1625915643-5d53d156c9525bd62bd0d3434ecdc231 (843955), страница 95
Текст из файла (страница 95)
These include genderspecific changes in distribution of body and facial hair,breast development, body fat distribution, muscle development, and pitch of voice. Estrogen is primarily responsible for female characteristics, whereas testosterone isthe major stimulus for development of male secondarysexual characteristics.The linear growth spurt. This event is attributed mainlyto estradiol in both sexes; estrogenic hormones causeaccelerated growth of long bones but also promote theeventual closure of the epiphyseal plates of these bones,and thus the end of growth in height.THE MENSTRUAL CYCLE AND FEMALEREPRODUCTIVE ENDOCRINOLOGYIn late puberty, adolescent girls experience their first menstruation, an event known as menarche, and menstrual cyclescommence, continuing until menopause (unless interruptedby pregnancy).
These cycles last an average of 28 days andconsist of three phases (Fig. 31.3):■■■The follicular phase, characterized by proliferation ofthe endometrium of the uterus and development ofovarian follicles.The ovulatory phase, during which one follicle that hasfully matured ruptures and releases an ovum.The luteal phase, characterized by transformation of follicular cells into a corpus luteum and further proliferation of the endometrium.
Unless implantation of afertilized ovum takes place, the corpus luteum regressesand menses follows, during which the proliferated endothelium is sloughed off and bleeding occurs for a periodof 3 to 5 days.Follicular PhaseThe development of follicles within the ovary is illustrated inFigure 31.4. During each cycle, after the onset of menses (byconvention, “day 1” of the cycle), several primordial ovarianfollicles begin to undergo further development under theinfluence of FSH, and hence, the term follicular phase is usedto describe the first half of the cycle.
Within the developingfollicles, theca interna cells secrete androgens, which are converted to estradiol by the granulosa cells of the follicles. Thisconversion is stimulated by LH. Estradiol causes endometrialproliferation, as well as development of glands and growth ofspiral arteries within the endometrium, in preparation forpossible implantation of a fertilized egg. For this reason, thefollicular phase is also called the proliferative phase. In addition, estradiol promotes secretion of watery cervical mucus,through which sperm can enter the uterus. Ultimately, one ofthe developing follicles predominates and becomes a maturefollicle (graafian follicle), and the others regress.Ovulatory PhaseEstradiol exerts negative feedback on the hypothalamic secretion of GnRH and anterior pituitary secretion of FSH (LHis not suppressed during this period) through much of thefollicular phase.
Additionally, granulosa cells of developingfollicles secrete a peptide hormone, inhibin, which has negative feedback effects specifically on FSH. Toward the end ofthe follicular phase, estradiol rises to a level at which positivefeedback is triggered (Fig. 31.5). A surge in LH, and to alesser extent, FSH, takes place and produces ovulation atmidcycle, releasing a mature ovum, which is carried by ciliaryaction into the fallopian tube (see Fig. 31.3). Interestingly, amature ovum is produced in alternating ovaries from monthto-month, but if a woman has only one functional ovary,that one ovary will normally produce a mature ovummonthly.Luteal PhaseIn the ensuing luteal phase of the cycle, the ruptured follicleundergoes involution, forming the corpus luteum.
Progesterone and inhibin production by the corpus luteum rise, as doesestradiol production to a lesser degree. Estrogens, progesterone, and inhibin now contribute to negative feedback on thehypothalamus and anterior pituitary (see Fig. 31.5). Furtherproliferative and secretory changes take place in the endometrium, stimulated by progesterone; the luteal phase is alsocalled the secretory phase for this reason.
In the cervix, secretions become thicker, making passage of sperm into the uterusmore difficult. Conception must take place within a day ortwo of ovulation, because the ovum is viable for only a shortperiod after release from the graafian follicle (normally, conception takes place while the egg is in transport within a fallopian tube).
