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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.

Embryology of the male genital tract

Many of the structures of the male genital tract are derived from the paired Wolffian or mesonephric ducts . These give rise to the right and left epididymis, vas deferens, ejaculatory duct and seminal vesicle. The prostate gland develops from the urogenital sinus whilst the testes have a separate origin, developing from mesoderm high in the posterior abdominal wall and later descending into the scrotum via the processus vaginalis. Testicular descent begins after the third month of pregnancy and is generally complete by term. Failure of this process may cause cryptorchidism . Uncorrected, this can be associated with later subfertility. If the processus vaginalis fails to close and regress after testicular descent, this can predispose to the development of a hydrocoele (a collection of fluid around the testis) or an indirect inguinal hernia , where abdominal contents can pass along the inguinal canal to the scrotum.

The testes

Most of the structures of the male reproductive system arise from the Wolffian (mesonephric) duct system as described above (see textbox, ‘ Embryology of the male genital tract ’), however the testes arise from the genital ridge , a thickening in the mesothelium high on the posterior wall of the peritoneal cavity. The organs later descend to their adult position in the scrotum. As a result, the blood supply and lymphatic drainage of the testes is derived from the upper abdomen, with the paired testicular arteries arising from the abdominal aorta, just distal to the origin of the renal arteries. This embryological fact is important in understanding why testicular (and also ovarian) tumours typically spread to para-aortic lymph nodes, rather than to local lymph nodes in the pelvic or inguinal area.

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.

Testicular pathology

Common disorders affecting the testis include infective and inflammatory processes (causing orchitis ), torsion (twisting of the organ which blocks off the blood supply) and a variety of tumours. These can arise in any of the normal components of the testis but the majority are derived from the multipotent cells of the seminiferous tubules and these are called germ cell tumours . This is a very diverse group of tumours but, in broad terms, most of these are malignant tumours and they may be divided very broadly into seminoma (resembling the normal cells in the seminiferous tubules) and non seminomatous germ cell tumours (resembling a variety of embryonal or extra-embryonal structures). Combinations of these types are common. This division reflects important differences in tumour management and prognosis.

e-Fig. 18.1, Torsion of the testis (LP)

e-Table 18.1
Classification of germ cell tumours of the testis
(From O’Dowd G, Bell S, Wright S: Wheater’s Pathology, 6e, Table 19.1, 2020, Elsevier, Ltd.)
Tumour type WHO (2016) classification
Germ cell tumours derived from GCNIS

  • Seminoma

  • Teratoma (post-pubertal type)

  • Embryonal carcinoma

  • Yolk sac tumour (endodermal sinus tumour)

  • Choriocarcinoma

Germ cell tumours not derived from GCNIS

  • Spermatocytic tumour

  • Pre-pubertal teratomas, e.g. epidermoid cyst

Combination of two or more of any type Mixed germ cell tumour

E-Fig. 18.2, Classical seminoma (HP)

E-Fig. 18.3, Mixed testicular germ cell tumours

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

M myofibroblast S A type A spermatogonium S B type B spermatogonium S 1 primary spermatocyte S 3 spermatid S 4 spermatozoon St Sertoli cell

AC acrosomal cap An annulus AV acrosomal vesicle Ax axoneme BM basement membrane C chromatin body Cy cytoplasm F outer dense fibres G Golgi apparatus M myofibroblast Mi mitochondrion Nu nucleolus PM plasma membrane Rb fibrous ribs Rn fibrous rings S spermatogonium St Sertoli cell S 1 primary spermatocyte S 3 spermatid S 4 spermatozoon

A appendix testis B basal cell DE ductulus efferens L Leydig cells RT rete testis S spermatozoa SM smooth muscle ST seminiferous tubule

A adipose tissue AA artery C verumontanum Cap capsule CI intermediate circular muscle layer CZ central zone ED ejaculatory duct I inner longitudinal muscle layer L lipofuscin granules M muscular wall O outer longitudinal muscle layer PZ peripheral zone St fibrous stroma Sp fibrous septum TZ transitional zone U urethra V vein

Common prostatic disease

The most common disease of the prostate is called benign prostatic hyperplasia and occurs in men over 50. The prostatic glands around the urethra (transition zone) become greatly increased in size and number and the gland lumina become distended by secretions and corpora amylacea. At the same time, the stromal smooth muscle fibres become greatly enlarged. This increase in bulk enlarges the prostate gland as a whole, and compresses the urethra, leading to interference with bladder emptying.

E-Fig. 18.4, Benign prostatic hyperplasia

Fig. 18.17, Prostate gland

B basal cell CC corpus cavernosum CS corpus spongiosum F fibrocollagenous capsule G gland HA helicine artery P para-urethral gland S skin Si vascular sinus SS supporting stroma U urethra

Review

Fig. 18.1, Male reproductive system

Fig. 18.3, Testis, monkey H&E (LP)

Fig. 18.4, Seminiferous tubules H&E (MP)

Fig. 18.5, Seminiferous tubule (a) H&E (HP) (b) Diagram

Fig. 18.6, Spermiogenesis

Fig. 18.7, Spermatozoa

Fig. 18.8, Sertoli cell EM ×3400

Fig. 18.9, Sertoli cells (HP)

Fig. 18.10, Interstitial (Leydig) cells of the testis H&E (HP)

Fig. 18.11, Rete testis H&E (LP)

Fig. 18.16, Seminal vesicle

Fig. 18.18, Prostate gland, dog H&E (LP)

Fig. 18.19, Prostate gland

Fig. 18.20, Penis H&E (LP)

Fig. 18.21, Corpus spongiosum H&E (LP)

Fig. 18.22, Penile erectile tissue H&E (LP)

Fig. 18.23, Penile urethra H&E (HP)

E-Fig. 18.5, Adenocarcinoma of the prostate

Fig. 18.2, Testis

Fig. 18.12, Appendix testis H&E (LP)

Fig. 18.13, Ductulus efferens H&E (MP)

Fig. 18.24, Semen H&E (HP)

Table 18.1
Review of the male genital tract
Organ Main components Cell types Functions Figures
Testis Seminiferous tubules Spermatogenic series cells Production of male gametes, spermatozoa 18.3–18.5 , 18.7
Sertoli cells Support cells for spermatogenesis 18.5 , 18.8 , 18.9
Interstitium Leydig cells Synthesis of androgenic hormones, principally testosterone 18.9 , 18.10
Rete testis Cuboidal epithelium with cilia and smooth muscle coat Convey spermatozoa to ductuli efferentes and thence to epididymis 18.11
Epididymis Head, body, tail Columnar epithelium with stereocilia and smooth muscle coat Store and mature spermatozoa 18.14
Vas deferens Columnar epithelium and smooth muscle coat, three layers Carry sperm to urethra during ejaculation 18.15
Prostate Central, transition and peripheral zones and anterior fibromuscular stroma Epithelium with two cell layers, luminal tall columnar layer and basal cell layer Produces secretions that mix with seminal fluid 18.18 , 18.19
Seminal vesicle Cuboidal to columnar epithelium with muscular wall Produce seminal fluid 18.16
Penis Corpus spongiosum and corpora cavernosa Spongy fibrous tissue containing anastomosing vascular sinuses Erectile tissue 18.20–18.22
Urethra Lined by urothelium proximally and pseudostratified columnar epithelium distally Duct for ejaculation (and micturition) 18.23

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