MALE REPRODUCTIVE SYSTEN

The internal male genitalia consist of the testes with the adjoining epididymis, the vas deferens and the accessory sex glands, namely the seminal vesicles, the prostrate and the bulbourethral glands (the latter sometimes are included in the external genitalia).

Testes

The testes have, like the ovaries, two functions: they produce the male gametes or spermatozoa, and they produce male sexual hormone, testosterone, which stimulates the accessory male sexual organs and causes the development of the masculine extragenital sex characteristics.

The testis is surrounded by a thick capsule, the tunica albuginea, from which a conical mass of connective tissue, the mediastinum testis, projects into the testis. The tunica albuginea is covered externally by a serosa.

From the mediastinum, delicate fibrous septa radiate towards the tunica albuginea and divide the parenchyma of the testis into about 300 lobuli testis, which communicate peripherally. Each lobule contains 1-4 convoluted seminiferous tubules (about 150-300 µm in diameter, 30-80 cm long).

Interstitial tissue between the convoluted tubules is continuous with a layer of loose vascular connective tissue, the tunica vasculosa testis, which is found beneath the tunica albuginea.

Each seminiferous tubule continues near the mediastinum into a straight tubule, a tubulus rectus. The straight tubules continue into the rete testis, a labyrinthine system of cavities in the mediastinum.

The Convoluted Seminiferous Tubules

These tubules are enclosed by a thick basal lamina, which is surrounded by 3-4 layers of smooth muscle cells (or myoid cells). The insides of the tubules are lined with seminiferous epithelium, which consists of two general types of cells: spermatogenic cells and Sertoli cells.

Spermatogenic cells:
Spermatogonia

are the first cells of spermatogenesis. They originate in the 4th week of foetal development in the endodermal walls of the yolk sac and migrate to the primordium of the testis, where they differentiate into spermatogonia. Spermatogonia remain dormant until puberty. They are always in contact with the basal lamina of the tubule.

Two types of spermatogonia can be distinguished in the human seminiferous epithelium:
Type A spermatogonia have a rounded nucleus with very fine chromatin grains and one or two nucleoli. They are stem cells which divide to form new generations of both type A and type B spermatogonia.
Type B spermatogonia have rounded nuclei with chromatin granules of variable size, which often attach to the nuclear membrane, and one nucleolus. Although type B spermatogonia may divide repeatedly, they do not function as stem cells and their final mitosis always results in the formation of

Primary spermatocytes

which lie in the cell layer luminal to the spermatogonia. They appear larger than spermatogonia. They immediately enter the prophase of the first meiotic division, which is extremely prolonged (about 22 days!). A large number of primary spermatocytes is always visible in cross-sections through seminiferous tubules. Cell divisions, from the formation of primary spermatocytes and onwards, to the production of the spermatocytes, are incomplete. The cells remain connected by bridges of cytoplasm. The completion of the first meiotic division results in the formation of

Secondary spermatocytes,

which are smaller than primary spermatocytes. They rapidly enter and complete the second meiotic division and are therefore seldom seen in histological preparations. Their division results in the formation of

Spermatids,

which lie in the luminal part of the seminiferous epithelium. They are small (about 10 µm in diameter) with an initially very light (often eccentric) nucleus. The chromatin condenses during the maturation of the spermatids into spermatozoa, and the nucleus becomes smaller and stains darker.

The terminal phase of spermatogenesis is called spermiogenesis and consists of the differentiation of the newly formed spermatids into

