Male reproductive system

Introduction

The male reproductive system is responsible for the production of spermatozoa and their delivery into the female reproductive tract and may be divided into four major functional components:

• The testes or male gonads, paired organs lying in the scrotal sac, are responsible for production of the male gametes, spermatozoa, and secretion of male sex hormones, principally testosterone.

• A system of ducts consisting of ductuli efferentes, epididymis, ductus (vas) deferens and ejaculatory duct collects, stores and carries spermatozoa from each testis. The ejaculatory ducts converge on the urethra, from which spermatozoa are expelled into the female reproductive tract during copulation.

• Two exocrine glands, the paired seminal vesicles and the single prostate gland, secrete a nutritive and lubricating fluid medium called seminal fluid in which spermatozoa are conveyed to the female reproductive tract. Semen, the fluid expelled during ejaculation, consists of seminal fluid and spermatozoa, plus some desquamated duct-lining cells.

• The penis is the organ of copulation. A pair of small accessory glands, the bulbourethral glands of Cowper, secrete a fluid which lubricates the urethra for the passage of semen during ejaculation.

FIG. 18.1 Male reproductive system

FIG. 18.2 Testis
During embryological development, each testis with the first part of its duct system, blood vessels, lymphatics and nerves descends from the posterior wall of the peritoneal cavity to the scrotum. During migration, the testis carries with it an investing layer of peritoneum so that in the scrotum the testis is almost completely surrounded by a double layer of mesothelium, enclosing a potential space. This double lining is called the tunica vaginalis and, like the pleura, consists of visceral and parietal layers, separated by a thin layer of serous fluid. The fluid is secreted by the mesothelial cells and acts as a lubricant, allowing the testis to move freely in the scrotal sac. The visceral layer of the tunica vaginalis rests on the capsule of the testis, the tunica albuginea, which gives rise to numerous incomplete collagenous septa. These divide the testis into about 250 testicular lobules. Within each lobule, there are one to four highly convoluted tubes, the seminiferous tubules, in which spermatozoa are produced. The seminiferous tubules converge upon a plexus of channels, the rete testis. From the rete testis, 15 to 20 small ducts called the ductuli efferentes carry spermatozoa to the extremely tortuous first part of the ductus deferens, which is known as the epididymis.

FIG. 18.3 Testis, monkey
H&E (LP)
This micrograph illustrates the macroscopic features of a testis; cut in the sagittal plane, it shows the relationship to the epididymis E, which lies on its posterior aspect. The testis is packed with coiled seminiferous tubules which can just be seen in various planes of section at this magnification. Groups of up to four seminiferous tubules are segregated into testicular lobules by fine interlobular septa S.
The dense fibrous capsule which invests the testis, and which is continuous with many of the interlobular septa, is called the tunica albuginea TA. It contains fibroblasts and abundant myofibroblasts and smooth muscle cells, particularly in the posterior aspect close to the rete testis, which subject the seminiferous tissue to rhythmic contractions. Scattered Leydig cells are also found within the tunica albuginea. The deepest layer of the tunica albuginea consists of loose connective tissue containing blood and lymphatic vessels, sometimes called the tunica vasculosa.
Spermatozoa pass from the seminiferous tubules into the rete testis RT, which is connected to the epididymis via the ductuli efferentes at the upper posterior pole of the testis; the ductuli are not included in the plane of this section. The epididymis is a tightly coiled tube which forms a compact mass extending down the whole length of the posterior surface of the testis and is the major site of storage of newly formed spermatozoa. At the lower pole of the testis, the epididymal tube becomes continuous with the relatively straight ductus (vas) deferens, not seen in this section.

Gametogenesis

In all somatic cells, cell division (mitosis) results in the formation of two daughter cells, each one genetically identical to the mother cell. Somatic cells contain a full complement of chromosomes (the diploid number) which function as homologous pairs (see Ch. 2). The process of sexual reproduction involves the fusion of specialised male and female cells called gametes to form a zygote, which has the diploid number of chromosomes. Each gamete contains only half the diploid number of chromosomes, one representative of each pair; this half complement of chromosomes is known as the haploid number.

The production of haploid cells involves a unique form of cell division called meiosis, which occurs only in the germ cells of the gonads during the formation of gametes; meiotic cell division is thus also called gametogenesis. Meiosis involves two cell division cycles, of which only the first is preceded by duplication of chromosomes (see Ch. 2). Thus, meiotic cell division of a single diploid germ cell gives rise to four haploid gametes. In the male, each of the four gametes undergoes morphological development into a mature spermatozoon. In contrast, in the female, unequal distribution of the cytoplasm during meiosis results in one gamete gaining almost all the cytoplasm from the mother cell, while the other three acquire almost no cytoplasm; the large gamete matures to form an ovum and the other three, called polar bodies, degenerate.

The primitive germ cells of the male, the spermatogonia, are present only in small numbers in the male gonads before sexual maturity. After puberty, spermatogonia multiply continuously by mitosis to provide a supply of cells which then undergo meiosis to form male gametes. In contrast, the germ cells of the female, called oogonia, multiply by mitosis only during early fetal development, thereby producing a fixed complement of cells with the potential to undergo gametogenesis. Gametogenesis in the female is discussed more fully in Ch. 19. The production of male gametes is called spermatogenesis and the subsequent development of the male gamete into a motile spermatozoon is called spermiogenesis, the whole process taking approximately 70 days; both these processes occur within the testes, although final maturation of spermatozoa occurs in the epididymis.

FIG. 18.4 Seminiferous tubules
H&E (MP)
This micrograph illustrates seminiferous tubules cut in various planes of section. The seminiferous tubules are highly convoluted and are lined by:

• Germ cells in various stages of spermatogenesis and spermiogenesis, which are collectively referred to as the spermatogenic series

• Non–germ cells, called Sertoli cells, which support and nourish the developing spermatozoa are also found within the seminiferous tubules.

• In the interstitial spaces between the tubules, endocrine cells called Leydig cells are found either singly or in groups in the supporting tissue.

In this micrograph of normal testis at medium power, note the seminiferous tubules Se cut in various planes of section, giving round and ovoid profiles. Between the seminiferous tubules the interstitium I contains Leydig cells (which cannot be discerned at this magnification) and small capillaries C. Larger arteries and veins are found in the fibrous septa that divide the organ into lobules.

C capillary E epididymis I interstitium RT rete testis S interlobular septum Se seminiferous tubule TA tunica albuginea