Anatomical And Morphological Relationships Of The Male Reproductive System

Ejaculation By Command

Curing Premature Ejaculation Permanently

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A. Introduction

The male reproductive system comprises the gonads (two testes), excretory ducts (epididymis, vas deferens, and ejaculatory duct), and several accessory structures (prostate, seminal vesicles, bulbourethral glands, and penis). These are illustrated in Figure 12-1.

The key steps of organogenesis of the sexually "indifferent," yet chromosomally determined male gonad are discussed in Chapter 14. The presentation of this chapter will begin with the generation of the morphologically identifiable testes.

B. Testes

The male gonads consist of two testes contained in the scrotum or pouch, which is suspended outside the abdominal cavity between the thighs. The scrotum, besides housing the testes, plays an important role in temperature regulation necessary for normal testicular function. The formation of the male sex cells optimally occurs only at the scrotal temperature of 32.5°C. A key developmental step in the male, prior to birth, is the translocation of the gonads from the abdominal cavity into the scrotum; this process is androgen dependent. Failure of the testes to descend into the scrotum is termed cryptorchidism and results in sterility.

Each adult human testis is 4-5 cm long and is an ellipsoid of 35-45 g. Structurally each testis consists of the parenchyma or seminiferous tubules, which are surrounded by a capsule composed of three layers (see Figure 12-2).

The tunica albuginea divides the parenchymal seminiferous tubules into discrete lobules. The seminiferous tubules are the site of production of the male sex cells, the spermatozoa, and have a total length of ~800 m (one-half mile). The seminiferous tubules are organized in a highly convoluted irregular pattern, with some blind pouches, ultimately terminating in the tu-bulus recti, which in turn empties into the rete testis and the epididymal duct.

The seminiferous tubule walls are composed of germinal epithelial cells and Sertoli cells, which are the site of the spermatogenesis process (discussed in Section II.E). The newly formed spermatozoa are transported through the lumen of the seminiferous tubules, where they empty into a highly convoluted network of ducts known as the rete testis. Next, the spermatozoa are transported, as a consequence of ciliary motion and testicular fluid pressure, through the efferent ductules to the epididymis, where they are stored.

The walls of the seminiferous tubules have a complex structure in that there is an array of many layers of epithelial germ cells interspersed with the Sertoli or "nurse" cells. The specialized Sertoli cells (see Figure 12-3) line the basement membrane of the seminiferous tubules; their cytoplasm is in close contact with the innermost layer of the basement membrane, which in a convoluted fashion frequently surrounds the germ cells as they undergo their maturation process.

The Sertoli-Sertoli cell tight junctions (see Figure 12-3) tend to form a blood-testis barrier (analogous to the blood-brain barrier) that divides the germinal epithelium into an adluminal and a basal compartment. As shown in Figure 12-3, the basal compartment contains the immature spermatozoa or spermatogonia, and the adluminal compartment contains the remainder of the germ cells. An important function of the blood-testis barrier is to prevent the back-diffusion of autoantigens from the interior of the tubule to the bloodstream. The spermatozoa which carry surface antigens can stimulate an antibody response even in the host male. Thus, the Sertoli-Sertoli membrane barrier is an important protective mechanism. While the detailed functions of the Sertoli cells in the adult male are not known, it is believed that they perform both a nutritive and a phagocytic role, being responsible for the clearing of damaged germ cells from the seminifer

Deatiled Male Reproductive Organ

Scrotum figure 12-1 Male reproductive organs (midsagittal view).

Scrotum figure 12-1 Male reproductive organs (midsagittal view).

ous epithelium. It has been demonstrated that FSH can stimulate the Sertoli cells to secrete an androgen-binding protein (ABP) into the seminiferous lumen.

As discussed in Chapter 14, in the male fetus the Sertoli cell secretes the Miillerian Inhibitory Factor (MIF), which acts to induce involution of the Miillerian ducts and also to block the development of the uterus and fallopian tubes. Thus, the Sertoli cell has two major categories of biological action, depending upon whether it is operative in a fetal (production of MIF) or adult testis (gamete production).

figure 12-2 Cross-sectional schematic diagram of the testis and epididymis. Note the communication between the testicular lobules and the convoluted seminferous tubules inside the testicular lobules. Reproduced with permission from Junqueira, L. C. Carneiro, J. and Kelley, R. O. (1991). "Basic Histology" 7th ed., p. 392. Appleton & Lange, Norwalk, CT.

The interstitial tissue (see Figure 12-3), which makes up 5% of the total testicular volume, contains the primary endocrine cells of the testes, the Leydig cells. Leydig cells under the action of LH are the major site of production of testosterone. The Leydig cells are located in clusters that lie in the connecting tissue stroma between the seminiferous tubules. The histology of a Leydig cell is comparable to that of other steroid-secreting cells, particularly the lutein cells of the corpus luteum (see Figure 13-3).