Spermatozoa

The mature human spermatozoon is about 60 µm long and actively motile. It is divided into head, neck and tail.
The head (flattened, about 5 µm long and 3 µm wide) chiefly consists of the nucleus (greatly condensed chromatin!). The anterior 2/3 of the nucleus is covered by the acrosome, which contains enzymes important in the process of fertilisation. The posterior parts of the nuclear membrane form the so-called basal plate.
The neck is short (about 1 µm) and attached to the basal plate. A transversely oriented centriole is located immediately behind the basal plate. The neck also contains nine segmented columns of fibrous material, which continue as the outer dense fibres into the tail.
The tail is further divided into a middle piece, a principal piece and an end piece. The axonema (the generic name for the arrangement of microtubules in all cilia) begins in the middle piece. It is surrounded by nine outer dense fibres, which are not found in other cilia. In the middle piece (about 5 µm long), the axonema and dense fibres are surrounded by a sheath of mitochondria. The middle piece is terminated by a dense ring, the annulus. The principal piece is about 45 µm long. It contains a fibrous sheath, which consists of dorsal and ventral longitudinal columns interconnected by regularly spaced circumferential hoops. The fibrous sheath and the dense fibres do not extend to the tip of the tail. Along the last part (5 µm) of the tail, called the end piece, the axonema is only surrounded by a small amount of cytoplasm and the plasma membrane.

It takes about 48 days from the time cells enter meiosis until morphologically mature spermatozoa are formed. Depending on the length of reproduction of spermatogonia (which is not precisely determined) it takes approximately 64 days to complete spermatogenesis.

Sertoli cells

are far less numerous than the spermatogenic cells and are evenly distributed between them. Their shape is highly irregular (columnar is the best approximation). Sertoli cells extend from the basement membrane to the luminal surface of the seminiferous epithelium. Processes of the Sertoli cells extend in between the spermatogenic cells (cell limits are therefore not clearly visible in the LM). The nucleus of Sertoli cells is ovoid or angular, large and lightly stained and often contains a large nucleolus. The long axis of the nucleus is oriented perpendicular to wall of the tubule. A fold in the nuclear membrane is characteristic for Sertoli cells but not always visible in the LM (well ... actually ... it's not that difficult to find, but not that easy either ....).
Lateral processes of Sertoli cells are interconnected by tight junctions, which are likely to be the structural basis for the blood-testis barrier. Spermatogonia and primary spermatocytes are located in the basal compartment, other cellular stages of spermatogenesis are located in the adluminal compartment. Tight junctions may temporarily open to permit the passage of spermatogenic cells from the basal into the adluminal compartment. Sertoli cells provide mechanical support for the spermatogenic cells. They also have a nutritive function.

Interstitial tissue

Leydig cells (15-20 µm), located in the interstitial tissue between the convoluted seminiferous tubules, constitute the endocrine component of the testis. They synthesise and secrete testosterone. Ledig cells occur in clusters , which are variable in size and richly supplied by capillaries. The cytoplasm is strongly acidophilic and finely granular. The nucleus is large, round and often located eccentric in the cell.

Ducts of the Testis

Spermatozoa pass via the tubuli recti (low columnar epithelium) and the rete testis (flattened or cuboidal epithelium) into numerous ductuli efferentes, which are lined by a columnar epithelium, which consists of both absorptive and ciliated cells. The height of the cells forming the epithelium of the ductuli efferentes is variable which gives the lumen a characteristic wavy outline.

The ductuli efferentes leave the testis and open into a common duct, the ductus epididymidis (about 6 m long!). It is lined by a very tall pseudostratified columnar epithelium. Most cells of the epithelium, also called principal cells, have long stereocilia. Stereocilia are non-motile structures, which in the EM resemble large microvilli. Towards the basal lamina we see a number of small nuclei, which belong to the basal cells of the ductus epididymidis. These cells regenerate the epithelium.

Peristaltic contractions of smooth muscle cells surrounding the ductus epididymidis move the spermatozoa towards the middle segment of the duct, which is the site of final functional maturation of the spermatozoa - now they are motile. The terminal segment of the ductus epididymidis is the site of storage of the mature spermatozoa. Smooth muscle fibres of the terminal part of the ductus epididymidis do not contract spontaneously. They contract during sexual stimulation concurrently with the contraction of the musculature of the duct into which it opens, the vas deferens.

