The Reproductive System

1. The Reproductive System The reproductive system consists of: 1- The primary sex organs, or gonads are the testes in males and the ovaries in females. The gonads produce sex cells, or gametes and secrete a variety of steroid hormones commonly called sex hormones. 2- The accessory reproductive organs :ducts, glands, and external genitalia . -Although male and female reproductive organs are quite different, their common purpose is to produce offspring.

2. A sperm and egg may fuse to form a fertilized egg, the first cell of the new individual, from which all body cells will arise. Once fertilization has occurred, the uterus provides the protective environment in which the embryo develops until birth. • Sex hormones–androgens in males and estrogens and progesterone in females–play vital roles both in the development and function of the reproductive organs and in sexual behavior and drives.

3. Anatomy of the Male Reproductive System • The sperm-producing testes or male gonads, lie within the scrotum. • From the testes, the sperm are delivered to the body exterior through a system of ducts including (in order) the epididymis, the ductus deferens, the ejaculatory duct, and finally the urethra, which opens to the outside at the tip of the penis. • The accessory sex glands, which empty their secretions into the ducts during ejaculation, are the seminal vesicles, prostate, and bulbourethral glands.

5. The ScrotumThe scrotum is a sac of skin ,covered with sparse hairs, and contains paired oval testes. A midline septum divides the scrotum, providing a compartment for each testis. Because viable sperm cannot be produced in abundance at core body temperature (37°C), the superficial location of the scrotum, which provides a temperature about 3°C lower, is an essential adaptation.

6. Furthermore, the scrotum responds to temperature changes: • When it is cold, the testes are pulled closer to the warmth of the body wall, and the scrotum becomes shorter and heavily wrinkled, increasing its thickness to reduce heat loss. • When it is warm, the scrotal skin is flaccid and loose to increase the surface area for cooling (sweating) and the testes hang lower, away from the body trunk.

8. The TestesEach testis is approximately 4 cm (1.5 inches) long and 2.5 cm (1 inch) in width and is surrounded by the tunica albuginea,the fibrous capsule of the testis. Septa extending from the tunica albuginea divide the testis into 250 to 300 wedge-shaped lobules ,each containing one to four tightly coiled seminiferous tubules, the actual “sperm factories.” Surrounding each seminiferous tubule are smooth muscle cells .By contracting rhythmically, these muscles squeeze sperm and testicular fluids through the tubules and out of the testes.

11. The seminiferous tubules of testis converge into the rete testis (re′te), a tubular network on the posterior side of the testis. From the rete testis, sperm leave the testis and enter the epididymis. •  Lying in the soft connective tissue surrounding the seminiferous tubules are the interstitial cells, also called Leydig cells .These cells produce androgens (testosterone). • Thus, the sperm-producing and hormone-producing functions of the testis are carried out by completely different cell populations.

13. testicular arteries, which branch from the abdominal aorta superior to the pelvis supply the testes. • The right testicular vein drains into the inferior vena cava ,the left drains into the left renal vein superiorly. •  The testes are served by both divisions of the autonomic nervous system. • Associated sensory nerves transmit impulses that result in agonizing pain and nausea when the testes are hit forcefully. • The nerve fibers are enclosed, along with the blood vessels and lymphatics, in a connective tissue sheath called the spermatic cord which passes through the inguinal canal .

15. The Male Duct SystemIn order (proximal to distal), the accessory ducts are :1-The EpididymisThe cup-shaped epididymis is about 3.8 cm long . Its head caps the superior aspect of the testis. Its body and tail are on the posterolateral area of the testis. It`s an uncoiled length is about 6 m. • The immature, nearly nonmotile sperm as it moves along its tortuous course (a trip that takes about 20 days), the sperm gain the ability to swim.

16. Sperm are ejaculated as the smooth muscle in the epididymis walls contracts, expelling sperm into the next segment of the duct system, the ductus deferens. • Sperm can be stored in the epididymis for several months, but if held longer, they are eventually phagocytized by epithelial cells of the epididymis.

17. 2-The Ductus Deferens and Ejaculatory DuctThe ductus deferens or vas deferens, is about 45 cm long. It runs upward as part of the spermatic cord through the inguinal canal into the pelvis .It then descends along the posterior bladder wall, then it joins with the duct of the seminal vesicle (a gland) to form the short ejaculatory duct. Each ejaculatory duct enters the prostate; there it empties into the urethra.

18. Its muscular layer is extremely thick and the duct feels like a hard wire when squeezed between the fingertips. At the moment of ejaculation, the thick layers of smooth muscle in its walls create strong peristaltic waves that rapidly squeeze the sperm forward along the tract and into the urethra. • Part of the ductus deferens lies in the scrotal sac. Some men opt to take full responsibility for birth control by having a vasectomy. Sperm are still produced, but they can no longer reach the body exterior. Eventually, they deteriorate and are phagocytized.