C. Duct System

The duct system connects each testis to the urethra and functions to transport the mature spermatozoa during ejaculation. It is composed of the epididymis, ductus deferens, and ejaculatory duct.

The epididymis is a small narrow structure attached to the surface of the testis. Approximately 12 days are required for the transit of the spermatozoa through the epididymis; during this interval they undergo the morphological and functional changes necessary to provide them with the capacity to fertilize an ovum. The mature spermatozoa then enter the vas deferens.

The vas deferens is a 7- to 8-cm tubule connecting the epididymis with the ejaculatory duct (see Figure 12-1). The passage of sperm through the vas deferens is accomplished by peristaltic contractions of smooth muscle in the duct wall. Vasectomy, which is the bilateral ligation of the vas deferens, has been established as an effective and safe male contraceptive procedure. However, the current success rate for the reversal of vasectomy (as judged by the subsequent production

Seminiferous tubuie

Testicular lobule

Tunica albugínea

Testicular Lobule

Ductus deferens

Ductuli efferentes

Ductus epididymis

Rete testis Tubulus rectus figure 12-2 Cross-sectional schematic diagram of the testis and epididymis. Note the communication between the testicular lobules and the convoluted seminferous tubules inside the testicular lobules. Reproduced with permission from Junqueira, L. C. Carneiro, J. and Kelley, R. O. (1991). "Basic Histology" 7th ed., p. 392. Appleton & Lange, Norwalk, CT.

Ductus deferens

Seminiferous tubuie

Testicular lobule

Ductuli efferentes

Ductus epididymis

Tunica albugínea

Rete testis Tubulus rectus

Primary spermatocyte

Spermatozoa

Adiuminal side (lumen)

Primary spermatocyte

Adiuminal side (lumen)

Testicular Lobule

Fibroblast

Basal side (blood)

Myoid Interstitial ce|| (Leydig) cell

Capillary

FIGURE 12-3 Schematic diagram of the wall of a seminiferous tubule and adjacent interstitial tissue. The tight junctions between the walls of adjacent Sertoli cells form a blood-testis barrier that prevents large molecules from passing into the lumen of the seminiferous tubules. Note that the germ cells are surrounded by two adjacent Sertoli cells, and that as the germ cells mature, they move from the basement membrane side (spermatogonium) to the adiuminal side (early spermatids). The interstitial or Leydig cells are the source of testosterone. Reproduced with permission from Junqueira, L. C., Carneiro, J., and Kelley, R. O. (1991). "Basic Histology", 7th ed., p. 392. Appleton & Lange, Norwalk, CT.

Secondary spermatocyte

Mitosis Blood-testis barrier Spermatogonium

Basement membrane

Fibroblast

Basal side (blood)

Myoid Interstitial ce|| (Leydig) cell

Capillary

FIGURE 12-3 Schematic diagram of the wall of a seminiferous tubule and adjacent interstitial tissue. The tight junctions between the walls of adjacent Sertoli cells form a blood-testis barrier that prevents large molecules from passing into the lumen of the seminiferous tubules. Note that the germ cells are surrounded by two adjacent Sertoli cells, and that as the germ cells mature, they move from the basement membrane side (spermatogonium) to the adiuminal side (early spermatids). The interstitial or Leydig cells are the source of testosterone. Reproduced with permission from Junqueira, L. C., Carneiro, J., and Kelley, R. O. (1991). "Basic Histology", 7th ed., p. 392. Appleton & Lange, Norwalk, CT.

of pregnancy) is only 50%. Also, approximately half of the men who elect to undergo a vasectomy develop antibodies against their own spermatozoa.

The ejaculatory duct is located at the merger of the vas deferens and the seminal vesicles and extends through the prostate to the urethra.

D. Accessory Structures

The accessory structures include the seminal vesicles, prostate gland, bulbourethral glands, and penis.

In adult males, the two seminal vesicles are 1020 cm in length and lie distal to the ejaculatory duct. Under the actions of androgens they produce the mucoid secretions that constitute the major fluid volume of the ejaculate.

The prostate gland is a muscular organ containing 30-50 tubuloalveolar glands that encompass the junction of the ejaculatory duct with the urethra. The prostatic secretions, which are highly responsive to androgens, are rich in the enzyme acid phosphatase, which is a constituent of the ejaculate.

The two bulbourethral glands, sometimes referred to as Cowper's glands, are 2-3 mm in diameter and secrete prostaglandins and an alkaline mucoid solution into the urethra to neutralize the acidity and coat the urethral lining immediately prior to the arrival of the ejaculate.

