Physiological Actions Of Ovarian Steroid Hormones

Bump To Birth

Natural Childbirth Options

Get Instant Access

As described above, intraovarian actions of estradiol and progesterone are intimately connected to ovulation and formation of the corpus luteum. In general, extra-ovarian actions of these hormones ensure that the ovum reaches its potential to develop into a new individual. Ovarian steroids act on the reproductive tract to prepare it for fulfilling its role in fertilization, implantation, and development of the embryo, and they induce changes elsewhere that equip the female physically and behavior-ally for conceiving, giving birth, and rearing the child. Although estrogens, perhaps in concert with progesterone, drive females of subprimate species to mate, andro-gens, rather than estrogens, are responsible for libido in humans of either sex. Estrogens and progesterone tend to act in concert and sometimes enhance or antagonize each other's actions. Estrogen secretion usually precedes progesterone secretion and primes the target tissues to respond to progesterone. Estrogens induce the synthesis of progesterone receptors. Without estrogen priming,

46. Hormonal Control of Reproduction in the Female: The Menstrual Cycle TABLE 1 Effects of Estrogen and Progesterone on the Reproductive Tract

Organ

Estrogen

Progesterone

Oviducts Lining

Muscular wall Uterus

Endometrium Myometrium Cervical glands Vagina

" Cilia formation and activity " Contractility

" Proliferation " Growth and contractility Watery secretion

" Epithelial proliferation " Glycogen deposition

" Secretion

# Contractility

" Differentiation and secretion

# Contractility Dense, viscous secretion

" Differentiation

# Proliferation progesterone has little biological effect. Conversely, progesterone downregulates its own receptors and estrogen receptors in some tissues and thereby decreases responses to estrogens.

Effects on the Reproductive Tract

At puberty estrogens promote growth and development of the oviducts, uterus, vagina, and external genitalia. Estrogens stimulate cellular proliferation in the mucosal linings as well as in the muscular coats of these structures. Even after they have matured, maintenance of size and function of internal reproductive organs requires continued stimulation by estrogen and progesterone. Prolonged deprivation after ovariectomy results in severe involution of both muscular and mucosal portions. Dramatic changes are also evident, especially in the mucosal linings of these structures, as steroid hormones wax and wane during the reproductive cycle. These effects of estrogen and progesterone are summarized in Table 1.

Menstruation

Nowhere are the effects of estrogen and progesterone more obvious than in the endometrium. Estrogens secreted by developing follicles increase the thickness of the endometrium by stimulating growth of epithelial cells in terms of both number and height. Endometrial glands form and elongate. Endometrial growth is accompanied by increased blood flow, especially through the spiral arteries, which grow rapidly under the influence of estrogens. This stage of the uterine cycle is known as the proliferative phase and coincides with the follicular phase of the ovarian cycle. Progesterone secreted by the corpus luteum causes the newly proliferated endometrial lining to differentiate and become secretory. This action is consistent with its role of preparing the uterus for nurture and implantation of the newly fertilized ovum if successful mating has occurred.

The so-called uterine milk secreted by the endometrium is thought to nourish the blastocyst until it can implant. This portion of the uterine cycle is called the secretory phase and coincides with the luteal phase of the ovarian cycle (Fig. 9).

Maintaining the thickened endometrium depends on the continued presence of the ovarian steroid hormones. After the regressing corpus luteum loses its ability to produce adequate amounts of estradiol and progesterone, the outer portion of the endometrium degenerates and is sloughed into the uterine cavity. The mechanism for shedding the uterine lining is incompletely understood, although prostaglandin F2„ appears to play an important role, perhaps in producing vascular spasm and ischemia and in stimulating release of lysosomal proteases. Loss of the proliferated endometrium is accompanied by bleeding. This monthly vaginal discharge of blood is known as menstruation. The typical menstrual period lasts 3 to 5 days and the total flow of blood seldom exceeds 50 ml. The first menstrual bleeding, called menarche, usually occurs at about age 13. Menstruation continues at monthly intervals until menopause, normally interrupted only by periods of pregnancy.

