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Unit 8 - Reproductive System - Full Textbook Notes
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Department
Biology
Course
BIOL 373
Professor
Heidi Engelhardt
Semester
Summer

Description
Unit 8: Reproductive System  Puberty: the period when a person makes the transition from being non-reproductive to being reproductive  Humans have internal fertilization, mating and courtship rituals, internal development within the uterus (which provides protection for the growing embryo) and we are sexually dimorphic  Human fetuses are exposed to sex hormones while in the uterus, but it is unclear how much influence these hormones have on behavior later in life  Gametes form a fertilized egg (zygote); the zygote begins to divide and becomes an embryo (0-8 weeks), then a fetus (8 weeks until birth) Sex Determination  Sex organs consist of 3 sets of structures: o Gonads: the organs that produce gametes (eggs and sperm)  The male gonads are testes, which produce sperm  The female gonads are ovaries, which produce eggs or ova  Undifferentiated gonadal cells are germ cells o Internal genitalia consists of accessory glands and ducts that connect the gonads with the outside environment o External genitalia includes all external reproductive structures  46 = 2n = diploid o 22 matched homologous pairs or autosomes > direct development of the human body and characteristics o One pair of sex chromosomes > direct development of internal and external sex organs o zygotes  23 = n = haploid o Include eggs and sperm  There must always be an X chromosome for an individual to live  XO = turner’s syndrome  Barr body: the inactivated X chromosome in each female cell; random th  Reproductive structures do not begin to differentiate until the 7 week of development [figure 26.2] o Before, the embryonic tissues are considered bipotential because they cannot be morphologically identified  The bipotential gonad has an outer cortex and an inner medulla  the medulla will develop into testes OR in the absence of that signal, the cortex will differentiate into ovarian tissue  the internal genitalia consists of 2 pairs of accessory ducts: 1. Wolffian ducts derived from the embryonic kidney 2. Müllerian ducts  One pair will develop and the other will disintegrate  External genitalia consist of a genital tubercle, urethral folds, urethral groove, and labioscrotal swellings  Sex determination depends on the sex-determining region of the Y chromosome (SRY gene)  Male Embryonic Development o SRY gene produces protein (testis-determining factor or TDF) that binds with DNA and activates additional genes, including SOX9, WT1 (Wilms’ tumor protein), and SF1 (steroidogenic factor) o The products of these genes direct development of the gonadal medulla into a testis (does not require testosterone) o Once differentiation occurs, testes secrete 3 hormones that influence development:  Anti-Müllerian hormone (AMH) is secreted in testicular sertoli cells  Causes the embryonic Müllerian ducts to regress  Testosterone and its derivative dihydrotestosterone (DHT) is secreted by Leydig cells  Testosterone and DHT are the dominant steroid hormones in males; they both bind to the same androgen receptor, but the two ligands elicit different responses  Testosterone converts the Wolffian ducts into male accessory structures: epididymis, vas deferens, and seminal vesicle; later in fetal development it will control migration of the testes from the abdomen into the scrotum, or scrotal sac  Remaining sex characteristics are controlled by DHT (ie. External genitalia)  Exposure of nongenital tissues to testosterone have masculinising effects (ie. Sexual behaviour and gender identity))  In pseudohermaphrodites, men inherit a defective gene for 5α-reductase, which catalyzes the conversion of testosterone to DHT; as a result, male external genitalia and prostate gland fail to develop fully during fetal development o At birth infants appear to be female o At puberty however, the testes begin to secrete testosterone, causing masculinisation of the external genitalia, pubic hair growth, and deepening voice  Female embryonic development o The cortex of the bipotential gonad develops into ovarian tissue o Female development is more complex than originally thought, with multiple genes required for the development of functional ovaries o In the absence of testicular AMH, the Müllerian ducts develop into the upper portion of the vagina, the uterus, and the fallopian tubes (aka oviducts) o Without DHT, the external genitalia take on female characteristics Basic Patterns of Reproduction  Eggs are some of the largest cells in the body o Nonmotile, and must be moved through the reproductive tract on currents created by smooth muscle contract or the beating of cilia  Sperm are quite small o Flagellated cells in the body, and are highly motile so that they can swim up the female reproductive tract  Gametogenesis: gamete production o Women are born with all the eggs (oocytes) they will ever have > during reproductive years, eggs mature in a cyclic pattern and