chapter 20. endocrine regulation of reproduction 20-19

Chapter 20

Endocrine Regulation of Reproduction 20-19

Interactions Between the Hypothalamus, Anterior Pituitary, and Gonads Hypothalamus controls release of LH and FSH from anterior pituitary (Ant Pit) with GnRH LH and FSH stimulate production of sperm and eggs, and gonadal sex steroids; and maintain size of gonads Sex steroids provide negative feedback on hypothalamus and Ant Pit Gonads also secrete hormone inhibin which negatively feeds back on FSH secretion 20-20

Onset of Puberty FSH and LH secretion is high for 1st 6 months of life, but falls to very low levels until puberty At puberty hypothalamus increases GnRH secretion This stimulates increased LH and FSH Which stimulates sex steroid secretion Which drives changes in secondary sex characteristics and menarche (1st menstrual flow) Growth of pubic and axiliary hair is due to androgen secretion from adrenal cortex 20-21

Growth as a Function of Sex and Age At beginning of female puberty, high E stimulates growth High pubertal Testost. in boys causes growth spurt that lags that of girls Age of puberty in girls depends on % body fat and physical activity Girls with low body fat and high activity levels enter puberty at older age 20-22

Excitation phase (arousal) is characterized by increased muscle tone and engorgement of sexual organs with blood Includes nipples, clitoris, penis, and labia minor Vagina lubricates During plateau phase clitoris is partially hidden by swelling labia Areolae continue swelling Outer third of vagina engorges 4 Phases of Human Sexual Response 20-24

During orgasm, men ejaculate and women have analogous contractions of uterus and vagina In the resolution phase, body returns to preexcitation conditions Men enter refractory period 4 Phases of Human Sexual Response continued 20-25

Male Reproductive System 20-26

Male Reproductive System Testes contain seminiferous tubules (STs) where spermatogenesis occurs; and interstitial tissue housing Testost.-secreting Leydig cells Sertoli cells of STs contain receptors for FSH FSH stimulates spermatogenesis Leydig cells contain LH receptors LH stimulates secretion of Testost. 20-27

Control of LH and FSH Secretion Is by negative feedback Testost. inhibits LH and GnRH secretion but not FSH Inhibin from Sertoli cells provides negative feedback on FSH 20-28

Testosterone Derivatives Effects of Testost. in brain are mediated by its derivatives Testost. can be converted to DHT and others Or can be converted to estradiol by aromatase Estradiol mediates negative feedback effects of Testost. 20-29

Testosterone and Age Secretion of Testost. declines gradually and varyingly in men> 50 Causes are unknown Not due to low GnRH, LH, or FSH because their levels are elevated 20-30

Interactions Between the 2 Comparments of the Testes Testost. Secreted by Leydig cells stim. spermatogenesis in tubules Leydig cells may also secrete ACTH, MSH, and -endorphin Secretion of inhibin by tubules may affect sensitivity of Leydig cells to LH stim. 20-31

Spermatogenesis Germ cells that migrate from yolk sac during development become spermatogonia (stem cells) Spermatogonia replicate selves throughout life by mitosis Give rise to haploid sperm by meiosis 20-32

Spermatogenesis continued Occurs in wall of Seminiferous Tubules Spermatogonia and primary spermatocytes are located in outer part of ST Spermatids and mature spermatozoa are located toward lumen Tails of spermatozoa are in lumen 20-33

Spermiogenesis = maturation spermatids into spermatozoa Requires participation of Sertoli cells Most of their cytoplasm is pinched off and ingested by surrounding cytoplasm of Sertoli cells 20-34

Spermiogenesis continued During spermiogenesis, proteins called protamines replace the DNA-associated histones Cause extreme compaction of chromatin, changing nuclear shape Sertoli cells phagocytize cytoplasm of developing sperm A flagellum and acrosome (cap of digestive enzymes) develop At end of spermiogenesis, spermatozoa are released into the lumen 20-35

Sertoli Cell Function Autoimmune destruction of developing sperm is prevented by blood-testes barrier created by Sertoli cells and by Sertoli-secreted FAS ligand that triggers apoptosis of T lymphocytes Spermatogonia and developing spermatozoa are embedded in, and nurtured by, adjacent Sertolis 20-36

Sertolis secrete androgen-binding protein (ABP) into lumen of Seminiferous Tubules ABP binds testosterone, concentrating it in tubules FSH stimulates spermiogenesis through its receptors on Sertolis Sertolis provide negative feedback on FSH via production of inhibin Sertoli Cell Function continued 20-37

