20 SPERMATOGENESIS

The male reproductive system is responsible for (1) the continuous production, nourishment, and temporary storage of the haploid male gamete (spermatozoa [sing. spermatozoon], or sperm); and (2) the synthesis and secretion of male sex hormones (androgens).

The male reproductive system consists of (1) the testes, which produce sperm and synthesize and secrete androgens; (2) the epididymis, vas deferens, ejaculatory duct, and a segment of the male urethra, which form the excurrent duct system responsible for the transport of spermatozoa to the exterior; (3) accessory glands, the seminal vesicle, the prostate gland, and the bulbourethral glands of Cowper, whose secretions form the bulk of the semen and provide nutrients to ejaculated spermatozoa; and (4) the penis, the copulatory organ, formed of erectile tissue.

The testes, epididymis, and the initial part of the vas deferens are located in the scrotal sac, a skin-covered pouch enclosing a mesothelium-lined cavity—the tunica vaginalis.

THE TESTES

The testes are paired organs located in the scrotum, outside the abdominal cavity. This location enables maintenance of the testes at a temperature 2°C to 3°C below body temperature. A temperature of 34°C to 35°C is essential for normal spermatogenesis.

The posterior surface of the mature testes is associated with the epididymis. Both testes and epididymis are suspended in the scrotal sac by the spermatic cord, which contains the vas deferens, the spermatic artery, and the venous and lymphatic plexuses.

The testes is enclosed by the tunica albuginea, which is thickened to form the mediastinum where the rete testis is located (Figure 20-1). Fibrous septa from the mediastinum project into the testicular mass, dividing the tissue into 250 to 300 lobules. Each lobule contains one to four seminiferous tubules.

Figure 20-1 Testes, epididymis, and vas deferens

Each seminiferous tubule is about 150 μm in diameter and 80 cm long; it is U-shaped with the two ends opening in the rete testis. The rete testis is a network of channels that collects the products of the seminiferous epithelium (testicular sperm, secretory proteins, and ions).

The seminiferous tubule (Figure 20-2) consists of a central lumen lined by a specialized seminiferous epithelium containing two distinct cell populations: (1) the somatic Sertoli cells and (2) the spermatogenic cells (spermatogonia, spermatocytes, and spermatids).

Figure 20-2 General organization of the seminiferous tubules

The seminiferous epithelium is encircled by a basement membrane and a wall formed by collagenous fibers, fibroblasts, and contractile myoid cells. Myoid cells are responsible for the rhythmic contractile activity that propels the nonmotile sperm to the rete testis. Sperm acquire forward motility after they have passed through the epididymal duct.

The space in between the seminiferous tubules is occupied by abundant blood vessels (arterioles, capillaries and venules) and lymphatic channels encircling each seminiferous tubule, and aggregates of the androgen-producing Leydig cells in close proximity to the lymphatic and blood circulation (see Figure 20-2). The general histologic structure of the testes is shown in Figure 20-3.

Figure 20-3 General histologie structure of the testes

Seminiferous epithelium

The seminiferous epithelium can be classified as a stratified epithelium with rather unusual characteristics not found in any stratified epithelium of the body. In this stratified epithelium, somatic columnar Sertoli cells interact with mitotically dividing spermatogonia, meiotically dividing spermatocytes, and a haploid population of spermatids undergoing a differentiation process called spermiogenesis.

Figure 20-4 illustrates relevant aspects of a mammalian spermatogenic cycle.

1. A prespermatogonium (also called gonocyte), derived from a primordial germinal cell in the fetal testes, divides by mitosis at puberty to generate two daughter cells. One daughter cell initiates a spermatogenic cycle. The other cell becomes a stem cell with self-renewal capacity and able to initiate soon another spermatogenic cycle. We have seen in Chapter 3, Cell Signaling, that stem cells can self-renew and give rise to either another stem cell or a cell entering a terminal differentiation pathway. The same rule applies to prespermatogonia.

2. After cell division, all spermatogenic cells remain interconnected by intercellular bridges because cytokinesis is incomplete.

3. Spermatogonia, spermatocytes, and spermatids complete their proliferation and differentiation sequence in a timely manner. The spermatogenic cell cohorts proliferate and differentiate synchronously.

4. Stem cells periodically initiate spermatogenic cell cycles to ensure the continuous production of sperm. We will study later how spermatogenic cell cycles overlap in a segment of a seminiferous tubule and generate constant combinations of spermatogenic cells called cellular associations.

