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Organs of the Male Reproductive System

The organs of the male reproductive system are specialized to produce and maintain the male sex cells, or sperm cells; to transport these cells, together with supporting fluids, to the female reproductive tract; and to secrete male sex hormones.

The primary sex organs (gonads) of this system are the two testes in which the sperm cells (spermatozoa) and the male sex hormones are formed. The other structures of the male reproductive system are termed accessory sex organs (secondary sex organs). They include the internal reproductive organs and the external reproductive organs (fig. 22.1).

Testes

The testes (sing., testis) are ovoid structures about 5 centimeters in length and 3 centimeters in diameter. Both testes, each suspended by a spermatic cord, are contained within the cavity of the saclike scrotum (see fig. 22.1 and reference plate 52).

Descent of the Testes

In a male fetus, the testes originate from masses of tissue posterior to the parietal peritoneum, near the developing kidneys. Usually a month or two before birth, these organs descend to the lower abdominal cavity and pass through the abdominal wall into the scrotum.

The male sex hormone testosterone, which the developing testes secrete, stimulates the testes to descend. A fibromuscular cord called the gubernaculum (gu"ber-nak'u-lum) aids movement of the testes. This cord is attached to each developing testis and extends into the inguinal region of the abdominal cavity. It passes through the abdominal wall and is fastened to the skin on the outside of the scrotum. The testis descends, guided by the gubernaculum, passing through the inguinal canal (ing'gwi-nal kah-nal') of the abdominal wall and entering the scrotum, where it remains anchored by the gubernaculum. Each testis carries a developing vas deferens, blood vessels, and nerves. These structures later form parts of the spermatic cord by which the testis is suspended in the scrotum (fig. 22.2).

If the testes fail to descend into the scrotum, they will not produce sperm cells because the temperature in the abdominal cavity is too high. If this condition, called cryptorchidism, is left untreated, the cells that normally produce sperm cells degenerate, and the male is infertile.

Urinary bladder

Symphysis pubis

Vas deferens

Urethra

Penis

Glans penis -Prepuce-

Large intestine

Seminal vesicle Ampulla

Ejaculatory duct

Prostate gland

Bulbourethral gland

Anus

Epididymis Testis

Urinary bladder

Symphysis pubis

Vas deferens

Urethra

Penis Abdominal Cavity

Large intestine

Epididymis Testis

Figure 22.1

(a) Sagittal view of male reproductive organs. (b) Posterior view.

Figure 22.1

(a) Sagittal view of male reproductive organs. (b) Posterior view.

Testis

Rectum

Gubernaculum

Symphysis pubis

- Abdominal wall

- Lower abdominal cavity

- Developing penis

Peritoneum

Testis Inguinal canal Gubernaculum

Vaginal process

Vaginal process

Gubernaculum

Vas deferens

Scrotum

Figure 22.2

Spermatic cord Testis

Gubernaculum

Vas deferens

Scrotum

Figure 22.2

During fetal development, each testis descends through an inguinal canal and enters the scrotum (a-c).

What is the function of the gubernaculum, both during and after the descent of the testes?

^9 What happens if the testes fail to descend into the scrotum?

Structure of the Testes

A tough, white, fibrous capsule called the tunica albugínea encloses each testis. Along its posterior border, the connective tissue thickens and extends into the organ, forming a mass called the mediastinum testis. From this structure, thin layers of connective tissue, called septa, pass into the testis and subdivide it into about 250 lobules.

A lobule contains one to four highly coiled, convoluted seminiferous tubules, each of which is approximately 70 centimeters long when uncoiled. These tubules course posteriorly and unite to form a complex network of channels called the rete testis. The rete testis is located within the mediastinum testis and gives rise to several ducts that join a tube called the epididymis. The epi-didymis, in turn, is coiled on the outer surface of the testis.

