12
The Reproductive Systems
Chapter 27
CHAPTER SUMMARY
This chapter begins with a brief general introduction to the reproductive systems. The male reproductive system is considered separately from the female reproductive system. In the male reproductive system, the important characteristics of the scrotum, testes, ducts, accessory sex glands, semen, and penis are described. In the female reproductive system, the important characteristics of the ovaries, uterine (Fallopian) tubes, uterus, vagina, vulva, perineum, mammary glands, and the uterine and ovarian cycles are described. Birth control methods are explained. The development of the reproductive systems is portrayed. The effects of aging on the reproductive systems are described. A glossary of key medical terms associated with the reproductive systems is provided. This chapter concludes with a thorough study outline, an excellent self-quiz, critical thinking questions, and answers to questions that accompany chapter figures.
STUDENT OBJECTIVES
1. Explain why, although they are very different from each other, both male and female reproductive systems produce gametes.
2. Describe the location, structure, and functions of the scrotum and testes.
3. Explain the principal events of spermatogenesis.
4. Describe the location, structure, and functions of the seminiferous tubules, straight tubules, rete testis, ductus epididymis (within the epididymis), ductus (vas) deferens (as a component of the spermatic cord), ejaculatory duct, and urethra as ducts of the male reproductive system.
5. Describe the location, structure, and functions of the accessory sex glands: seminal vesicles, prostate gland, and bulbourethral (Cowper’s) glands and explain the characteristics of semen.
6. Explain the structure and functions of the penis.
7. Describe the location, structure, and functions of the ovaries.
8. Explain the principal events of oogenesis.
9. Explain the location, structure, and functions of the uterine (Fallopian) tubes, uterus and vagina.
10. Describe the components of the vulva and explain their functions.
11. Explain the structure and functions of the mammary glands.
12. Describe the principal events of the ovarian and uterine cycles.
13. Compare the various kinds of birth control methods and their effectiveness.
14. Describe the development of the male and female reproductive systems.
15. Describe the effects of aging on the reproductive systems.
LECTURE OUTLINE
A. Introduction (p. 835)
1. Sexual reproduction is the process by which organisms produce offspring by means of germ cells called gametes; when a male gamete unites with a female gamete during fertilization, the resulting cell contains one set of chromosomes from each parent.
2. The organs of the reproductive systems may be grouped by function:
i. gonads produce gametes and secrete sex hormones
a. testes produce sperm cells
b. ovaries produce ova
ii. ducts receive, store, and transport gametes
iii. accessory sex glands produce substances that protect gametes and facilitate their movement
iv. supporting structures assist delivery and joining of gametes and, in females, the growth of the fetus during pregnancy
3. Gynecology is the medical specialty concerned with diagnosis and treatment of diseases of the female reproductive system; although urology is the study of the urinary system, urologists also diagnose and treat diseases and disorders of the male reproductive system.
B. Male Reproductive System (p. 836)
1. The male reproductive system includes:
i. testes (male gonads), which produce sperm and secrete hormones
ii. system of ducts, which stores and transports sperm to the exterior
iii. accessory sex glands, which produce secretions that mix with sperm to form semen
iv. supporting structures including the scrotum and penis
2. Scrotum: (p. 836)
i. the scrotum is an outpouching of the abdominal wall consisting of loose skin and superficial fascia that hangs from the root of the penis
ii. externally, it has a median ridge called a raphe which separates the pouch into two lateral portions
iii. internally, it has a vertical scrotal septum that divides it into two sacs, each containing a single testis
a. the septum consists of superficial fascia and smooth muscle tissue called the dartos muscle
- dartos muscle is also found in the subcutaneous tissue of the scrotum
- it causes wrinkling of the skin of the scrotum
iv. the scrotum provides the testes with an environment that is about 2-3 degrees Celsius lower than core body temperature
a. this lower temperature is essential for proper production and survival of sperm
b. the cremaster muscle, a continuation of the internal oblique muscle, assists by elevating a testis on exposure to cold and during sexual arousal
c. the dartos also contracts in response to cold to help assure the ideal temperature
v. the scrotum receives blood from the internal pudendal branches of the internal iliac arteries, the cremasteric branches of the inferior epigastric arteries, and the external pudendal arteries from the femoral arteries; blood drains into scrotal veins that follow the arteries
