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Endocrine problems involving thereproductive system are relatively common. For example,infertility, often a result of an endocrine abnormality, affectsapproximately 10% of the population.FETAL DEVELOPMENT OF THE REPRODUCTIVEORGANS AND DIFFERENTIATION OF GENITALIAAn individual’s sex can be characterized in terms of genetics(the male XY genotype vs. the female XX genotype), gonads(testes vs. ovaries), and phenotype (external appearance of“maleness” vs. “femaleness”). Although genetic sex is determined at the time of conception, the fetus remains undifferentiated in terms of gonadal sex for the first 5 weeks ofdevelopment (Fig.
31.1).Development of Gonadal SexUnder the control of the SRY gene on the Y chromosome, theundifferentiated gonads develop into testes; full developmentof gonadal sex requires several other genes on the Y chromosome and various autosomal chromosomes. By 8 to 9 weeksof development, the Leydig cells of the testes begin to secretetestosterone. Development of ovaries, on the other hand,begins at week 9 of gestation and requires the presence of twoX chromosomes. Germ cells within the ovaries give rise tooogonia, which proliferate and soon enter meiosis, leading tothe primary oocyte stage, at which meiosis is arrested untilactivated during sexual cycles after puberty. The developingovary produces the estrogens.During the undifferentiated stage of fetal development, twopairs of ducts, the wolffian ducts and the müllerian ducts,develop in both sexes (see Fig. 31.1). Under the influence oftestosterone, the wolffian ducts develop into male structuresincluding the epididymis, vas deferens, and seminal vesicles.Meanwhile, müllerian inhibitory factor, a dimeric glycopro-Synthesis and secretion of testosterone by the fetal testesis stimulated initially by a placental hormone known aschorionic gonadotropin, the same hormone that is essential inmaintaining maternal ovarian progesterone production duringpregnancy.
After the first trimester, fetal pituitary luteinizinghormone (LH) is required to maintain androgen production ina male fetus. Similarly, in a female fetus, estrogens are secretedby the ovaries in response to pituitary LH. After birth, the pituitary gonadotrophs become quiescent until puberty. The role ofgonadotropins in pubertal development, reproductive function,and pregnancy is discussed below.tein hormone secreted by the Sertoli cells of the fetal testes,causes regression of the müllerian ducts.
The combination ofthese two series of events leads to the development of the malesystem. In the absence of testes and their secretions, the müllerian ducts persist and develop, forming the fallopian tubes,uterus, and the upper vagina, whereas the wolffian ductsdegenerate.Development of Genital SexThe external genitalia are also present in an undifferentiatedform during early development (Fig.
31.2). Differentiationbegins at 9 to 10 weeks of gestation. In the absence of androgens, female genitalia are produced. In the male fetus, testosterone produced by the testes and secreted into the circulationis converted to dihydrotestosterone (DHT) in the primitivegenital structures; DHT stimulates the formation of themale genitalia (conversion of testosterone to DHT is notinvolved in the actions of testosterone on wolffian ductdifferentiation).Phenotypic SexPhenotypic sex is the outward appearance of maleness orfemaleness. Whereas genetic sex is based on the presence ofthe XY or XX genotype, and gonadal sex is determined byspecific genes on the X chromosome as well as several autosomal genes, phenotypic sex develops in response to gonadalhormones. In the presence of testosterone, male genitaliadevelop; in its absence, the female form of genitalia prevail.Other phenotypic characteristics of femaleness are promoted356Endocrine PhysiologyUndifferentiatedGonadsMesonephros(wolffian body)Mesonephric(wolffian) ductsMüllerian ductsTestosterone from the fetaltestes acts locally on thewolffian ducts causing themto persist and differentiate.Müllerian-inhibiting factor,also secreted by the fetaltestes, causes degenerationof the müllerian ducts.Bladder(pulled aside)Urogenital sinusFemaleMaleOvaryTestisWolffian ductdegeneratesandmüllerian ductpersistsin absenceof testosteroneDegeneratingmüllerian ductPersistentwolffian duct(vas deferens)Fallopian tubeGartner's ductEpoöphoronAppendixvesiculosaParoöphoronOvaryUterusRound ligUpper vaginaWolffian ductremnantUrethraLower vaginaSkene's ductBartholin's glandVas deferensSeminal vesicleProstatic utricleProstate glandBulbourethralglandVas deferensAppendixepididymidisAppendix testisEpididymisVasa efferentiaTestisGubernaculumFigure 31.1 Gonad and Genital Duct Formation The undifferentiated gonads and ducts of theearly embryo differentiate into the male or female gonads and duct systems under the influence of variousproducts encoded by the X and Y chromosomes.