5. Sertoli cells represent a stable somatic cell population. They facilitate the displacement of differentiating spermatogenic cells farther away from the periphery of the seminiferous tubule and closer to the lumen.

Figure 20-4 Outline of the spermatogenic cycle

Sertoli cells

Sertoli cells are the predominant cell type of the seminiferous epithelium until puberty. After puberty, they represent about 10% of the cells lining the seminiferous tubules. In elderly men, when the population of spermatogenic cells decreases, Sertoli cells again become the major component of the epithelium.

Sertoli cells are columnar cells extending from the basal lamina to the lumen of the seminiferous tubule (Figure 20-5). They act as bridge cells between the intertubular space and the lumen of the seminiferous tubule.

Figure 20-5 Two compartments of the seminiferous epithelium

The apical and lateral plasma membranes of Sertoli cells have an irregular outline because they provide crypts to house the developing spermatogenic cells.

The nucleus displays indentations and a large nucleolus with associated heterochromatin masses. The cytoplasm contains smooth and rough endoplasmic reticulum, mitochondria, lysosomes, lipid droplets, an extensive Golgi apparatus, and a rich cytoskeleton (vimentin, actin, and microtubules).

At their basolateral domain, Sertoli cells form tight junctions with adjoining Sertoli cells.

Basolateral tight junctions (1) subdivide the seminiferous epithelium into a basal compartment and an adluminal compartment (see Figure 20-5) and (2) are the determining components of the so-called blood-testes barrier, which protects developing spermatocytes and spermatids from autoimmune reactions.

The functions of Sertoli cells are (1) to support, protect, and nourish developing spermatogenic cells; (2) to eliminate by phagocytosis excess cell portions, called residual bodies, discarded by spermatids at the end of spermiogenesis; (3) to facilitate the release of mature spermatids into the lumen of the seminiferous tubule by actin-mediated contraction, a process called spermiation; and (4) to secrete a fluid rich in proteins and ions into the seminiferous tubular lumen.

Sertoli cells respond to follicle-stimulating hormone (FSH) stimulation. FSH regulates the synthesis and secretion of androgen-binding protein (ABP).

ABP is a secretory protein with high binding affinity for the androgens testosterone and dihydrotestosterone. The androgen-ABP complex, whose function is unknown at present, is transported to the proximal segments of the epididymis (see Figure 20-16).

Note that although both ABP and the androgen receptor have binding affinity for androgens, they are distinct proteins. ABP is a secretory protein, whereas the androgen receptor is a cytoplasmic and nuclear protein.

Sertoli cells secrete inhibin and activin subunits (α and β subunits). Inhibin (an αβ heterodimer) exerts a negative feedback on gonadotropin-releasing factor and FSH release by the hypothalamus and anterior hypophysis. Activin (an αα or ββ homodimer) exerts a positive feedback on the release of FSH (see Chapter 18, Neuroendocrine System).

Sertoli cells are postmitotic after puberty. No mitotic cell division is observed in the adult testes.

Spermatogonia

Spermatogonia are diploid spermatogenic cells directly in contact with the basal lamina in the basal compartment (Figures 20-5 to 20-7). They are located below the inter–Sertoli cell occluding junctions and therefore outside the blood-testes barrier.

Figure 20-6 Identification of seminiferous epithelial cells

Figure 20-7 Human seminiferous epithelium

Spermatogonia derive from spermatogonial stem cells and undergo successive rounds of mitotic cell divisions starting at puberty.

Two major morphologic spermatogonial cell types can be observed:

1. Type A spermatogonia display an oval euchromatic nucleus and a nucleolus attached to the nuclear envelope (see Figures 20-5 and 20-7). Subclasses of type A spermatogonia (with a dark nucleus, called A dark spermatogonium, and with a lighter nucleus, called A pale spermatogonium) are observed in human testes.

2. Type B spermatogonia have a round nucleus, masses of heterochromatin attached to the nuclear envelope, and a central nucleolus (see Figure 20-5).

Spermatogonial stem cells have important implications for male fertility. They are relatively quiescent and therefore resistant to radiation and cancer chemotherapy. Mitotically dividing spermatogonia, meiotically dividing spermatocytes, and differentiating spermatids are sensitive to radiation and cancer chemotherapy. After cessation of radiotherapy or anticancer chemotherapy, spermatogonial stem cells can reestablish the spermatogenic process. Postmitotic Sertoli cells are highly resistant to these therapies.