are released from the ovaries roughly once a month > after ~40 years, female reproductive cycles cease (menopause) o Men manufacture sperm continuously from the time they reach reproductive maturity > sperm production and testosterone secretion diminish with age but do not cease o In both females and males, germ cells in the embryonic gonads first undergo a series of mitotic divisions to increase their number; after that, the germ cells are ready to undergo meiosis, the cell division process which forms gametes  In the first step of meiosis, the germ cell’s DNA replicates until each chromosome is duplicated (46  96); the cell is now called a primary spermatocyte or primary oocyte  Each duplicated chromosome forms two identical sister chromatids, linked together at a region known as the centromere  in the first meiotic division, one primary gamete divides into 2 secondary gametes (secondary spermatocyte or secondary oocyte) > each gamete gets one copy of each duplicated autosome plus one sex chromosome  in the second meiotic division, the sister chromatids separate  in males, the cells split during the second meiotic division, resulting in 2 haploid sperm from each secondary spermatocyte  in females, the second meiotic division creates one egg and one small cell called a polar body > what happens after depends on whether the egg is fertilized  Male Gametogenesis o At birth, the testes of a newborn boy have not progressed beyond mitosis and contain only immature germ cells > After birth, the gonads become quiescent (relatively inactive) until puberty, the periof in the early teen years when the gonads mature o At puberty, germ cell mitosis resumes > from this point onward, germ cells are known as spermatogonia, and have two possible fates:  Some continue to undergo mitosis throughout the male’s reproductive life  Other are destined to start meiosis and become primary spermatocytes  Each primary spermatocyte creates 4 sperm  in the first meiotic division, it divides into 2 secondary spermatocytes  in the secondary meiotic division, each secondary spermatocyte divides into 2 spermatids  each spermatid has 23 single chromosomes; they mature into sperm  Female Gametogenesis o In the embryonic ovary, germ cells are called oögonia > oögonia complete mitosis and the DNA duplication stage of meiosis by the 5 month of fetal development > at this time, germ cell mitosis ceases and no further oocytes can be formed o At birth each ovary contains about half a million primary gametes, or primary oocytes o In the ovary, meiosis does not resume until puberty > each primary oocyte divides into 2 cells, a large egg (secondary oocyte) and a tiny first polar body > the egg and polar body each contain 23 chromosomes; the first polar body disintegrates o The egg begins the second meiotic division > after the sister chromatids separate from each other, meiosis pauses > the final step of meiosis, in which sister chromatids go to separate cells, does not take place unless the egg is fertilized o The ovary releases the mature egg during ovulation  If the egg isn’t fertilized, meiosis never goes to completion, and the egg disintegrates or passes out of the body  If fertilization has occurred, the final step of meiosis takes place > half the sister chromatids remain in the fertilized egg (zygote), the other half is released in a second polar body that disintegrates o Each primary oocyte gives rise to only one egg  Gametogenesis is under the control of hormones from the brain and from endocrine cells in the gonads  The pathways that regulate reproduction begin with secretion of peptide hormones by the hypothalamus and anterior pituitary o These trophic hormones control gonadal secretion of the steroid sex hormones, including androgens, estrogens, and progesterone  Sex steroids are closely related to each other and come from the same steroid precursors; both sexes produce androgens (dominate in males) and estrogens (dominate in females)  In males, most testosterone is secreted by the testes but about 5% comes from the adrenal cortex > testosterone is converted in peripheral tissues to its more potent derivative, DHT  Males synthesize some estrogens, but the feminizing effects of these hormones are usually not obvious in males  Both testes and ovaries contain the enzyme aromatase, which converts androgens to estrogens  In women, the ovary produces estrogens (particulary estradiol and estrone) and progestins, particularly progesterone > the ovary and the adrenal cortex produce small amounts of androgens  Control Pathways o Hormonal control of reproduction in both sexes follows the basic hypothalamus-anterior pituitary- peripheral gland pattern o Gonadotropin-releasing hormone (GnRH) from the hypothalamus controls secretion of 2 anterior pituitary gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH)  These hormones then act on the gonads: FSH is required to initiate and maintain gametogenesis and LH acts on endocrine cells, stimulating producing of the steroid sex hormones o Both ovary and testis secrete peptide hormones that feed back to act directly on the