The seminiferous tubules are lined with a specialized stratified epithelium, which includes the spermato-genic cells that give rise to the sperm cells. Other specialized cells, called interstitial cells (cells of Leydig), lie in the spaces between the seminiferous tubules. Interstitial cells produce and secrete male sex hormones (figs. 22.3, 22.4, and 22.5a).

The epithelial cells of the seminiferous tubules can give rise to testicular cancer, a common cancer in young men. In most cases, the first sign is a painless testis enlargement or a scrotal mass that attaches to a testis.

If a biopsy (tissue sample) reveals cancer cells, surgery is performed to remove the affected testis (orchiectomy). Radiation and/or chemotherapy often prevent the cancer from recurring.

During the descent of a testis, a pouch of peritoneum, called the vaginal process, moves through the inguinal canal and into the scrotum. In about one-quarter of males, this pouch remains open, providing a potential passageway through which a loop of intestine may be forced by great abdominal pressure, producing an indirect inguinal hernia. If the protruding intestinal loop is so tightly constricted within the inguinal canal that its blood supply stops, the condition is called a strangulated hernia. Without prompt treatment, the strangulated tissues may die.

What are the primary sex organs of the male reproductive system?

Describe the descent of the testes.

U Describe the structure of a testis.

^9 Where are the sperm cells produced within the testes? ^9 Which cells produce male sex hormones?

Formation of Sperm Cells

The epithelium of the seminiferous tubules consists of supporting cells (sustentacular cells, or Sertoli cells) and spermatogenic cells. The sustentacular cells are tall, columnar cells that extend the full thickness of the epithelium from its base to the lumen of the seminiferous tubule. Many thin processes project from these cells, filling the spaces between nearby spermatogenic cells. The sustentacular cells support, nourish, and regulate the spermatogenic cells, which give rise to sperm cells (spermatozoa).

- Vas deferens

- Vas deferens

Lumen Seminiferous Tubules

Lumen of seminiferous tubule

Basement membrane

Sperm cells

Testis Seminiferous tubules

Interstitial cells Germinal epithelial cell

Testis Seminiferous tubules

Lumen of seminiferous tubule

Basement membrane

Interstitial cells Germinal epithelial cell

Sperm cells

Figure 22.3

(a) Sagittal section of a testis. (b) Cross section of a seminiferous tubule.

Figure 22.3

(a) Sagittal section of a testis. (b) Cross section of a seminiferous tubule.

Figure

Scanning electron micrograph of cross sections of human seminiferous tubules (150x).

Figure

Scanning electron micrograph of cross sections of human seminiferous tubules (150x).

Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, by R. G. Kessel and R. H. Kardon. ©1979 W. H. Freeman and Company.

In the male embryo, the spermatogenic cells are undifferentiated and are called spermatogonia. Each of these cells contains 46 chromosomes in its nucleus, the usual number for human cells (fig. 22.5).

During embryonic development, hormones stimulate the spermatogonia to become active. Some of the cells undergo mitosis, giving rise to new spermatogonia and providing a reserve supply of these undifferentiated cells. Other cells enlarge and become primary spermatocytes. Sperm production (spermatogenesis) (fig. 22.5) is arrested at this stage. At puberty, testosterone secretion rises, and the primary spermatocytes then reproduce by a special type of cell division called meiosis (mi-o'sis).

Meiosis includes two successive divisions, called the first and second meiotic divisions. The first meiotic division (meiosis I) separates homologous chromosome pairs. Homologous pairs are the same, gene for gene. They may not be identical, however, because a gene may have more than one variant, and the chromosome that comes from the person's mother may carry a different variant for the corresponding gene from the father's homologous chromosome. Before meiosis I, each homologous chromosome is replicated, so that it consists of two complete DNA strands called chromatids. The chromatids of a replicated chromosome attach at regions called centromeres.

Each of the cells that undergo the second meiotic division (meiosis II) emerges with one member of each homologous pair, a condition termed haploid. This second

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