The Vas deferens (or ductus deferens)

The mucosa of the vas deferens forms low longitudinal folds. It is lined by a pseudostratified columnar epithelium. Similar to the epididymis, cells have long stereocilia. The lamina propria is unusually rich in elastic fibres. The muscularis is well developed (up to 1.5 mm thick) and consists of a thick circular layer of smooth muscle between thinner inner and outer longitudinal layers. The muscularis is the structure which makes the vas deferens palpable in the spermatic cord. The vas deferens is surrounded by an adventitia, which is slightly denser than usual.

Male Accessory Reproductive Glands

The accessory (or secondary) male sex glands consist of the seminal vesicles, the prostrate and the bulbourethral glands.

Prostate

The prostate is the largest accessory sex gland in men (about 2x3x4 cm). It contains 30 - 50 tubuloalveolar glands, which empty into 15 - 25 independent excretory ducts. These ducts open into the urethra. The glands are embedded into a fibromuscular stroma, which mainly consists of smooth muscle separated by strands of connective tissue (rich in collagenous and elastic fibres). The muscle forms a dense mass around the urethra and beneath the fairly thin capsule of the prostrate.

The secretory alveoli of the prostate are very irregularly shaped because of papillary projections of the mucosa into the lumen of the gland. The epithelium is cuboidal or columnar. Basal cells are again present, and the epithelium may look pseudostratified where they are found. The secretory cells are slightly acidophilic and secretory granules may be visible in the cytoplasm. Small extensions of the apical cytoplasm into the lumen of the alveoli may represent cells which  release their secretory products (secretion is apocrine/merocine). The secretion of the prostate contains citric acid, the enzyme fibrinolysin (liquifies the semen), acid phosphatase, a number of other enzymes and lipids. The secretion of the prostate is the first fraction of the ejaculate.

The epithelium of the secretory ducts of the prostate is simple columnar but changes to a transitional epithelium near their openings into the urethra.

A characteristic feature of the prostate is the appearance of corpora amylacea in the secretory alveoli. They are rounded eosinophilic bodies. Their average diameter is about 0.25 mm (up to 2 mm). They appear already in the seventh month of development. Their number increases with age - in particular past 50. They may undergo calcification. Corpora amylacea may appear in semen.

Macroscopically the prostrate can be divided into lobes, but they are inconspicuous in histological sections. In good histological sections it is possible to distinguish three concentric zones, which surround the prostatic part of the urethra.

• The peripheral zone contains large, so-called main glands, whose ducts run posteriorly to open into the urethra.
• The internal zone consists of the so-called submucosal glands, whereas
• the innermost zone contains mucosal glands.

This subdivision of the prostate is of clinical importance. With age the prostate becomes enlarged due to benign nodular hyperplasia (more than 95% of men older than 45 exhibit hyperplastic changes). This condition affects the mucosal glands. Cancer of the prostate, which is the second most common malignant tumour in western males, involves the peripheral zone.

Seminal Vesicles

The seminal vesicles develop from the vas deferens. Their histological organisation resembles to some extent that of the ductus deferens. They are elongated sacs (about 4 cm long and 2 cm wide), which taper where they unite with the ductus deferens. Each seminal vesicle consists of one coiling tube (about 15cm long). All the lumina visible in sections of the seminal vesicle are in continuity in the intact organ.

The mucosa shows thin, branched, anastomosing folds. The structure of the epithelium is variable appearing columnar or pseudostratified columnar (columnar cells and basal cells). The lamina propria of the mucosa is fairly thin and loose. The muscularis consists of inner circular and outer longitudinal layers of smooth muscle.

Seminal vesicles were thought to store semen - hence there name. This turned out to be wrong. They are glands, whose secretion constitutes a 60-70 % of the ejaculate. The secretory product of the columnar cell, which may be seen in the lumen of the seminal vesicles, is strongly acidophilic. It contains large amounts of fructose which is utilised by the spermatozoa as a source of energy. Furthermore, the secretion contains prostaglandins, flavins (yellow fluorescing pigment - of use in forensic medicine to detect semen stains) and several other proteins and enzymes.

The coctail of compounds release by the seminal vesicles has three main functions:

The secretion of the seminal vesicles is the third fraction of the ejaculate (the spermatozoa are released with the second fraction - the contents of the vas deferens).