Knowledge of the functions of key hormones associated with reproduction is essential to an understanding of the female menstrual cycle. The following discussion centers on hormones that play a major role in the process of human reproduction.
HORMONES RELEASED BY THE HYPOTHALAMUS
Since hormones released by the hypothalamus stimulate the release of other hormones, they are termed “releasing factors.” Factor hormones act on the anterior pituitary and stimulate the release of hormones from the pituitary.
Releasing factors from the hypothalamus include the following:
• Gonadotropin-releasing hormone (GnRH): Stimulates the release of the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. These hormones are released when a decrease in ovarian hormones (estrogen and progesterone) is detected by the hypothalamus. In the premenopausal female, GnRH exerts an ovarian infl u- ence: it affects the cyclic process of follicular growth, ovulation, and maintenance of the corpus luteum. In the male, GnRH affects spermatogenesis, the process of meiosis in the testes to produce sperm cells.
• Corticotropin-releasing hormone (CRH): Regulates adre- nocorticotropic-stimulating hormone (ACTH) secre- tion by the anterior pituitary to activate the sympa- thetic nervous system. CRH is also released by the pregnant woman and her embryo soon after implanta- tion. CRH appears to provide a protective action by minimizing a maternal immunological rejection that could result in a miscarriage. Other effects of CRH relate to the woman’s response to stress.
• Growth hormone-releasing hormone (GH-RH): Stimu- lates the production and release of growth hormone (GH) by the anterior pituitary.
• Growth hormone-inhibiting hormone (GH-IH), also known as somatostatin: Inhibits the release of GH.
• Thyrotropin-releasing hormone (TRH): Regulates thyroid hormones (T3 and T4) by stimulating the ante- rior pituitary to release thyroid-stimulating hormone (TSH). TRH also stimulates the release of prolactin.
• Prolactin-inhibiting factor (PIF), also known as prolactostatin: Inhibits the synthesis and release of prolactin by the pituitary gland. Dopamine, another hormone released by the hypothalamus, also inhibits prolactin.
HORMONES RELEASED BY THE PITUITARY GLAND
The anterior pituitary produces the following hormones:
• Thyroid-stimulating hormone (TSH), also known as thyrotropin: Regulates the endocrine function of the thyroid gland.
• Adrenocorticotropic hormone (ACTH), also known as cor- ticotropin: Controls the development and functioning of
Skin Adipose tissue
Areola Nipple Lactiferous duct
Lactiferous glands
Intercostal muscles Pectoralis major Ribs
muscle
Figure 5-13 Mammary gland shown in a midsagittal section
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the adrenal cortex, including its secretion of glucocorti- coids and androgens (Venes, 2009).
• Prolactin (PRL): Stimulates the maturation of the mammary glands during pregnancy; initiates milk production and provides some inhibition to the stimulation of FSH and LH.
• Growth hormone (GH), also known as somatotropin:
Stimulates growth and cell reproduction (e.g., height growth during childhood) in humans. Growth hormone is also responsible for increased muscle mass, calcium retention and bone mineralization, the growth of various organ systems, protein syn- thesis, stimulation of the immune system, reduced uptake of glucose in the liver, and the promotion of lipolysis.
• Gonadotropins (gonad-stimulating hormones):
Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate and inhibit the ovaries. These two hormones help to regulate the menstrual cycle by producing a positive and negative feedback of estrogen and progesterone by the ovaries. The feedback systems stimulate the hypothalamic secretion of releasing hor- mones that act on the anterior pituitary gland.
The posterior pituitary releases oxytocin. Oxytocin stimulates uterine contractions and the release of milk from milk ducts in the breasts during lactation. A syn- thetic form of oxytocin can be administered during labor to enhance uterine contractions and after birth to pro- mote expulsion of the placenta and minimize uterine bleeding.
Collectively, the pituitary hormones are essential in the regulation of gonadal, thyroid, and adrenal function, lactation, body growth, and somatic development (Mos- kosky, 1995; Venes, 2009).
A & P review Hormonally Mediated Events and the Menstrual Cycle
The menstrual cycle is hormonally mediated through events that take place in the hypothalamus, anterior pitu- itary gland, and the ovaries. The hypothalamus stimu- lates the anterior pituitary gland to produce gonadotro- pin. FSH, one of these hormones, stimulates the growth and development of the graafi an follicle, which secretes estrogen. Estrogen stimulates proliferation of the endo- metrial lining of the uterus. After ovulation, the anterior pituitary gland secretes LH, which stimulates develop- ment of the corpus luteum. Progesterone secreted by the corpus luteum prompts further development of the lining of the uterus in preparation for the fertilized ovum.
