Copyright 2007 by Saunders/Elsevier. All rights reserved.

Chapter 21:

Male Reproductive System

Color Textbook of Histology, 3rd ed.

Gartner & Hiatt Copyright 2007 by Saunders/Elsevier. All rights reserved.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Male Reproductive System

The male reproductive system consists of the two testes suspended in the scrotum, a system of intratesticular and extratesticular genital ducts, associated glands, and the male copulatory organ, the penis. The testes are responsible for the formation of the male gametes, known as spermatozoa, as well as for the synthesis, storage, and release of the male sex hormone, testosterone.

The glands associated with the male reproductive tract are the paired seminal vesicles, the single prostate gland, and the two bulbourethral glands (of Cowper). These glands form the non-cellular portion of semen (spermatozoa suspended in the secretions of the accessory glands), which not only nourishes the spermatozoa but also provides a fluid vehicle for their delivery into the female reproductive tract. The penis has a dual function: It delivers semen to the female reproductive tract during copulation and serves as the conduit of urine from the urinary bladder to outside the body.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–1  The male reproductive system.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

TestisEach testis is surrounded by a capsule known as the tunica albuginea. Immediately deep to this layer is a highly vascularized loose connective tissue, the tunica vasculosa. The posterior aspect of the tunica albuginea is somewhat thickened, forming the mediastinum testis, from which connective tissue septa radiate to subdivide each testis into approximately 250 compartments known as the lobuli testis

Each lobule has one to four blindly ending seminiferous tubules and small conglomerations of endocrine cells, the interstitial cells (of Leydig) which synthesize testosterone.

Spermatozoa, produced by the seminiferous epithelium of the seminiferous tubules, enter short straight ducts, tubuli recti, that connect the open end of each seminiferous tubule to the rete testis, a system of labyrinthine spaces housed within the mediastinum testis. The spermatozoa leave the rete testis through 10 to 20 short tubules, the ductuli efferentes, which eventually fuse with the epididymis.

The vascular supply of each testis is derived from the testicular artery, which descends with the testis into the scrotum accompanying the ductus deferens (vas deferens).

The capillary beds of the testes are collected into several veins, the pampiniform plexus of veins, which are wrapped around the testicular artery.

Blood in the pampiniform plexus of veins is cooler than that in the testicular artery, reduces the temperature of the arterial blood, thus forming a countercurrent heat exchange system, keeping the temperature of the testes a few degrees lower (35˚ C), than that of the remainder of the body. At this cooler temperature spermatozoa develop normally; at body temperature, spermatozoa that develop are sterile.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–2  The testis and epididymis. Lobules and their contents are not drawn to scale.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Seminiferous Tubule

The wall of the seminiferous tubule is composed of a slender connective tissue layer, the tunica propria, and a thick seminiferous epithelium, separated from each other by a well-developed basal lamina.

The seminiferous epithelium is several cell layers thick and is composed of two types of cells: Sertoli cells and spermatogenic cells. The latter cells are in various stages of maturation.

The lateral cell membranes of adjacent Sertoli cells form occluding junctions with each other, thus subdividing the lumen of the seminiferous tubule into basal and adluminal compartments. Thus, the zonulae occludentes of these cells establish a blood-testis barrier that isolates the adluminal compartment from connective tissue influences, thereby protecting the developing gametes from the immune system..

Sertoli cells function in: supporting the developing spermatogenic cells; establishing the blood-testis barrier; phagocytosis of cytoplas shed by developing spermatogenic cells; manufacturing the following substances: androgen binding protein, antimullerian hormone, inhibin, testicular transferrin, and a fructose-rich medium.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–5 Seminiferous epithelium.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Spermatogenesis

Most of the cells composing the thick seminiferous epithelium are spermatogenic cells in various stages of maturation. Some of these cells, spermatogonia, are located in the basal compartment, whereas most of the developing cells—primary spermatocytes, secondary spermatocytes, spermatids, and spermatozoa—occupy the adluminal compartment.

Spermatogonia are diploid cells that undergo mitotic division to form more spermatogonia as well as primary spermatocytes, which migrate from the basal into the adluminal compartment.

Primary spermatocytes enter the first meiotic division to form secondary spermatocytes, which undergo the second meiotic division to form haploid cells known as spermatids.

Spermatids are transformed into spermatozoa by shedding of much of their cytoplasm, rearrangement of their organelles, and formation of flagella.

The maturation process is divided into three phases:

– Spermatocytogenesis: spermatogonia differentiate into primary spermatocytes

– Meiosis: reduction division whereby diploid primary spermatocytes reduce their chromosome complement, forming haploid spermatids

– Spermiogenesis: transformation of spermatids into spermatozoa (sperm)

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–5 Seminiferous epithelium.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

SpermatozoonSpermatids discard much of their cytoplasm and form a flagellum to become transformed into spermatozoa, a process known as spermiogenesis.

The spermatozoa (sperm) are long cells (~65 μm), composed of a head, housing the nucleus, and a tail, which accounts for most of its length

The tail of the spermatozoon is subdivided into four regions: neck, middle piece, principal piece, and end piece. The plasmalemma of the head is continuous with the tail’s plasma membrane.

The neck (~5 μm long) connects the head to the remainder of the tail. It is composed of the cylindrical arrangement of the nine columns of the connecting piece that encircles the two centrioles, one of which is usually fragmented. The posterior aspects of the columnar densities are continuous with the nine outer dense fibers.