Toward the end of the luteal phase, unlesspregnancy occurs, steroid and inhibin secretion fall, andmenses results.Hormones of the Reproductive SystemFOLLICULAROVULATORYOvumLUTEALOvariancycleDeveloping folliclesMenstruationMaturefollicleRupturedfollicleCorpus luteumSecretoryProliferativeGlandSpiralarteryVenous lakesBleedingUterinecycleVeinBleeding4IU/LDays14Days4020LH020IU/LGonadotropic hormone levels6010FSH0ng/mL10Progesteronepg/mL0200100Estrogen040IU/LOvarian hormone levels2020Inhibin0Figure 31.3 Menstrual Cycle During the female menstrual cycle, changes take place in the ovariesand uterus, under the control of the hypothalamus and anterior pituitary gland. During the follicular phase,several primary follicles undergo further development in response to FSH and synthesize androgens, whichare converted to estradiol under the influence of LH.
Ultimately, one follicle fully matures and the othersregress. The uterine endometrium proliferates in response to estradiol. Near midcyle, estradiol rises to alevel that initiates positive feedback, and thus a surge in LH and FSH release by the anterior pituitary, whichresults in ovulation.
During the ensuing luteal phase, the mature follicle becomes the corpus luteum, whichsecretes progesterone and estradiol. The uterus undergoes further proliferative and secretory changes.Unless pregnancy occurs, endometrial sloughing and menstruation eventually occur, marking the beginningof a new cycle. FSH, follicle-stimulating hormone; LH, luteinizing hormone.28359360Endocrine PhysiologySuperficial (germinal)epithelium (cuboidalcells)OvumTunicaalbugineaEpithelialcordgrowing inZona pellucidaCorona radiataCumulus oöphorusPrimaryovaCortexFluid-filled follicularcavityPrimordialfolliclesGranulosaTheca internaTheca externaInfant ovaryRipening folliclePrimary follicleEpithelial cordPrimordial follicle growing inDeveloping folliclesSuperficial (germinal) epithelium(cuboidal cells)Mature(graafian)follicleBlood vesselsentering ovaryRuptured follicle(corpus hemorrhagicum)Corpus albicansDischargedovumMature corpus luteumFibrinBlood clotLuteal cellsEarly corpus luteumStages of ovum and follicleFigure 31.4 Ovary, Ova, and Follicles Until puberty, the ovary contains numerous primordial folliclesthat remain in a dormant state.
After puberty, several follicles begin ripening with each menstrual cycle, instages illustrated in the bottom panel. Only one follicle becomes a mature follicle; the others ultimatelyregress. After ovulation and release of the ovum, the mature follicle involutes to form the corpus luteum,which persists to the end of the cycle.CLINICAL CORRELATEThe Rhythm Method and Oral ContraceptivesThe average menstrual cycle is 28 days in length (beginning withthe first day of menses), but this may vary both between womenand for an individual woman. Ovulation takes place at aboutmidcycle, but more precisely, it occurs 14 days before the onset ofthe next menses.
In other words, in a 26-day cycle, ovulation takesplace on day 12, but in a 30-day cycle, ovulation takes place onday 16. Sperm are viable in the female reproductive tract for a fewdays, while mature ova are viable for only a short time afterrelease.For pregnancy to occur, intercourse needs to take place betweenapproximately 5 days before ovulation and at most a day afterovulation. However, because it is not possible to predict exactlywhen ovulation will take place (because the date of onset of thenext menses cannot be predicted with certainty), “rhythmmethods” of contraception have low reliability compared withmost forms of birth control.
In various rhythm methods, intercourse is avoided for several days before and after the predicteddate of ovulation. For example, in the Standard Days Method,women whose cycles are between 26 and 32 days in length avoidintercourse between days 8 and 19 of their cycles. In general, therhythm method has a failure rate of several percent annually whenused perfectly, and up to 25% otherwise.With the use of oral contraceptives, most commonly a combination of an estrogen and a progestin (progesterone-like drug),cycles are controlled by the drug. Typically, hormone-containingpills are taken for 21 days, followed by a week of no pills or dailysugar pills.
Pregnancy is prevented primarily by inhibition of ovulation, due to inhibition of gonadotropin release by the oral hormones. Bleeding (menstruation) occurs upon withdrawal fromthe hormones after 21 days. With perfect use, the annual rate ofconception is 0.3%, although the failure rate is actually severalpercent annually.Hormones of the Reproductive SystemCLINICAL CORRELATEAbnormal Uterine BleedingNormal menstrual bleeding follows the luteal phase of the cycleand is caused by the fall in gonadal hormone levels. Heavy bleeding during menses (menorrhagia) or irregular bleeding during thecycle may be caused by hormonal imbalance.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