Have oval-shaped head that contains DNA and the acrosome (a cap of digestive enzymes) Have a midpiece and flagellar tail Tail will become motile in epididymus Spermatozoa 20-38

Hormonal Control of Spermatogenesis Formation of primary spermatocytes and entry into early prophase I, begin during embryonic development Spermatogenesis is then arrested until puberty Testosterone and its metabolites are required for completion of meiosis and spermatid maturation A number of paracrines may also be involved FSH is not essential for spermatogenesis It enhances Testost.-stimulated spermatogenesis And at puberty acts with Testost. to stimulate proliferation of Sertolis 20-39

Male Accessory Sex Organs Spermatozoa and fluids exit Seminiferous Ttubules via rete testis Pass through efferent ductules to epididymis and leave through vas deferens 20-40

Male Accessory Sex Organs continued Spermatozoa entering epididymis are non-motile and cannot fertilize In part because pH is low Spermatozoa mature and become motile in epididymis Prostatic fluid neutralizes pH during ejaculation 20-41

Male Accessory Sex Organs continued Vas deferens carries sperm into pelvic cavity Seminal vesicles add fluid (constituting 60% of ejaculate) to that coming from epididymis Contains fructose for energy for sperm Vas deferens becomes ejaculatory duct which merges with urethra in prostate 20-42

Male Accessory Sex Organs continued Fluid becomes semen when prostate adds secretions containing citric acid, Ca 2+ and coagulation proteins Which coagulate semen during ejaculation 20-43

Erection Is controlled by hypothalamus and sacral cord Occurs as result of Parasymp-induced blood flow into erectile tissues of penis NO is the neurotransmitter Erectile tissues include corpus cavernosum and spongiosum Venous outflow is partially occluded, aiding erection 20-44

Emission and Ejaculation Emission is movement of semen into urethra Ejaculation is forcible expulsion of semen from urethra out of penis Both are stimulated by sympathetic activity Which also causes peristalsis of tubular system, contractions of seminal vesicles, prostate, and muscles at base of penis 20-45

Male Fertility Normal volume of ejaculate is 1-5 ml With 60-150 million sperm/ml Sperm count of < 20 million/ml constitutes oligospermia Associated with decreased fertility Caused by heat, lead or arsenic poisoning, and drugs Including marijuana, cocaine, and anabolic steroids 20-46

Male Contraception Efforts have targeted gonadotropin secretion (with limited usefulness because of side effects) Gossypol interferes with sperm development (but also has side effects) Vasectomy is one of most widely used methods Each vas deferens is cut and tied Sperm cant enter urethra; accumulate in crypts along reproductive tract Where sperm are phagocytosed by immune system 70% develop anti-sperm antibodies 20-47

Female Reproductive System 20-48

Female Reproductive System Ovaries contain large number of follicles that produce female gametes (eggs or ova) in ovarian cycle Extensions of fallopian tubes called fimbriae partially cover each ovary The cilia of its lining draw in ovulated eggs 20-49

Female Reproductive System continued Uterus has 3 layers: Perimetrium is outer layer of connective tissue Myometrium is middle layer of smooth muscle Endometrium is hormonally-responsive inner epithelial layer that is shed during menstruation Between uterus and vagina is cervix 20-50

Accessory Sex Organs Consist of vagina, uterus, and fallopian tubes Vaginal opening is just posterior to urethra Both are covered by labia minora and majora Erectile clitoris is at anterior margin of labia minora 20-51

Production of Oocytes Germ cells that migrate into ovaries from yolk sac multiply so that at 5 mo. gestation, ovaries contain 6-7 million oogonia Production of oogonia then ceases forever Toward end of gestation, oogonia (now 1 o oocytes) begin meiosis but arrest in prophase I There is a continual loss of 1 o oocytes throughout life At birth are 2 million left At puberty 400,000 are left 400 oocytes are ovulated during reproductive years; rest undergo apoptosis 20-52

An Ovary Containing Follicles at Different Stages of Development Primary oocytes are contained in primary follicles Consisting of oocyte + follicle cells In response to FSH some follicles enter ovarian cycle and grow, producing layers of granulosa cells Some primary follicles continue, developing vesicles and becoming secondary follicles 20-53

Ovary continued One follicle continues growth Vesicles fuse, forming fluid-filled cavity called an antrum Is now called a graafian follicle 20-54