Spermatocytes

Type B spermatogonia enter meiotic prophase immediately after completing the last S phase (DNA synthesis). This last round of major DNA synthetic activity in the lifetime of spermatogenic cells determines that a primary spermatocyte starting meiotic prophase I will have two times the amount of DNA of a spermatogonium. The primary spermatocyte has a 4C DNA value, where 1C equals about 1.5 pg of DNA per cell.

Spermatocytes divide by two successive meiotic cell divisions (Figure 20-8) and are located in the adluminal compartment of the seminiferous epithelium, just above the inter—Sertoli cell occluding junctions. Therefore, meiosis occurs inside the blood-testes barrier.

Figure 20-8 Meiosis in the male

A primary spermatocyte undergoes the first meiotic division (or reductional division) without significant DNA synthesis (only repair DNA synthesis occurs) to produce two secondary spermatocytes. The secondary spermatocytes rapidly undergo the second meiotic division (or equational division). Each secondary spermatocyte forms two spermatids that mature without further cell division into sperm.

By the end of the first meiotic division, the original 4C DNA content of a primary spermatocyte is reduced to 2C in a secondary spermatocyte. By the end of the second meiotic division, the 2C DNA content is reduced to 1C. The resulting spermatids are the haploid spermatids and initiate a complex differentiation process called spermiogenesis.

Because the first meiotic division is a long process (days) and the second meiotic division is very short (minutes), primary spermatocytes are the most abundant cells observed in the seminiferous epithelium. For comparison, Figure 20-9 illustrates the meiotic process of the female gamete that is initiated in the ovary during fetal development (see Chapter 23, Fertilization, Placentation, and Lactation).

Figure 20-9 Meiosis in the female

Meiosis

After the last mitotic division of type B spermatogonia, the resulting daughter cells synthesize DNA (S phase), advance into the G₂ phase, and start the first meiotic division with a 4C DNA content. The first meiotic division is characterized by a long prophase, lasting about 10 days.

The prophase substages of the first meiotic division are the leptotene (threadlike), zygotene (pairing), pachytene (thickening), diplotene (appearing double), and diakinesis (moving apart) stages (Figures 20-10 and 20-11).

Figure 20-10 First meiotic division (prophase stage): From leptotene to pachytene

Figure 20-11 First meiotic division (prophase stage): From diplotene to diakinesis

These substages are characterized by four major events: (1) the formation of a synaptonemal complex (see Box 20-A) during zygotene-pachytene to facilitate the pairing or synapsis of homologous chromosomes (autosomes and sex chromosomes X and Y); (2) the pairing of homologous chromosomes (synapsis); (3) crossing over (the exchange of genetic information between non-sister chromatids of homologous chromosomes); and (4) disjunction (the separation of paired homologous chromosomes).

Box 20-A Synaptonemal complex

• The function of the synaptonemal complex is to facilitate the synapsis of homologous chromosomes by stabilizing their axial alignment and association.

• Sister chromatids are maintained in close contact by the cohesin protein complex.

• The separation between synapsed homologous chromosomes is 100 nm.

• A synaptonemal complex consists of two lateral elements (closely associated to chromosomal chromatin loops) and a central element.

• The lateral elements are formed by the cohesin protein complex (Rec8, SCM1, and SCM3 proteins), SCP2, and SCP3 (SCP stands for synaptonemal complex protein).

• The lateral elements are bridged by transverse fibrous SCP1 dimers, whose terminal globular regions overlap in the center of the synaptonemal complex to form the central element.

• Recombination nodules are present along the synaptonemal complex during pachytene. They represent sites where genetic recombination between nonsister chromatids (called reciprocal exchange) will occur.

After this prolonged prophase, pairs of sister chromatids pass through metaphase, anaphase, and telophase and are separated into daughter cells—the secondary spermatocytes.

During the second meiotic division, prophase, metaphase, anaphase, and telophase separate sister chromatids into daughter cells—the spermatids.

In the female (see Figure 20-9), a primary oocyte (with a 4C DNA content) completes the first meiotic division at ovulation to produce a secondary oocyte (2C DNA content) and the first polar body. When fertilization occurs, the secondary oocyte completes the second meiotic division to reach the haploid state (1C DNA content), and a second polar body is generated.

The three most important consequences of meiosis are: (1) Sperm and oocytes contain only one representative of each homologous pair of chromosomes. (2) Maternal and paternal chromosomes are randomly assorted. (3) Crossing over increases genetic variation.