pituitary  Inhibins inhibit FSH secretion, and related peptides called activins stimulate FSH secretion  Activins also promote spermatogenesis, oocyte maturation, and development of the embryonic nervous system  AMH is also made in both the ovaries and testes after birth  Inhibins, activins, and AMH are part of a group of related growth and differentiation factors known as transforming growth factor-β family  Feedback pathways o Follow the general patterns for long-loop and short-loop feedback for trophic hormones o Gonadal hormones alter secretion of GnRH, FSH, and LH in a long-loop response, and the pituitary gonadotropins inhibit GnRH release from the hypothalamus by a short-loop path o When gonadal steroids are low, the pituitary secretes FSH and LH  As steroid levels increase, negative feedback usually inhibits gonadotropin release: as androgen levels go up, FSH and LH secretion decreases o Higher levels of estrogen can exert either positive or negative feedback  Low levels of estrogen have no feedback effect  If estrogen rises rapidly to a threshold level and remains high for at least 36 hours, feedback switches from negative to positive, and gonadotropin release is stimulated  Some evidence suggests that high levels of estrogen increase the number of GnRH receptors in the anterior pituitary making it more sensitive to GnRH; other evidence points to estrogen influencing GnRH release by altering the release of a peptide called kisspeptin form hypothalamic neurons  Pulsatile GnRH release o Tonic GnRH release from the hypothalamus occurs in small pulses every 1-3 hours in both sexes o GnRH pulse generator is the region of the hypothalamus that contains the GnRH neuron cell bodies > it coordinates the periodic pulsatile secretion of GnRH o Steady high levels of GnRH cause down-regulation of the GnRH receptors on gonadotropin cells, making the pituitary unable to respond to GnRH  The basis for the therapeutic use of GnRH in treating certain disorders  Patients suffering from prostate and breast cancers which are stimulated by androgens and estrogens, may be given GnRH agonists to slow the growth of the cancer cells  after a brief increase in FSH and LH, the pituitary becomes insensitive to GnRH  then FSH and LH secretion decreases, and gonadal output of steroid hormones also falls  the GnRH agonist creates chemical castration that reverses when the drug is no longer administered  Environmental effects: o In men, factors that influence gametogensis are difficult to monitor short of requesting periodic sperm counts o Disruption of the normal reproductive cycle in women is easier to study because physiological uterine bleeding in the menstrual cycle is easily monitored  Factors include stress, nutritional status, and changes in the day-night cycle, such as those that occur with travel across time zones or with shift work o The hormone melatonin from the pineal gland mediates reproduction in seasonally bleeding animals; researchers are investigating whether melatonin also plays a role in seasonally and daily rhythms in humans o Environmental estrogens are naturally occurring compounds, such as the phytoestrogens of plants, or synthetic compounds that have been released into the environment  Some of these compounds bid to estrogen receptors in both sexes and mimic estrogen’s effects  Others are anti-estrogens that block estrogen receptors or interfere with second messenger pathways or protein synthesis  Some of these endocrine disruptors can adversely influence developing embryos and even have their effects passed down to subsequent generations Male Reproduction [pg.621-863]  Consists of the testes, the internal genitalia (accessory glands and ducts), and the external genitalia o The external genitalia consists of the penis and the scrotum (a sac-like structure that contains the testes)  The tip of the penis is enlarged into a region called the glans that at birth is covered by a layer of skin called the foreskin or prepuce  Circumcision is the removal of the foreskin; claims to be necessary for good hygiene, and evidence suggests that the incidence of penile cancer, sexually transmitted diseases, and urinary tract infections is lower in circumcised men; helps prevent the spread of HIV/AIDS  Scrotum is necessary for the development of sperm, which requires a temperature that is 2-3°F lower than core body temperature  Men with low sperm count are advised to where boxers instead of briefs o The urethra serves as a common passageway for sperm and urine, not simultaneously  it runs through the ventral aspect of the shaft of the penis and is surrounded by a spongy column of tissue known as the corpus spongiosum  corpus spongiosum and two columns of tissue called the corpora cavernosa constitute the erectile tissue of the penis  Cryptorchidism: failure of one or both testes to descend; occurs in 1-3% of newborn males o If left alone, 80% of cryptoorchidism testes spontaneously descend later o Those that remain in the abdomen through puberty become sterile and are unable to produce sperm o They can produce androgens, indicating that