Ovarian Cycle continued As graafian follicle develops, primary oocyte completes meiosis I This is an asymmetric division because 1 daughter cell (the secondary oocyte) gets all cytoplasm Other daughter becomes a small polar body which will degenerate secondary oocyte arrests at metaphase II Only fertilized eggs complete meiosis II 20-55

Ovary continued Secondary oocyte is part of graafian follicle Granulosa cells form layer around outside of follicle Oocyte sits on mound in this layer called cumulus oophorus secondary oocyte is enclosed by ring of granulosa cells called corona radiata Between oocyte and radiata is gelatinous layer called zona pellucida Which forms barrier to sperm penetration 20-56

Ovary continued By 10-14 days after menstruation begins, only 1 follicle survives Others become atretic follicles (degenerate) Surviving graafian follicle forms bulge on surface of ovary Secretes increasing levels of Estrogen 20-57

Ovary continued Graafian follicle releases egg into fallopian tube at ovulation 20-58

Ovary continued If fertilized, oocyte completes meiosis II with formation of another polar body oocyte degenerates in 2 days if not fertilized 20-59

Ovary continued LH causes empty follicle to become corpus luteum (CL) Which secretes progesterone (P) and Estrogen If no fertilization, becomes corpus albicans (non- functional remnant) 20-60

Menstrual Cycle 20-61

Menstrual Cycle Is ~month-long cycle of ovarian activity seen in higher primates Characterized by shedding of endometrial lining and bleeding (menstruation) And sexual receptivity throughout cycle Other animals have estrous cycles in which there is no shedding of endometrium and receptivity is limited In estrous animals that bleed (dogs and cats), it is due to high Estrogen that accompanies receptive period 20-62

Menstrual Cycle continued In humans is ~28 days Day 1 is taken to be first day of menstruation Days 1 thru ovulation constitute the follicular phase Time from ovulation to menstruation is luteal phase Endometrial changes are called: menstrual, proliferative, and secretory phases 20-63

Follicular Phase Lasts from day 1 to about 13 Is dominated by growth and death of a cohort of primary follicles into secondary follicles With 1 survivor becoming a graafian follicle Which will undergo ovulation As follicles grow, secrete larger amounts of Estrogen Reaching peak about day 12 20-64

Follicular Phase continued Follicular growth and Estrogen secretion are dependent on FSH from Ant Pit FSH and Estrogen induce formation of FSH receptors in granulosa cells Causing follicles to become increasingly sensitive to same level of FSH At same time, FSH and Estrogen recruit LH receptors in graafian follicle 20-65

Follicular Phase continued Rapidly rising Estrogen secretion causes: Hypothalamus to increase pulses of GnRH And Ant Pit sensitivity to GnRH to increase, resulting in greater LH secretion A positive feedback between Estrogen and Ant Pit secretion develops Resulting in LH surge that peaks 16 hrs before ovulation And causes ovulation 20-66

The Cycle of Ovulation and Menstruation After ovulation, LH causes empty follicle to become the corpus luteum (CL) which secretes Estrogen and Progesterone 20-67

Luteal Phase continued Progesterone levels rise and peak about a week after ovulation Development of new follicles and another ovulation are inhibited by: High Progest. and Estrogen exert strong negative feedback on LH and FSH Inhibin from Corpus Luteum further suppresses FSH 20-68

With no fertilization, CL regresses Consequently Estrogen and Progesterone levels decline Causing menstruation and allowing new cycle of follicle development Luteal Phase continued 20-69

Cyclic Changes in the Endometrium Are driven by cyclic changes in Estrogen and Progesterone levels Proliferative phase occurs during follicular phase; Estrogen levels increase Estrogen stimulates growth of endometrial lining Spiral arteries develop Estrogen causes cervical mucus to become thin and watery to allow sperm penetration 20-70

Secretory phase occurs during luteal phase; endometrium becomes ready for implantation Progest. stimulates development of uterine glands Progest. and Estrogen cause endometrium to become thick, vascular, and spongy Progest causes cervical mucus to thicken and become sticky Cyclic Changes in the Endometrium continued 20-71

Menstrual phase results from drop in Progest and Estrogen following Corpus Luteum degeneration Low Progest. causes constriction of spiral arteries Blood flow stops, causing necrosis and sloughing of endometrium Cyclic Changes in the Endometrium continued 20-72

20-73 Endocrine Control of the Ovarian Cycle:

Factors Affecting Menstrual Cycle Release of GnRH is regulated not only by hormonal feedback but also by input from higher brain centers Olfactory system can send activity to hypothalamus in response to pheromones Can cause the dormitory effect (Ch 11) in which cycles of roommates become synchronized 20-74

