nuclear changes also occur as spermatozoa traverse the epididymis. Nuclear proteins play a role in morphogenesis and in the stabilization of the head of the sperm cell. Protamine staining increases as spermatozoa pass from the rete testes to the corpus of the epididymis and sharply declines thereafter. These observations, taken together, underscore the significance of the epididymis for sperm maturation, a prerequisite for the acquisition of fertilizing capacity.

The epididymis empties into the vas deferens, which is responsible for the movement of sperm along the tract. The vas deferens contains well-developed muscle layers that facilitate sperm movement. The vas deferens passes through the inguinal canal, traverses the ureter, and continues medially to the posterior and inferior aspect of the urinary bladder, where it is joined by the duct arising from the seminal vesicle; together, they form the ejaculatory duct. The ejaculatory duct enters the prostatic portion of the urethra after passing through the prostate. Sperm are stored in the epididymis as well as in the proximal end of the vas deferens. All these accessory structures depend on androgens secreted by the testis for full functional development.

Spermatozoa are the only independently motile cells in the human body

Spermatozoa are highly specialized cells consisting of a head, midpiece, and tail (Fig. 54-9). The head contains the nucleus and is essentially devoid of cytoplasm, which is lost during spermatogenesis. Within the nucleus, the haploid number of chromosomes are packaged as tightly coiled chromatin fibers. The anterior segment of the head contains the acrosome, which lies in front of and around the anterior two thirds of the sperm nucleus, much like a motorcycle helmet fits over one's head. The acrosome is essentially a large secretory vesicle, originating from the Golgi complex in the spermatid and containing enzymes that will facilitate penetration of the ovum. The midpiece is the engine of sperm. It consists of multiple mitochondria arranged as spirals around a filamentous core. The energy for sperm motility comes from the metabolism of fructose present in the seminal fluid. The filamentous core in the midpiece forms a drive-shaft connection to the tail. The tail is a specialized flagellum, powered by the midpiece, that executes rapid lashing movements, propelling the cell forward.

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FIGURE 54-9 Anatomy of a spermatozoon.

The accessory male sex glands—the seminal vesicles, prostate, and bulbourethral glands—produce the seminal plasma

Only 10% of the volume of semen (i.e., seminal fluid) is sperm cells. The normal concentration of sperm cells is >20 million per milliliter and the typical ejaculate volume is >2 mL. The typical ejaculate content varies between 150 and 600 million spermatozoa.

Aside from the sperm cells, the remainder of the semen (i.e., 90%) is seminal plasma, the extracellular fluid of semen (Table 54-3). Very little seminal plasma accompanies the spermatozoa as they move through the testes and epididymis. The seminal plasma originates primarily from the accessory glands (the seminal vesicles, prostate gland, and bulbourethral glands). The seminal vesicles contribute ~70% of the volume of semen. Aside from the sperm, the remaining ~20% represents epididymal fluids, as well as secretions of the prostate gland and bulbourethral glands. However, the composition of the fluid exiting the urethral meatus during ejaculation is not uniform. The first fluid to exit is a mixture of prostatic secretions and spermatozoa with epididymal fluid. Subsequent emissions are composed mainly of secretions derived from the seminal vesicles. The first portion of the ejaculate contains the highest density of sperm; it also usually contains a higher percentage of motile sperm cells.

TABLE 54-3

Normal Parameter Values for Semen

PARAMETER

VALUE

Volume of ejaculate

2–6 mL

Viscosity

Liquefaction in 1 hr

pH

7–8

Count

≥20 million/mL

Motility

≥50%

Morphology

60% normal

Seminal plasma is isotonic. The pH in the lumen of the epididymis is relatively acidic (6.5 to 6.8) as the result of H+ secretion by clear cells that are analogous to intercalated cells in the nephron (see p. 729). Addition of the relatively alkaline secretions of the seminal vesicles raises the final pH of seminal plasma to between 7.3 and 7.7. Spermatozoa generally tolerate alkalinity better than acidity. A pH near neutrality or slightly higher is optimal for the motility and survival of sperm cells in humans and in other species.