The middle piece (~5 μm long) is located between the neck and the principal piece. It is characterized by the presence of the mitochondrial sheath, which encircles the outer dense fibers and the centralmost axoneme. The middle piece stops at the annulus. Two of the nine outer dense fibers terminate at the annulus; the remaining seven continue into the principal piece.

The principal piece (~45 μm long) is the longest segment of the tail and extends from the annulus to the end piece. The axoneme of the principal piece is continuous with that of the middle piece. Surrounding the axoneme are the seven outer dense fibers that are continuous with those of the middle piece and are surrounded, in turn, by the fibrous sheath.

The end piece (~5 μm long) is composed of the central axoneme surrounded by plasmalemma. The axoneme is disorganized in the last 0.5 to 1.0 μm.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–9  Spermiogenesis and a mature spermatozoon.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Leydig Cells

Luteinizing hormone (LH), a gonadotropin released from the anterior pituitary gland, binds to LH receptors on the Leydig cells, activating adenylate cyclase to form cyclic adenosine monophosphate (cAMP). Activation of protein kinases of the Leydig cells by cAMP induces inactive cholesterol esterases to become active and cleave free cholesterol from intracellular lipid droplets. The first step in the pathway of testosterone synthesis is also LH-sensitive because LH activates cholesterol desmolase, the enzyme that converts free cholesterol into pregnenolone.

The various products of the synthetic pathway are shuttled between the smooth endoplasmic reticulum and mitochondria until testosterone, the male hormone, is formed and is ultimately released by these cells.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–13  Testosterone synthesis by the interstitial cells of Leydig. ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; CoA, coenzyme A; LH, luteinizing hormone; SER, smooth endoplasmic reticulum.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Hormonal Relationships

Because blood testosterone levels are not sufficient to initiate and maintain spermatogenesis, FSH induces Sertoli cells to synthesize and release androgen-binding protein (ABP). ABP binds testosterone, thereby preventing the hormone from leaving the region of the seminiferous tubule and elevating the testosterone levels in the local environment sufficiently to sustain spermatogenesis.

Release of LH is inhibited by increased levels of testosterone and dihydrotestosterone, whereas release of FSH is inhibited by the hormone inhibin, which is produced by Sertoli cells.

Testosterone is also required for the normal functioning of the seminal vesicles, prostate, and bulbourethral glands as well as for the appearance and maintenance of the male secondary sexual characteristics. The cells that require testosterone possess 5α-reductase, the enzyme that converts testosterone to its more active form, dihydrotestosterone.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–14  Hormonal control of spermatogenesis. FSH, follicle-stimulating hormone; LH, luteinizing hormone; LHRH, luteinizing hormone–releasing hormone. (Adapted from Fawcett, DW: Bloom and Fawcett’s A Textbook of Histology, 10th ed. Philadelphia, WB Saunders, 1975.)

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Prostate

The prostate gland, the largest of the accessory glands, is pierced by the urethra and the ejaculatory ducts. The slender capsule of the gland is composed of a richly vascularized, dense irregular collagenous connective tissue interspersed with smooth muscle cells. The connective tissue stroma of the gland is derived from the capsule and is, therefore, also enriched by smooth muscle fibers in addition to their normal connective tissue cells.

The prostate gland, a conglomeration of 30 to 50 individual compound tubuloalveolar glands, is arranged in three discrete, concentric layers mucosal, submucosal, and main.

The mucosal glands are closest to the urethra and thus are the shortest of the glands. The submucosal glands are peripheral to the mucosal glands and are consequently larger than the mucosal glands. The largest and most numerous of the glands are the peripheralmost main glands, which compose the bulk of the prostate.

The lumina of the tubuloalveolar glands frequently house round to oval prostatic concretions (corpora amylacea), composed of calcified glycoproteins, whose numbers increase with a person’s age.

The prostatic secretion constitutes a part of semen. It is a serous, white fluid rich in lipids, proteolytic enzymes, acid phosphatase, fibrinolysin, and citric acid. The formation, synthesis, and release of the prostatic secretions are regulated by dihydrotestosterone, the active form of testosterone.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–18  Human prostate gland.

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Penis

The penis is composed of three columns of erectile tissue, each enclosed by its own dense, fibrous connective tissue capsule, the tunica albuginea.

Two of the columns of erectile tissue, the corpora cavernosa, are positioned dorsally; their tunicae albugineae are discontinuous in places, permitting communication between their erectile tissues. The third column of erectile tissue, the corpus spongiosum, is positioned ventrally. Because the corpus spongiosum houses the penile portion of the urethra, it is also called the corpus cavernosum urethrae. The corpus spongiosum ends distally in an enlarged, bulbous portion, the glans penis (head of the penis). The tip of the glans penis is pierced by the end of the urethra as a vertical slit.

The three corpora are surrounded by a common loose connective tissue sheath, but no hypodermis, and are covered by thin skin. Skin continues distal to the glans penis to form a retractable sheath, the prepuce. When an individual is circumcised, it is the prepuce that is removed.

Erectile tissue of the penis contains numerous variably shaped, endothelially lined spaces. The vascular spaces of the corpora cavernosa are larger centrally and smaller peripherally, near the tunica albuginea. However, the vascular spaces of the corpus spongiosum are similar in size throughout its extent.

For more information see the Introduction section in Chapter 21 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 21–21  The penis in cross section.