vi. the scrotal nerves are derived from the pudendal, posterior cutaneous of the thigh, and ilioinguinal nerves
3. Testes or testicles: (p. 837)
i. the testes are paired oval glands which develop in the posterior portion of the embryo’s abdomen and usually begin their descent into the scrotum through the inguinal canals during the latter half of the seventh month of fetal development
ii. the testes are partially covered by the tunica vaginalis
a. this serous membrane covers a white fibrous capsule called the tunica albuginea; this capsule has inward extensions called septa which divide each testis into 200 to 300 lobules
iii. each lobule contains one to three tightly coiled seminiferous tubules where spermatogenesis occurs
a. seminiferous tubules are lined with spermatogenic cells in various stages of development; in order of advancing maturity, they are:
1. spermatogonia (which develop from primordial germ cells) line
the periphery of the tubules
2. primary spermatocytes
3. secondary spermatocytes
4. spermatids
5. sperm cells or spermatozoa, located in the lumen
b. embedded between the spermatogenic cells are sustentacular or Sertoli cells which extend from the basement membrane to the lumen
- tight junctions between these cells form the blood-testis barrier which prevents an immune response against the spermatogenic cells
- Sertoli cells perform several important functions:
1. support and protect developing spermatogenic cells
2. nourish spermatocytes, spermatids, and spermatozoa
3. phagocytize excess spermatid cytoplasm
4. control movements of spermatogenic cells and release of spermatozoa into the lumen
5. secrete fluid for sperm transport
6. secrete inhibin which decreases the rate of spermatogenesis
c. between the seminiferous tubules are clusters of Leydig cells or interstitial endocrinocytes which secrete testosterone
d. spermatogenesis:
- spermatogenesis takes about 65-75 days and involves the following sequence of events:
1. diploid spermatogonia, which develop from primordial germ cells, are stem cells which undergo mitosis
2. some of the daughter cells differentiate into diploid primary spermatocytes
3. primary spermatocytes undergo meiosis I which results in the formation of haploid secondary spermatocytes
4. the secondary spermatocytes undergo meiosis II which results in the formation of haploid spermatids which lie close to the lumen
5. spermatids undergo maturation, i.e., spermiogenesis, to form spermatozoa; the release of a spermatozoon from its connection to a Sertoli cell into the lumen is called spermiation
- sperm migrate from the seminiferous tubules to the ductus epididymis where they complete their maturation and become capable of fertilizing an ovum; sperm are also stored in the ductus (vas) deferens where they may retain their fertility for up to several months
- spermatozoa mature at a rate of about 300 million per day and, once ejaculated, have a life expectancy of about 48 hours within the female reproductive tract
- each spermatozoon has several regions:
1. head, which contains the nucleus and an acrosome
- the acrosome is a specialized lysosome that contains enzymes which aid penetration into the secondary oocyte (immature ovum)
2. tail, which consists of:
- neck, which contains centrioles
- middle piece, which contains numerous mitochondria that provide ATP for locomotion
- principle piece, which is the longest portion of the tail
- end piece, which is the terminal, tapering portion
4. Reproductive System Ducts in Males: (p. 843)
i. in the testis, spermatozoa and fluid produced by the Sertoli cells move through the following sequence of ducts:
a. seminiferous tubules
b. straight tubules
c. rete testis
d. efferent ducts
ii. efferent ducts carry the sperm out of the testis and into the tightly coiled ductus epididymis located within the epididymis (plural is epididymes)
a. the epididymis is a cup-shaped organ located along the posterior border of each testis
b. the epididymis has:
1. a large superior portion called the head where the efferent ducts join the ductus epididymis
2. narrow midportion called the body
3. smaller, inferior portion called the tail
c. the ductus epididymis is lined by pseudostratified columnar epithelium having stereocilia which increase the surface area for reabsorption of degenerated spermatozoa
- it is the site where sperm motility increases while sperm are being stored
- during ejaculation, the smooth muscle layers in the wall of the ductus epididymis undergo peristaltic contractions to propel sperm into the ductus (vas) deferens
iii. the ductus or vas deferens ascends from the tail of the epididymis along the posterior border of the epididymis, passes through the inguinal canal, and enters the pelvic cavity where it travels over the side and then down the posterior surface of the urinary bladder