hormone production is not as temperature sensitive as sperm production o Doctors recommend that un-descended testes be moved to the scrotum with testosterone treatment, or surgically  Accessory glands and ducts include the prostate gland, the seminal vesicles, and the bulbourethral (Cowper’s) glands o The bulbourethral glands and seminal vesicles empty their secretions into the urethra through ducts o The individual glands of the prostate open directly into the urethral lumen o The prostate gland is the best known of the 3 accessory glands because of its medical significance  Cancer of the prostate is the most common form of cancer in men, and benign prostatic hypertrophy (enlargement) creates problems for many men after age 50  The enlargement of the prostate gland causes difficulty in urinating by narrowing the passageway, since the prostate encircles the urethra  Fetal development of the prostate gland is under the control of dihydrotestosterone  Finasteride, a 5α-reductase inhibitor blocks the production of DHT and helps treat benign prostate hypertrophy; it also lowers chances of developing prostate cancer  Testes are paired ovoid structures about 5cm by 2.5cm o They have a tough outer fibrous capsule that encloses masses of coiled seminiferous tubules clustered into 250-300 compartments  Seminiferous tubules make up 80% of testicular mass in an adult  Each individual tubule is 0.3-1 meter long, and, if stretched out and laid end to end, the entire mass would extend for about the length of two and a half football fields o Interstitial tissue consisting primarily of blood vessel and the testosterone-producing Leydig cells lies between the tubules o Seminiferous tubules leave the testis and join the epididymus, a single duct that forms a tightly coiled cord on the surface of the testicular capsule  The epididymus becomes the vas deferens, also known as the ductus deferens > This duct passes into the abdomen, where it eventually empties into the urethra, the passageway from the urinary bladder to the external environment  Seminiferous tubules o The site of sperm production and contains two types of cells: spermatogonia in various stages of becoming sperm and sertoli cells o The developing spermatocytes stack in columns from the outer edge of the tubule to the lumen > between each column is a single Sertoli cell that extends from the outer edge of the tubule to the lumen o Surrounding the outside of the tubule is a basal lamina that acts as a barrier, preventing certain large molecules in the interstitial fluid from entering the tubule but allowing testosterone to enter easily o Adjacent sertoli cells in a tubule are linked to each other by tight junctions (Blood testis barrier) that form an additional barrier between the lumen of the tubule and the interstitial fluid outside the tubule  The blood testis barrier behaves much like the impermeable capillaries of the blood-brain barrier, restricting movement of molecules between 2 compartments o The basal lamina and tight junctions create 3 compartments: 1. The tubule lumen 2. A basal compartment on the basolateral side of the Sertoli cells 3. The interstitial fluid  Because of these compartments, the luminal fluid has a different composition then the interstitial fluid, with low concentrations of glucose and high concentrations of K+ and steroid hormones  Sertoli Cells (aka sustentacular cells) o Their functions is to regulate sperm development; they provide sustenance, or nourishment, for the developing spermatogonia o They manufacture and secrete proteins that range from the hormones inhibin and activin to growth factors, enzymes, and androgen-binding protein (ABP)  ABP is secreted into the seminiferous tubule lumen, where it binds to testosterone  Testosterone bound to protein is less lipophilic and cannot diffuse out of the tubule lumen  Leydig Cells o Located in the interstitial tissue between seminiferous tubules o Secrete testosterone o First become active in the fetus, when testosterone is needed to direct development of male characteristics o After birth, the cells inactivate o At puberty they resume testosterone production o They calso convert some testosterone to estradiol  Sperm Production o Spermatogonia are found clustered near the basal ends of the Sertoli cells, just inside the basal lamina of the seminiferous tubules  Here they undergo mitosis to create additional germ cells  Some spermatogonia will remain here to produce future spermatogonia  Others start meiosis and become primary spermatocytes  As spermatocytes differentiate into sperm, they move inward toward the tubule lumen, continuously surrounded by Sertoli cells  The tight junctions of the blood-testis barrier break and reform around the migrating cells, ensuring that the barrier remains intact  By the time spermatocytes reach the luminal ends of Sertoli cells, they have divided twice and become spermatids  Spermatids remain embedded in the apical membrane of Sertoli cells while they complete the transformation into sperm, losing most of their cytoplasm and developing a flagellated tail o The chromatin of