Notably, a product of the SRY gene of the Y chromosomeresults in differentiation of the gonads into testes. Production of testosterone by the testes results in persistence and differentiation of the wolffian ducts. Müllerian-inhibiting factor secreted by the testes causesmüllerian duct degeneration. In the female fetus, in the absence of testosterone the gonads develop intoovaries and the wolffian ducts degenerate.by the absence of androgens and presence of estrogen. Inandrogen insensitivity syndrome, for example, androgenreceptors are absent, and the female phenotype prevails.PUBERTYBeginning a year or two before the onset of puberty, production of androgens by the zona reticularis of the adrenal glandscommences.
The major adrenal androgens are dehydroepiandrosterone (DHEA) and androstenedione. This event isknown as adrenarche and is distinct from puberty. Secretionof adrenal androgens results in:■■■appearance of pubic and axillary hairadult body odoracne and oiliness of skinHormonal Regulation of PubertyThe actual process of puberty is associated with maturationof the hypothalamus and anterior pituitary gland. The hypo-Hormones of the Reproductive System357Genital tubercleUndifferentiatedGlans areaEpithelial tagUrethral foldUrethral grooveLateral buttress(labioscrotal swelling)Anal tubercleAnal pitTestosterone5α-ReductaseDihydrotestosteroneFemaleGlansEpithelial tagBody of clitorisUrethral foldsMaleGlansEpithelial tagBody of penisUrethral slitUrethral folds fusingUrogenital slitPenoscrotal raphéLabioscrotal swellingAnal tubercleAnal tubercleAnusAnusUrethral orificeGlans penisBody of clitorisPrepucePrepuceBody of penisGlans clitorisUrethral orificeRaphéof penisLabium minusScrotumLabium majusPerineal raphéVaginaPerineal raphéPerianal tissues,includingexternal sphincterAnusPerianal tissues,including externalsphincterAnusFigure 31.2 Differentiation of the External Genitalia In the absence of testosterone, the undifferentiated external genitalia develop into the female structures.
Testosterone, after conversion to dihydrotestosterone, stimulates the formation of male external genitalia from the undifferentiated structures.Homologies between male and female genital structures are color-coded.thalamic secretion of the decapeptide gonadotropin-releasinghormone (GnRH) increases and becomes pulsatile, as thehypothalamus becomes less sensitive to negative feedback bysex steroids.
Thus, a pulsatile pituitary secretion of the gonadotropins follicle-stimulating hormone (FSH) and luteinizinghormone (LH) is established, stimulating gonadal maturationand sex hormone synthesis. Estradiol (the major estrogen or“estrogenic hormone”) and progesterone are the predomi-nant sex hormones produced in women, whereas testosteroneis the main sex steroid of men (the details of the hypothalamicpituitary-gonadal axis are discussed later in the chapter).Adrenal androgens continue to have a physiological role inpuberty and beyond in women (effects of adrenal androgensare overshadowed by those of testosterone in men).
In males,some testosterone is converted to estradiol, and the latterhormone has important functions in male puberty. The358Endocrine PhysiologyThe pubertal growth spurt typically begins earlier in girlsthan in boys. Previously, it was believed that androgens(testosterone and adrenal androgens) were responsible for therapid increase in height. Recent studies, however, have demonstrated that although testosterone increases bone mass anddensity, estradiol is the hormone primarily responsible for thegrowth spurt in both sexes. Estradiol stimulates long bonegrowth but also promotes closure of the growth plates. Females,who enter puberty earlier, attain less adult height than males.physiological and anatomical changes occurring at pubertyare the result of various hormone actions and include:■■■■Gonadal maturation and sex steroid synthesis andrelease, as a result of increased gonadotropin secretion.Maturation of other sex organs and genitalia, mainlystimulated by estradiol in females and testosterone inmales.Development of secondary sexual characteristics (characteristics of “maleness” or “femaleness” not directlyassociated with reproduction).
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