a. the dilated terminal portion of this duct is called the ampulla
b. the ductus deferens is lined with pseudostratified columnar epithelium surrounded by three layers of smooth muscle tissue
c. it stores sperm and transports sperm from the epididymis into the ejaculatory duct during ejaculation by peristaltic contractions of its muscle layers
iv. the ductus deferens delivers sperm to the ejaculatory duct which is located posterior to the urinary bladder
a. each ejaculatory duct is formed by the union of the ductus deferens and the duct from the seminal vesicle
b. just prior to ejaculation, sperm and secretions of the seminal vesicles are ejected from the ejaculatory ducts into the prostatic urethra
v. the male urethra is a passageway for semen and urine; it consists of three regions:
a. prostatic urethra passes through the prostate gland
b. membranous urethra passes through the urogenital diaphragm, a muscular partition between the two ischial and pubic rami
c. spongy (penile) urethra passes through the corpus spongiosum of the penis and ends at the external urethral orifice
vi. alongside the ductus deferens, as it ascends through the scrotum, are:
1. testicular artery
2. autonomic nerves
3. veins that drain the testis
4. lymphatic vessels
5. cremaster muscle
- these structures constitute the spermatic cord
- the spermatic cord and ilioinguinal nerve pass through the inguinal canal which extends from the deep (abdominal) inguinal ring to the superficial (subcutaneous) inguinal ring
5. Accessory Sex Glands secrete most of the liquid portion of semen: (p. 847)
i. two seminal vesicles or seminal glands:
a. they are convoluted pouchlike structures located posterior to and at the base of the urinary bladder, and anterior to the rectum
b. they secrete an alkaline, viscous fluid that contains fructose, prostaglandins, and clotting proteins
c. this fluid is injected via two ducts into the ejaculatory ducts and it accounts for about 60% of the volume of semen
ii. single prostate (gland):
a. it is a doughnut-shaped gland located inferior to the urinary bladder and it surrounds the prostatic urethra
b. it secretes a milky, slightly acidic fluid that contains several acids and several enzymes
c. these secretions enter the prostatic urethra via many prostatic ducts and account for about 25% of the volume of semen
iii. two bulbourethral or Cowper’s glands:
a. they are pea-size glands located beneath the prostate gland on either side of the membranous urethra within the urogenital diaphragm
b. they secrete an alkaline fluid that contains mucus into the spongy urethra via two ducts
6. Semen: (p. 847)
i. semen is a mixture of sperm and seminal fluid, the secretions of the seminiferous tubules and accessory sex glands
ii. it has the following notable features:
a. the average volume that is ejaculated is 2.5 to 5 ml
b. the average number of sperm cells is 50 to 150 million sperm per ml
c. it provides sperm with a transportation medium and nutrients
d. it has a slightly alkaline pH of 7.2 to 7.7 which neutralizes the acidic environment in the male urethra and the vagina
e. it contains an antibiotic called seminalplasmin that destroys certain bacteria
7. Penis: (p. 848)
i. the urethra of the penis delivers semen into the vagina (and excretes urine)
ii. it consists of three regions:
a. root of the penis, which consists of three major components:
- bulb of the penis is the expanded base of the corpus spongiosum penis; it is attached to the inferior surface of the urogenital diaphragm and is enclosed by the bulbocavernosus muscle
- crura of the penis are the separated and tapered portions of the corpora cavernosa penis; each crus is attached to the ischial and inferior pubic rami and surrounded by the ischiocavernosus muscle
b. body of the penis, which contains three cylindrical masses of erectile tissue covered by fibrous tissue called the tunica albuginea; the three cylindrical masses are:
- two dorsolateral corpora cavernosa penis or corpora cavernosa
- one smaller midventral corpus spongiosum penis or corpus spongiosum which contains the spongy urethra
- sexual excitation causes large volumes of blood to enter their blood sinuses, resulting in erection which is a parasympathetic reflex
- prior to ejaculation, emission discharges a small volume of semen
- ejaculation, the propulsion of semen from the urethra to the exterior, is a sympathetic reflex
c. glans penis, which is the slightly enlarged distal end of the corpus spongiosum penis
- the margin of the glans penis is the corona
- the distal urethra enlarges within the glans penis and forms a terminal slitlike opening, the external urethral orifice
- encircling the glans penis is the loosely fitting prepuce or foreskin
iii. The weight of the penis is supported by the fundiform ligament and the suspensory ligament of the penis.
iv. the penis receives blood from the internal pudendal and femoral arteries; blood drains into corresponding veins