In the myometrium, estrogen increases expression of contractile proteins, gap junction formation, and spontaneous contractile activity. In its absence, uterine muscle is insensitive to stretch or other stimuli for contraction. Further estrogen increases the irritability of uterine smooth muscle and, in particular, increases its sensitivity to oxytocin, in part as a consequence of inducing uterine receptors for oxytocin (see Chapter 47). The latter phenomenon may be of significance during parturition. Progesterone counteracts these effects and decreases both the amplitude and frequency of spontaneous contractions. Withdrawal of progesterone prior to menstruation is accompanied by increased myometrial prostaglandin formation. Myometrial contractions in response to prostaglandins are thought to account for the discomfort that precedes menstruation.

FIGURE 9 Endometrial changes during a typical menstrual cycle. Simultaneous events in the ovary are also indicated. The endometrium thickens during the follicular phase; uterine glands elongate, and spiral arteries grow to supply the thickened endometrium. During the early luteal phase there is further thickening of the endometrium, marked growth of the coiled arteries, and increased complexity of the uterine glands. As the corpus luteum wanes, the endometrial thickness is reduced by loss of ground substance. Increased coiling of spiral arteries causes ischemia and finally sloughing of endometrium. (From Bartelmez GW. Am J Obstet Gynecol 1957; 74:931. With permission.)

FIGURE 9 Endometrial changes during a typical menstrual cycle. Simultaneous events in the ovary are also indicated. The endometrium thickens during the follicular phase; uterine glands elongate, and spiral arteries grow to supply the thickened endometrium. During the early luteal phase there is further thickening of the endometrium, marked growth of the coiled arteries, and increased complexity of the uterine glands. As the corpus luteum wanes, the endometrial thickness is reduced by loss of ground substance. Increased coiling of spiral arteries causes ischemia and finally sloughing of endometrium. (From Bartelmez GW. Am J Obstet Gynecol 1957; 74:931. With permission.)

Effects on the Mammary Glands

Development of the breasts begins early in puberty and is due primarily to estrogen, which promotes development of the duct system and growth and pigmentation of the nipples and areolar portions of the breast. In cooperation with progesterone, estrogen may also increase the lobuloalveolar portions of the glands, but alveolar development also requires the pituitary hormone prolactin (see Chapter 47). Secretory components, however, account for only about 20% of the mass of the adult breast. The remainder is stromal tissue and fat. Estrogen also stimulates stromal proliferation and fat deposition.

Other Effects of Ovarian Hormones

Estrogen also acts on the body in ways that are not necessarily related to reproduction. As already indicated (see Chapter 44), it contributes to the pubertal growth spurt and its termination by directly stimulating growth of cartilage progenitors, increased growth hormone secretion, and closure of the epiphyseal plates. In adolescent females and males, estrogens increase bone density, and in the adult they contribute to the maintenance of bone density by stimulating osteoblastic activity and inhibiting bone resorption. Estrogen can also cause selective changes in bone structure, especially widening of the pelvis, which facilitates passage of the infant through the birth canal. It promotes deposition of subcutaneous fat, particularly in the thighs and buttocks, and increases hepatic synthesis of steroid- and thyroid-hormone-binding proteins. Based on epidemiological evidence, estrogens are considered to have cardiovascular protective effects. Indeed, estrogen receptors are expressed in vascular smooth muscle and cardiac myocytes, and estrogens have been found to alter plasma lipid profiles. Estradiol also acts on various cells in the central nervous system and is responsible for some behavioral patterns, especially in lower animals.

Clinical Note

Treatment of Neoplastic Breast Disease

Responsiveness of breast tissue elements to growth-promoting effects of estrogen is of significance in treatment of neoplastic breast disease. Some forms of breast cancer continue to express estrogen receptors and remain partially or completely dependent on estrogens for growth. These cancers are sensitive to treatment with estrogen antagonists, such as tamoxifen, or with a newer class of drugs (aromatase inhibitors) that block the activity of the enzyme P450arom and therefore starve these estrogen-dependent tumors of estrogens.

Was this article helpful?

0 0
Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

Get My Free Ebook


Post a comment