19. The UrethraThe urethra is the terminal portion of the male duct system. It conveys both urine and semen (at different times), so it serves both the urinary and reproductive systems. Its three regions are • the prostatic urethra, the portion surrounded by the prostate; • the membranous (or intermediate part of the) urethra in the urogenital diaphragm; and (3) the spongy (penile) urethra, which runs through the penis and opens to the outside at the external urethral orifice(about 15 cm long) .

20. Accessory GlandsThe accessory glands produce the bulk of semen (sperm plus accessory gland secretions).1-The Seminal VesiclesTwo fairly large, hollow glands , about the shape and length (5–7 cm) of a little finger on the posterior bladder surface. • Its secretion is a yellowish viscous alkaline fluid( 60% of the volume of semen) containing fructose sugar, ascorbic acid and prostaglandins, as well as other substances that enhance sperm motility or fertilizing power. • The duct of each seminal vesicle joins that of the ductus deferens on the same side to form the ejaculatory duct. Sperm and seminal fluid mix in the ejaculatory duct and enter the prostatic urethra together during ejaculation.

22. 2-The Prostate The prostate (pros′tāt) is a single doughnut-shaped. It encircles the urethra just inferior to the bladder. The prostatic gland secretion enters the prostatic urethra via several ducts when prostatic smooth muscle contracts during ejaculation. It plays a role in activating sperm and accounts for up to one-third of the semen volume. It is a milky, slightly acid fluid that contains citrate (a nutrient source), several enzymes and prostate-specific antigen (PSA).

23. 3-The Bulbourethral GlandsThe bulbourethral glands are pea-sized glands inferior to the prostate gland .They produce a thick, clear mucus, some of which drains into the spongy urethra when a man becomes sexually excited and neutralizes traces of acidic urine in the urethra. • Semen is a milky white, somewhat sticky mixture of sperm and accessory gland secretions. The liquid provides a transport medium and nutrients. Mature sperm contains little cytoplasm or stored nutrients. Catabolism of the fructose in seminal vesicle secretion provides nearly all the fuel needed for sperm ATP synthesis.

24. -Prostaglandins in semen decrease the viscosity of mucus guarding the entry (cervix) of the uterus and facilitate sperm movement through the female reproductive tract. -The relative alkalinity of semen as a whole (pH 7.2–7.6) helps neutralize the acid environment of the male’s urethra and the female’s vagina, thereby protecting the delicate sperm and enhancing their motility. -Sperms are very sluggish under acidic conditions (below pH 6).

25. - Semen also contains substances that suppress the immune response in the female’s reproductive tract and an antibiotic chemical called seminalplasmin, which destroys certain bacteria. - Clotting factors found in semen coagulate it just after it is ejaculated. Soon, its contained fibrinolysin liquefies the sticky mass, enabling the sperm to swim out and begin their journey through the female duct system. -The amount of semen propelled out of the male duct system during ejaculation is 2–5 ml, but there are between 20 and 150 million sperm per milliliter.

26. Spermatogenesis (sperm formation) -The process begins around the age of 14 years in males, and continues throughout life. Every day, a healthy adult male makes about 400 million sperm.    The normal chromosome number in most body cells is referred to as the diploid chromosomal number of the organism, symbolized as 2n. In humans, this number is 46, and such diploid cells contain 23 pairs of similar chromosomes called homologous chromosomes. One member of each pair is from the male parent (the paternal chromosome); the other is from the female parent (the maternal chromosome).

27. The number of chromosomes present in human gametes is 23, referred to as the haploid chromosomal number ,or n; gametes contain only one member of each homologous pair. When sperm and egg fuse, they form a fertilized egg that reestablishes the typical diploid chromosomal number of human cells.   Gamete formation in both sexes involves meiosis, a unique kind of nuclear division that occurs only in the gonads. • Meiosis consists of two consecutive nuclear divisions, and its product is four daughter cells instead of two, each with half as many chromosomes as typical body cells. Thus, meiosis reduces the chromosomal number by half (from 2n to n) in gametes.

30. Mitosis of Spermatogonia: Forming Spermatocytes The outermost tubule cells are stem cells called spermatogonia . -The spermatogonia divide more or less continuously by mitosis and, until puberty, all their daughter cells become spermatogonia. - Spermatogenesis begins during puberty, and from then on, each mitotic division of a spermatogonium results in two distinctive daughter cells–types A and B. The type A daughter cell remains at the basement membrane to maintain the germ cell line. -The type B cell gets pushed toward the lumen, where it becomes a primary spermatocytedestined to produce four sperms.

31. Meiosis: Spermatocytes to Spermatids Each primary spermatocytegenerated during the first phase undergoes meiosis I, forming two smaller haploid cells called secondary spermatocytes. The secondary spermatocytes continue on rapidly into meiosis II, and their daughter cells, called spermatids. Spermiogenesis: Spermatids to Sperm   Each spermatid has the correct chromosomal number for fertilization (n), but is nonmotile. It still must undergo a process called spermiogenesis, during which it elongates, sheds its excess cytoplasmic baggage, and forms a tail. The resulting sperm, or spermatozoon has a head, a midpiece, and a tail.