The penis, which is an external genital organ, carries the urethra to the exterior of the body. Its erectile tissue is highly vascularized, so that when the penis is filled with blood it is in an erect and firm state, which facilitates deposition of the spermatozoa in the female vaginal canal.

E. Spermatozoa and Semen

A mature human spermatozoon, which contains the haploid number of chromosomes, including either the male, Y, or female, X, determining chromosome, is ~50-60 fim long. Anatomically, the spermatozoon is composed of the head and tail (see Figure 12-4). Under normal circumstances, in the adult human ~30 million germ cells are produced per day and available for maturation over an 8- to 10-week interval into spermatozoa.

The ejaculate or seminal fluid, which normally consists of 2.5-3.5 ml, contains an average of 100 million

Human Ejaculate Diagram

figure 12-4 Schematic diagram of a spermatozoon. The spermatozoon consists of two principal regions: the head, containing the nucleus, and the tail, which can be subdivided into four regions: the neck (Nk), middle piece (MP), principal piece (PP), and end piece (EP). The head contains the nucleus (N) with its condensed chromatin enclosed by a nuclear envelope; the anterior portion of the nucleus is enclosed by the acrosomal cap (Ac) or acrosome. The middle piece contains elongated mitochondria wound helically around the flagellar fibers (F). As shown in the inset, the axial filament of the sperm flagellum is composed of two central single fibers surrounded by nine double fibrils (ODF). Reproduced with permission from Lentz, T. L. (1971). "Cell Fine Structure," p. 247. W. B. Saunders Co., Philadelphia, PA.

figure 12-4 Schematic diagram of a spermatozoon. The spermatozoon consists of two principal regions: the head, containing the nucleus, and the tail, which can be subdivided into four regions: the neck (Nk), middle piece (MP), principal piece (PP), and end piece (EP). The head contains the nucleus (N) with its condensed chromatin enclosed by a nuclear envelope; the anterior portion of the nucleus is enclosed by the acrosomal cap (Ac) or acrosome. The middle piece contains elongated mitochondria wound helically around the flagellar fibers (F). As shown in the inset, the axial filament of the sperm flagellum is composed of two central single fibers surrounded by nine double fibrils (ODF). Reproduced with permission from Lentz, T. L. (1971). "Cell Fine Structure," p. 247. W. B. Saunders Co., Philadelphia, PA.

spermatozoa and a mixture of secretions from the various accessory genital glands. It is rich in fructose, ascorbic acid, prostaglandins, carnitine, and a variety of enzymes, including acid phosphatase, aminotransferases, muramidase, and dehydrogenases. Even though fertilization of an ovum requires only one sperm, 50% of men with sperm counts of 20-40 million/ml and all men with sperm counts under 20 million/ml are sterile.

III. CHEMISTRY, BIOCHEMISTRY, AND BIOLOGICAL RESPONSES

Table 12-1 tabulates the 9-10 hormones pertinent to male development and reproduction.

A. Steroid Hormones

1. Androgens

Androgens are steroid hormones that induce the differentiation and maturation of the male reproductive organs, the development of male secondary sex characteristics, and the behavioral manifestations consistent with the male role in reproduction. The two most important steroid hormones of the adult male are testosterone and 5a-dihydrotestosterone. The structures of these compounds are presented in Figure 12-5.

As reviewed in Chapter 2 (see Figures 2-3, 2-22, and 12-5), the naturally occurring androgens typically are 19-carbon steroids. Testosterone is the principal male androgen produced and secreted by the testes. In addition, a number of other androgen intermediates are released in low concentrations, particularly andro-stenedione and androstene-3/3,17-diol. Table 12-2 summarizes the secretion rates, plasma levels, and metabolic clearance rates of the principal sex steroids in the male.

The biochemical pathway for the production of androgens is discussed in Chapter 2. The side chain cleavage of cholesterol is confined to the mitochondria of the Ley dig cells (see Figure 2-21). The subsequent conversion of pregnenolone into testosterone requires four enzymatic reactions, which are divided into two parallel pathways: one proceeding via 17-hydroxypregnenolone, known as the A5-pathway, and the other proceeding via 17-hydroxyprogesterone, known as the A4-pathway (see Figure 2-22). The relative activities of the A5- versus A4-pathway vary among various mammalian species; in the rodent the A4-path-way is predominant, while in the human testes the A5-

Hormones, Second Edition TABLE 12-1 Hormones Related to Male Development and Spermatogenesis

Hormone

Site of production Principal target tissue

Principal biological function

Testosterone

5a-Dihydrotestosterone (DHT) Androstenediol Dehydroepiandrosterone Estradiol

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