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Development of the Reproductive Systems
- Differentiation of female and male genitalia begins around 7 to 8 weeks of embryonic development, when the gonads of genetically male embryos begin to secrete male sex hormones, primarily testosterone. Until that time, the primitive reproductive organs of males and females are homologous (the same).
- The structure and function of both male and female reproductive systems depend on interactions among the central nervous system (hypothalamus), the endocrine system (anterior pituitary), the gonads (ovaries, testes), and the hypothalamic-pituitary-gonadal (H-P-G) axis. A set of complex neurologic and hormonal interactions accelerate at puberty and lead to sexual maturation and reproductive capability.
- Extrahypothalamic factors cause the hypothalamus to secrete gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to secrete gonadotropin follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that stimulate the gonads (ovaries and testes) to secrete female or male sec hormones. Paracrine hormones (inhibin, active, and follistatin) influence the positive and negative feedback loops that occur along the H-P-G axis.
- Production of primitive female gametes (ova) occurs solely during fetal life. From puberty to menopause, one female gamete matures per menstrual cycle. Production of the male gametes (sperm) begins at puberty; after that, millions are produced daily, usually for life.
The Female Reproductive System
- The function of the reproductive system is to produce mature ova and, when they are fertilized, to protect and nourish them through embryonic and fetal life and expel them at birth.
- The external female genitalia are the mons pubis, labia majora, labia minora, clitoris, vestibule (urinary and vaginal openings), Bartholin glands, and Skene glands. They protect body openings and may play a role in sexual functioning.
- The internal female genitalia are the vagina, uterus, fallopian tubes, and ovaries. Although all these organs are needed for reproduction, the ovaries are the most essential because they produce the female gametes and female sex hormones.
- The vagina is a fibromusclar canal that receives the penis during sexual intercourse and is the exit route for menstrual fluids and products of conception. The vagina leads from the introitus (its external opening) to the cervical portion of the uterus.
- The uterus is the hollow, muscular organ in which a fertilized ovum develops until birth. The uterine walls have three layers: the endometrium (lining), myometrium (muscle layer), and perimetrium (outer covering, which is continuous with the pelvic peritoneum). The endometrium proliferates (thickens) and sloughs off in response to cyclic changes in levels of female sex hormones. The cervix is the narrow, lower portion of the uterus that opens into the vagina.
- The two fallopian tubes extend from the uterus to the ovaries. Their function is to conduct ova from the spaces around the ovaries to the uterus. Fertilization normally occurs in the distal third of the fallopian tubes.
- From puberty to menopause, the ovaries are the site of (a) ovum maturation and release and (b) production of female sex hormones (estrogen, progesterone) and androgens. The female sex hormones are involved in sexual differentiation and development, the menstrual cycle, pregnancy, and lactation. Although they are primarily male sex hormones, androgens in women are precursors of female sex hormones and contribute to the prepubertal growth spurt, pubic and axillary hair growth, and activation of sebaceous glands.
- Estrogen (primarily estradiol) is produced by cells in the developing ovarian follicle (structure that encloses the ovum). Progesterone is produced by cells of the corpus luteum, the structure that develops from the ruptured ovarian follicle after ovulation (ovum releases). Androgens are produced within the ovarian follicle, adrenal glands, and adipose tissue.
- The average menstrual cycle lasts 27 to 30 days and consists of three phases, which are named for ovarian and endometrial changes: the follicular/proliferative phase, the luteal/secretory phase, and menstruation.
- Ovarian events of the menstrual cycle are controlled by gonadotropins and follicular secretion of inhibin. High follicle-stimulating hormone (FSH) levels stimulate follicle and ovum maturation (follicular phase); then a surge of luteinizing hormone (LH) causes ovulation, which is followed by development of the corpus luteum (luteal phase).
- Uterine (endometrial) events of the menstrual cycle are caused by ovarian hormones. During the follicular phase of the ovarian cycle, estrogen produced by the follicle causes the endometrium to proliferate (proliferative phase). During the luteal phase, estrogen maintains the thickened endometrium, while progesterone causes it to develop blood vessels and secretory glands (secretory phase). As the corpus luteum degenerates, production of both hormones drops sharply and the “starved” endometrium degenerates and sloughs off, causing menstruation.