Seminal plasma contains a plethora of sugars and ions. Fructose and citric acid are contributed to the seminal plasma by the accessory glands, and their concentrations vary with the volume of semen ejaculated. The fructose is produced in the seminal vesicles. In a man with oligospermia (i.e., a low daily sperm output) and a low ejaculate volume (recall that more than half of the ejaculate comes from the seminal vesicles), the absence of fructose suggests obstruction or atresia of the seminal vesicles. Ascorbic acid and traces of B vitamins are also found in human seminal plasma. The prostate gland releases a factor—which contains sugars, sulfate, and a vitamin E derivative—that acts to prevent the clumping of sperm heads. In addition, human semen also contains high concentrations of choline and spermine, and is also rich in Ca2+, Na+, Mg2+, K+, Cl, Zn2+, and phosphate. Concentrations of Zn2+ and Ca2+ are higher in semen than in any other fluid and most other tissues. Calcium ions stimulate the motility of immature epididymal spermatozoa, but they inhibit the motility of spermatozoa in ejaculates obtained from humans. It appears that the diminished response of sperm to Ca2+ and the acquisition of progressive motility are functions of epididymal maturation.

Semen also contains free amino acids, low-molecular-weight polypeptides, and proteins. The free amino acids, which probably arise from the breakdown of protein after the semen is ejaculated, may protect spermatozoa by binding toxic heavy metals or by preventing the agglutination of proteins.

Human semen coagulates immediately after ejaculation. Coagulation is followed by liquefaction, which is apparently caused by proteolytic enzymes contained in prostatic secretions. Prostatic secretion is rich in acid phosphatase. The natural substrate for acid phosphatase is phosphorylcholine, which is contributed by the seminal vesicles. Hyaluronidase is also present in human semen, although its functional role remains to be clarified. Hyaluronidase is not a product of the accessory glands; rather, it is contained within the sperm cell cytoplasm and is rapidly released into the seminal plasma. Hyaluronidase may play a role in facilitating penetration of the oocyte by the sperm cell because of its ability to depolymerize hyaluronic acid. imageN54-9

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Congenital and Acquired Ductal Obstruction

Contributed by Ervin Jones

Genital duct obstruction may be congenital and may result from ductal absence or structural abnormality, or it may be acquired as a result of stricture, infection, or vasectomy. Genital duct obstruction is found in ~7% of infertile men. An uncommon cause of male infertility is congenital absence of the vas deferens, which accounts for as many as 50% of cases of congenital ductal obstruction. These patients generally have azoospermic ejaculates with low volume. Congenital absence of the vas deferens is common in male patients with cystic fibrosis (CF) and is sometimes the only manifestation of CF.

Epididymal abnormalities range from the presence of an incomplete epididymis to the presence of only small portions of the epididymis; in addition, the seminal vesicles are often absent. Spermatogenesis is thought to be normal inasmuch as testicular biopsy specimens demonstrate germ cells in several stages of development. Obstruction of the epididymis may also occur as a result of gonococcal or tuberculous epididymitis. Smallpox and filariasis are common causes of ductal obstruction in areas where these diseases are endemic. Inspissated secretions may occlude the epididymis in men with Young syndrome or CF.

Elective vasectomy, a simple surgical procedure in which a small segment of the vas deferens is removed to ensure male infertility, is currently the leading cause of ductal obstruction.

Azoospermia in men with normal testes is the hallmark of genital duct obstruction. However, when specimens of testicles from men who have had vasectomies are examined microscopically, interstitial fibrosis is found in as many as 20% of cases. This group exhibits low fertility after elective reversal of their vasectomy. When the seminiferous tubules are examined, increased thickness of the tubule wall, an increase in cross-sectional tubular area, and decreased numbers of Sertoli cells are usually noted. Testosterone and gonadotropin levels are normal in most patients with ductal obstruction.