When pregnancy does not occur, the corpus luteum degenerates, and the levels of estrogen and progesterone decline. The decreased levels of estrogen and progester- one cause the uterus to shed its lining during menstrua- tion. The decrease in estrogen and progesterone triggers a positive feedback to the hypothalamus, which stimu- lates the anterior pituitary gland to secrete FSH once again. A schema depicting the hormonal feedback mecha- nisms that regulate the menstrual cycle is presented in Figure 5-14. The interrelationships between the levels of hormone secretion, development of ovarian follicles, and changes in the uterine endometrium are presented in Figure 5-15. ◆
HORMONES RELEASED BY THE GONADS
The gonadal hormones are estrogen, progesterone, and testosterone. Estrogen and progesterone are primarily female hormones; testosterone is primarily a male hor- mone. These hormones are produced chiefl y by the gonads (ovaries and testes) and have important infl uences on sexual characteristics and the menstrual cycle. Fluctu- ating levels of estrogen and progesterone stimulate or suppress the hypothalamus or pituitary gland to release or cease releasing their hormones in a complex orchestration of events that regulate the menstrual cycle.
Estrogen
Estrogen, the primary female sex hormone, is present in high levels in women of childbearing age, and is also pres- ent in much smaller levels in men. In females, estrogen is responsible for development of the secondary sex charac- teristics (i.e., breast development, widening of the hips, deposition of fat in the buttocks and mons pubis). It also helps to regulate the menstrual cycle by stimulating pro- liferation of the endometrial lining in preparation for a pregnancy.
Progesterone
Progesterone also plays a role in regulation of the men- strual cycle. It decreases uterine motility and contractility (caused by estrogen) and prepares the uterus for implan- tation after fertilization. During pregnancy, progesterone readies the breasts for lactation.
Ovarian follicle
Figure 5-14 Hormonal feedback mechanisms that regulate the female menstrual cycle.
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126 unit two The Process of Human Reproduction
Testosterone
Testosterone, the primary male hormone, is produced by the testes (in men) and the ovaries (in women). Testoster- one is responsible for the development of the male genital tract and the secondary sex characteristics (body hair dis- tribution, growth and strength of the long bones, increase in muscle mass, deepening of the voice through enlarge- ment of the vocal cords). In both genders, testosterone enhances the libido, increases energy, boosts immune function, and helps protect against osteoporosis.
HUMAN CHORIONIC GONADOTROPIN, PROSTAGLANDINS, AND RELAXIN
Human chorionic gonadotropin (hCG) is an important hormone during early pregnancy. It is produced by the trophoblast (outermost layer of the developing blasto- cyst) and maintains the ovarian corpus luteum (remain- der of ovarian follicle after ovulation) by keeping levels of progesterone and estrogen elevated until the placenta has suffi ciently developed to assume that function.
Human chorionic gonadotropin may also play a role in limiting the maternal immune response to the preg- nancy. Serum or urine hCG levels are measured to diag- nose pregnancy.
Prostaglandins are unsaturated, oxygenated fatty acids classifi ed as hormones. Prostaglandins are found in many body tissues, occurring in high concentrations in the
female reproductive tract. Prostaglandins modulate hor- monal activity and have an effect on ovulation, fertility, and cervical mucus viscosity. Premenstrually, the release of prostaglandins in the uterus causes vasoconstriction and muscle contractions that lead to the tissue ischemia and pain associated with premenstrual syndrome (PMS).
During pregnancy, prostaglandins are believed to help maintain a reduced placental vascular resistance and most likely are involved in the biochemical process that initi- ates labor.
Relaxin is a hormone primarily produced by the cor- pus luteum, although the uterine decidua and the pla- centa are also believed to produce small amounts.
Relaxin may be detected in maternal serum by the time of the fi rst missed menstrual period. Although its role in pregnancy is not fully understood, relaxin aids in the softening and lengthening of the uterine cervix and works on the myometrial smooth muscle to promote uterine relaxation.
Now Can You— Discuss key hormones involved in reproduction?
1. Name two gonadotropins and describe their functions?
2. Explain the primary action of human chorionic gonadotropin (hCG)?
3. Identify one action of prostaglandins during pregnancy?
Endometrial cycle
Ovulation
2 4 6 8 12 14 16 20
Luteal phase Follicular phase
24 28
Menstrual phase
Follicle-stimulating hormone (FSH)
Blood levels:
Ovarian hormones Blood levels:
Gonadotropins
Estrogen Progesterone Ovary
Menstrual phase
Proliferative phase
Secretory phase
Ischemic phase Ovarian cycle
Primary follicle
Mature
follicle Ovulation
Corpus luteum
Degenerating corpus luteum Luteinizing hormone (LH)
2 4 6 8 12 14 16 20 24 28
Veins Spiral
artery Uterus
Pituitary
Figure 5-15 The female reproductive cycle. Levels of the hormones secreted from the anterior pituitary are shown relative to one another and throughout the cycle. Changes in the ovarian follicle are depicted. The relative thickness of the endometrium is also shown.
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