- Cyclic changes in hormone levels also cause thinning and thickening of the vaginal epithelium, thinning and thickening of cervical secretions, and changes in basal body temperature.
The Male Reproductive System
- The function of the male reproductive system is to produce male gametes (sperm) and deliver them to the female reproductive tract.
- The external male genitalia are the testes, epididymides, scrotum, and penis. The internal genitalia are the vas deferens, ejaculatory duct, prostatic and membranous sections of the urethra, seminal vesicles, prostate gland, and Cowper glands.
- The testes (male gonads) are paired glands suspended within the scrotum. The testes have two functions: spermatogenesis (sperm production) and production of male sex hormones (androgens, chiefly testosterone).
- The epididymis is a long, coiled tube arranged in a comma-shaped compartment that curves over the top and rear of the testis. The epididymis receives sperm from the testis and stores them while they develop further. Sperm travel the length of the epididymis and then are ejaculated into the vas deferens, which transports sperm to the urethra.
- The scrotum is a skin-covered, fibromusclar sac that encloses the testes and epididymides, which are suspended within the scrotum by the spermatic cord. The scrotum keeps these organs at optimal temperatures for sperm survival (about 1° to 2° lower than body temperature) by contracting in cold environments and relaxing in warm environments.
- The penis is a cylindrical organ consisting of three longitudinal compartments (two corpora cavernosa and one corpus spongiosum) and the urethra. The urethra runs through the corpus spongiosum. The corpora cavernosa and corpus spongiosum consist of erectile tissue. Externally the penis consists of a shaft and a tip, which is called the glans.
- The penis has two functions: delivery of sperm and elimination of urine.
- Sexual intercourse is made possible by the erectile reflex, in which tactile or psychogenic stimulation of the parasympathetic nerves causes arterioles in the corpora cavernosa and corpus spongiosum to dilate and fill with blood, cause the penis to enlarge and become firm.
- Emission, which occurs at the peak of sexual arousal, is the movement of semen from the epididymides to the penis. Ejaculation, which is a continuation of emission, is the pulsatile ejection of semen from the penis.
- Spermatogenesis is a continuous process because spermatogonia, the primitive male gametes, undergo continuous mitosis within the seminiferous tubules of the testes. Some spermatogonia develop into primary spermatocytes, which divide meiotically into secondary spermatocytes and then spermatids. The spermatids develop into sperm with the help of nutrients and hormonal signals from Sertoli cells.
- Production of the male sex hormones (like production of the female sex hormones) is controlled by interactions among the hypothalamus, anterior pituitary, and gonads. The male hormones are produced steadily rather than cyclically, however.
Structure and Function of the Breast
- Until puberty, the female and male breasts are similar, consisting of a small, underdeveloped nipple, some fatty and fibrous tissue, and a few ductlike structures under the areola. At puberty, however, a variety of hormones (estrogen, progesterone, prolactin, growth hormone, insulin, cortisol) cause the female breast to develop into a system of glands and ducts that is capable of producing and ejecting milk.
- The basic functional unit of the female breast is the lobe, a system of ducts that branches from the nipple to milk-producing units called lobules. The lobules contain acini cells, which are convoluted spaces lined with epithelial cells, that contract moving milk into the system of ducts that leads to the nipple.
- Each breast contains 15 to 20 lobes, which are separated and supported by Cooper ligaments.
- Milk production occurs in response to prolactin, a hormone that is secreted in larger amounts after childbirth. Milk ejection is under the control of oxytocin, another hormone of pregnancy and lactation.
- During the reproductive years, breast tissue undergoes cyclic changes in response to hormonal changes of the menstrual cycle. At menopause, the tissue involutes, fat deposits and connective tissue increase, and the breasts reduce in size and firmness.
- Perimenopause is the transitional period between reproductive and nonreproductive years in women.
- Menopause the point that marks 12 consecutive months of amenorrhea.
- Andropause is androgen deficiency in the aging male.