Factors Affecting Menstrual Cycle continued Limbic system (involved in emotions; Ch 8) input to the hypothalamus in times of stress can cause functional amenorrhea (cessation of menstruation) Also occurs in thin or athletic females with low body weight Appears to be related to reduced leptin secretion by small adipocytes 20-75

Contraceptive Methods Oral contraceptive pills usually contain synthetic Estrogen and Progest. taken daily for 3 wks after menstrual period Mimic Corpus Luteum so that negative feedback inhibits ovulation Placebo pills are taken in 4 th wk to permit menstruation 20-76

Contraceptive Methods continued Rhythm method involves daily measurement of oral basal body temperature (BT) upon awakening Because ovarian steroids cause BT changes Declining Estrogen on day of LH surge causes a slight drop in BT Rising Progest. on day after LH peak causes elevated BT for rest of luteal phase 20-77

Menopause Is cessation of ovarian activity and menstruation at ~50 years Ovaries are depleted of follicles and thus produce no Estrogen LH and FSH are high because of no negative feedback Lack of Estrogen from ovaries is most responsible for hot flashes, osteoporosis, and increased risk of atherosclerosis 20-78

Fertilization, Pregnancy, and Parturition Once fertilization has occurred, secondary oocyte completes meiosis It then undergoes mitosis resulting in a structure called a blastocyst Cells of blastocyst secrete HCG to maintain corpus luteum of mother and prevents menstruation Birth is dependent upon strong contractions of the uterus, stimulated by oxytocin

Fertilization During intercourse, the male ejaculates an ave. of 300 million sperm into the vagina of the female During their passage through the female reprod. Tract, 10% gain ability to fertilize an oocyte; this ability is called capacitation In order for the sperm to become capacitated, must be in female tract for at least 7 hours Capacitated sperm are guided up to the fallopian tube toward the oocyte by chemotaxis and thermotaxis.

Fertilization continued Fertilization normally occurs in the fallopian tubes Each sperm contains a large, enzyme filled vesicle above its nucleus called an acrosome The interaction of sperm with particular molecules in the zona pellucida surrounding the oocyte triggers an acrosomal reaction, which stim. fusion of acrosome with cell membrane (Figs. 20.38 & 20.39) Within 12 hrs. after fertilization, nuclear membrane of ovum disappears and the 23 chromosomes in the ovum is joined with the 23 chromosomes from the sperm. Now its a single cell with 23 pairs of chromosomes called a zygote.

Cleavage, Formation of the Blastocyst, and Implantation The diploid zygote undergoes several mitotic divisions, called cleavage to become a morula (ball of cells) and then a blastocyst (hollow ball of cells) Implantation of the blastocyst in the endometrium begins between the 5 th and 7 th day The trophoblast cells of the blastocyst secrete HCG (human chorionic gonadotropin), which functions like LH and maintains mothers corpus luteum for 1 st 10 weeks Trophoblast cells provide fetal contribution to placenta

The Placenta The placenta is formed from the trophoblast cells of the fetus and the adjacent maternal tissue in the endometrium Oxygen, nutrients, and wastes are exchanged by diffusion between the fetal and maternal blood The umbilical cord contains the fetuss blood and carries it to and from the fetus and the placenta The placenta secretes hormones: hCS (chorionic somatomammotropin and steroid hormones hCS is similar to prolactin and GH in its actions The major steroid secreted is estriol

Labor and Parturition Contractions of the uterus in labor stim. by oxytocin from the post. pit. and by prostaglandins, prod. within uterus Androgens, primarily DHEAS, secreted by fetal adrenal cortex are converted into estrogen by placenta Estrogen secretion by the placenta induces oxytocin synthesis, enhances uterine sensitivity to oxytocin, and promotes prostaglandin synth. in the uterus. These events culminate in labor and delivery (parturition = childbirth)

Lactation High levels of estrogen during pregnancy, act synergistically with other hormones to stim. growth and development of mammary glands Prolactin and the prolactin-like effects of hCS stim. the prod. of milk proteins Prolactin secretion and action are blocked during pregnancy by high levels of estrogen from placenta After birth, estrogen levels fall, prolactin stim. milk prod. The milk-ejection reflex is a neuroendocrine reflex. Stimulus is suckling of infant---causing reflex secretion of oxytocin This stim. contractions of lactiferous ducts and ejection of milk from the nipple

chapter 20. endocrine regulation of reproduction 20-19

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