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Reproductive System

Anatomy of the Male Reproductive System

1. The paired testes, male gonads, reside in the scrotum outside the abdominopelvic cavity (Figure 16.1). Testes have both an exocrine (sperm-producing) and an endocrine (testosterone-producing) function.

2. The male duct system includes the epididymis, vas deferens, and urethra. Sperm maturation occurs in the epididymis. When ejaculation occurs, sperm are propelled through duct passageways to the body exterior.

3. Accessory glands of the male include the seminal vesicles, prostate gland, and bulbourethral glands. Collectively, these glands produce a fluid that activates and nourishes sperm. Semen is composed sperm (1%), seminal fluid (70%), prostatic fluid (25%), and bulbourethral fluid (4%).

4. External genitalia:

   1. Scrotum - skin sac that hangs outside the abdominopelvic cavity and provides the proper temperature for producing viable sperm.

   2. Penis - consists of three columns of erectile tissue surrounding the urethra. Erectile tissue provides a way for the penis to become rigid so it may better serve as a penetrating device during sexual intercourse.

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Male Reproductive Functions

1. Spermatogenesis (sperm production) begins at puberty in seminiferous tubules in response to FSH (Figure 16.2). Spermatogenesis involves meiosis, a special nuclear division that halves the chromosomal number in resulting spermatids. An additional process that strips excess cytoplasm from the spermatid, called spermiogenesis, is necessary for production of functional, motile sperm.

2. Testosterone production begins in puberty in response to LH (Figure 16.3). Testosterone is a hormone produced by interstitial (Leydig) cells of the testes. Testosterone causes appearance of male secondary sex characteristics and is necessary for sperm maturation.

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Anatomy of the Female Reproductive System

1. The ovaries, female gonads, are located against the lateral walls of the pelvis (Figure 16.4). They produce female sex cells (exocrine function) and hormones (endocrine function).

2. The duct system:

   1. Uterine (Fallopian) tubes extend from the vicinity of an ovary to the uterus. Ends are fringed and "wave" to direct ovulated oocytes into uterine tubes, which conduct the oocyte (embryo) to the uterus by peristalsis and ciliary action.

   2. The uterus is a pear-shaped organ in which the embryo implants and develops. Its mucusa (endometrium) sloughs off each month in menses unless an embryo has become embedded in it. The myometrium contracts rhythmically during the birth of a baby.

   3. The vagina is a passageway between the uterus and the body exterior that allows a baby or the menstrual flow to leave the body. It also receives the penis and semen during sexual intercourse.

3. External genitalia of the female include labia majora and minora (skin folds), clitoris, and urethral and vaginal openings, collectively called the vulva (Figure 16.5).

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Female Reproductive Functions and Cycles

1. Oogenesis (production of female sex cells) occurs in ovarian follicles, which are activated at puberty by FSH and LH to mature and eject oocytes (ovulation) on a cyclic basis (Figure 16.6). The female egg (ovum) is formed only if sperm penetrates the secondary oocyte. In females, meiosis produces only one functional ovum (plus three nonfunctional polar bodies) as opposed to the four functional sperm per meiosis produced by males.

2. Hormone production: Estrogens are produced by ovarian follicles in response to FSH. Progesterone, produced in response to LH, is the main hormonal product of the corpus luteum. Estrogens stimulate development of female secondary sex characteristics.

3. The menstrual cycle concerns changes in the endometrium in response to fluctuating blood levels of ovarian hormones (Figure 16.7). The phases are

   1. Menses. Endometrium sloughs off and bleeding occurs. Ovarian hormones are at their lowest levels.

   2. Proliferative phase. Endometrium is repaired, thickens, and becomes well vascularized in response to increasing levels of estrogens.

   3. Secretory phase. Endometrial glands begin to secrete nutrients, and lining becomes more vascular in response to increasing levels of progesterone. If fertilization does not occur, the phases are repeated about every 28 days.

Mammary Glands

1. Mammary glands are milk-producing glands found in the breasts. They produce milk after birth of a baby in response to stimulation by the hormone prolactin.

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Survey of Pregnancy and Embryonic Development

1. An oocyte is fertilizable for up to 24 hours; sperm are viable within the female reproductive tract for up to 72 hours. Hundreds of sperm must release their acrosomal enzymes to break down the egg's corona radiata.

2. Following sperm penetration, the secondary oocyte completes meiosis II. Then ovum and sperm nuclei fuse (fertilization), forming a zygote (Figure 16.8).

3. If fertilization occurs, embryonic development begins immediately. Cleavage, a rapid series of mitotic divisions without intervening growth, begins with the zygote and ends with a blastocyst.

4. By day 14 after ovulation, the young embryo (blastocyst) has implanted in the endometrium, and the placenta is being formed. HCG released by the blastocyst maintains hormone production of the corpus luteum, preventing menses, until the placenta assumes its endocrine role.

5. The placenta serves respiratory, nutritive, and excretory needs of the embryo and produces hormones of pregnancy.

6. All major organ systems have been laid down by 8 weeks, and at 9 weeks the embryo is called a fetus. Growth and tissue/organ specialization are the major events of the fetal period.

7. A pregnant woman has increased respiratory, circulatory, and urinary demands placed on her system by the developing uterus. Good nutrition is necessary to produce a healthy baby.

8. Childbirth (parturition) includes a series of events called labor. It is initiated by several factors, but most importantly by rising levels of oxytocin and prostaglandins, which promote vigorous uterine contractions. The three stages of labor are dilation, expulsion, placental.