32. -The head of a sperm consists almost entirely of its flattened nucleus, which contains the compacted DNA. Adhering to the top of the nucleus is a helmetlikeacrosome . -Acrosome contains hydrolytic enzymes that enable the sperm to penetrate and enter an egg. - The sperm midpiece contains mitochondria spiraled tightly around the contractile filaments of the tail. -The long tail is a typical flagellum produced by the centriole near the nucleus. The mitochondria provide the metabolic energy (ATP) needed for the whiplike movements of the tail that will propel the sperm along its way in the female reproductive tract.

36. Hormonal Regulation of Male Reproductive FunctionThe Brain-Testicular AxisHormonal regulation of spermatogenesis and testicular androgen production involves interactions between the hypothalamus, anterior pituitary gland, and testes, a relationship sometimes called the brain-testicular axis.    The hypothalamus releases gonadotropin-releasing hormone (GnRH), which controls the release of the two anterior pituitary gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). (Both FSH and LH were named for their effects on the female gonad). 

37. - FSH stimulates spermatogenesis.  - LH stimulates the interstitial cells, prodding them to secrete testosterone (and a small amount of estrogen). • Locally, testosterone serves as the final trigger for spermatogenesis. Testosterone entering the bloodstream exerts a number of effects at other body sites(secondary male sex characters) . • Male secondary sex characteristicsare the features induced in the nonreproductiveorgans by the male sex hormones (mainly testosterone)–make their appearance at puberty.

39. These include the appearance of pubic, axillary, and facial hair, enhanced hair growth on the chest or other body areas in some men, and a deepening of the voice as the larynx enlarges. The skin thickens and becomes oilier (which predisposes young men to acne), bones grow and increase in density, and skeletal muscles increase in size and mass. -Testosterone is the basis of the sex drive (libido) in males and to some extent in females. -The testes are not the only source of androgens; the adrenal glands of both sexes also release androgens.

40. Female Reproductive SystemThe reproductive role of the female is far more complex than that of a male. Not only must she produce gametes, but her body must prepare to nurture a developing embryo for a period of approximately nine months. Ovaries, the female gonads, are the primary reproductive organs of a female, and like the male testes, ovaries serve a dual purpose: They produce the female gametes (ova) and sex hormones, the estrogens and progesterone .  The female’s accessory ducts, from the ovary to the body exterior, are the uterine tubes, the uterus, and the vagina.

42. The OvariesThe paired ovaries flank the uterus on each side .Shaped like an almond and about twice as large, each ovary is held in place within the peritoneal cavity by several ligaments. The ovarian ligament anchors the ovary medially to the uterus; the suspensory ligament anchors it laterally to the pelvic . In between , they are enclosed and held in place by a fold of peritoneum, the broad ligament.   The ovaries are served by the ovarian arteries, branches of the abdominal aorta and by the ovarian branch of the uterine arteries.

47. Embedded in the highly vascular connective tissue of the ovary cortex are many tiny saclike structures called ovarian follicles. Each follicle consists of an immature egg, called an oocyte surrounded by one or more layers of very different cells called follicle cells. Follicles at different stages of maturation are distinguished by their structure. • A primordial follicle, one layer of squamouslike follicle cells surrounds the oocyte. • A primary follicle has two or more layers of cuboidal or columnar-type cells enclosing the oocyte; it becomes a secondary follicle when fluid-filled spaces appear and then coalesce to form a central fluid-filled cavity called an antrum.

48. At the mature vesicular follicle, or Graafianfollicle (graf′e-an), stage, the follicle bulges from the surface of the ovary. • Each month in adult women, one of the ripening follicles ejects its oocyte from the ovary, an event called ovulation . • After ovulation, the ruptured follicle is transformed into a very different looking glandular structure called the corpus luteum(lu′te-um; plural: corpora lutea), which eventually degenerates. As a rule, most of these structures can be seen within the same ovary. • In older women, the surfaces of the ovaries are scarred and pitted, revealing that many oocytes have been released.

49. The Female Duct System1-The Uterine Tubes • The uterine tubes (u′ter-in), also called fallopian tubes or oviducts, form the initial part of the female duct system .They receive the ovulated oocyte and are the site where fertilization generally occurs. • Each uterine tube is about 10 cm (4 inches) long and extends medially from the region of an ovary to empty into the superolateral region of the uterus . • The distal end of each uterine tube expands as funnel-shaped infundibulum bearing ciliated, fingerlike projections called fimbriaethat partially surround the ovary.

50. Unlike the male duct system, which is continuous with the tubules of the testes, the uterine tubes have little or no actual contact with the ovaries. An ovulated oocyte is cast into the peritoneal cavity, and many oocytes are lost there. • The uterine tube performs complex movements to capture oocytes–it bends to drape over the ovary while the fimbriae stiffen and sweep the ovarian surface. The beating cilia on the fimbriae then create currents in the peritoneal fluid that tend to carry an oocyte into the uterine tube, where it begins its journey toward the uterus.