the nucleus condenses into a dense structure that fills most of the head, while a lysosome- like vesicle called an arosome flattens out to form a cap over the tip of the nucleus  The arosome contains enzymes essential for fertilization o Mitochondria concentrate in the midpiece of the sperm body, along with microtubules that extend into the tail, to produce energy for sperm movement o Sperm are released into the lumen of the seminiferous tubule, along with secreted fluid > from there, they are free to move out of the testis o This entire process takes ~64 days o At any given time, different regions of the tubule contain spermatocytes in different stages of development  The staggering of developmental stages allows sperm production to remain nearly constant at a rate of 200 million sperm per day which is about the same number of sperm released in a single ejaculation o Sperm released from Sertoli cells are not yet mature and are incapable of swimming >they are pushed out of the tubule lumen by other sperm and by bulk flow of the fluid secreted by Sertoli cells > sperm entering the epididymis complete their maturation during the 12 or so days of their transit time, aided by protein secretions from epididymal cells  Hypothalamic GnRH promotes release of LH and FSH from the anterior pituitary > FSH and LH in turn stimulate the testes o GnRH release in pulsatile, peaking every 1.5 hours, and LH release follows the same pattern o FSH levels are not as obviously related to GnRH secretion because FSH secretion is also influenced by inhibin and activin o FSH targets Sertoli cells > male germ cells do not have FSH receptors  FSH stimulates Sertoli synthesis of paracrine molecules needed for spermatogonia mitosis and spermatogenesis; it also stimulates production of androgen-binding protein and inhibin o The primary target of LH is the Leydig cells, which produce testosterone; testosterone feeds back to inhibit LH and GnRH release o Testosterone is essential for spermatogenesis, but its actions appear to be mediated by Sertoli cells, which have androgen receptors; spermatocytes lack androgen receptors and cannot respond directly to testosterone  The male reproductive tract has 3 accessory glands: 1. Bulbourethral glands  Contribute mucus for lubrication and buffers to neutralize the usually acidic environment of the vagina 2. Seminal vesicles  Contribute prostaglandins that appear to influence sperm motility and transport in both male and female reproductive tracts  Contribute to nutrients for sperm metabolism 3. Prostate  Contribute to nutrients for sperm metabolism o Their primary function is to secrete various fluid mixtures which mix with sperm after they leave the vas deferens = semen  Semen provides a liquid medium for delivering sperm  Contains zinc; when levels are low it is associated with male infertility o The accessory gland secretions help protect the male reproductive tract from pathogens that might ascend the urethra from the external environment  The secretions physically flush out the urethra and supply immunoglobulins, lysozyme, and other compounds with antibacterial action  Primary sex characteristics are the internal sexual organs and external genitalia that distinguish males from females o Androgens are responsible for the differentiation of male genitalia during embryonic development and for growth during puberty  Secondary sex characteristics are other traits that distinguish males from females o The male body shape: inverted triangle; broad shoulders and narrow waist and hips o The female body shape: pear shaped; broad hips and narrow shoulders o Androgens are responsible for such typically male traits as beard and body hair growth, muscular development, thickening of the vocal chords with subsequent lowering of the voice, and behavioral effects, such as the sex drive, also called libido o Androgens are anabolic hormones that promote protein synthesis, which gives them their street name of anabolic steroids  Widespread use of these drugs in athletes; adverse effects include liver tumors, infertility, and excessive aggression (‘roid rage’) and addictiveness  Withdrawal may be associated with behavioral changes such as depression, psychosis, or aggression Female Reproduction [pg.868-869]  The female external genitalia are known collectively as either the vulva or the pudendum o At the periphery are the labia majora which are fold of skin that arise from the same embryonic tissue as the scrotum o Within the labia majora are the labia minora which are derived from embryonic tissues  In the male they would be the shaft of the penis o The clitoris is a small bud of erectile, sensory tissue at the anterior end of the vulva, enclosed by the labia minora and an additional fold of tissue equivalent to the foreskin of the penis o The urethra opens to the external environment between the clitoris and the vagina, the cavity that acts as receptacle for the penis during intercourse o At birth, the external opening of the vagina is partially closed by a thin ring of tissue called the hymen or maidenhead  The hymen is external to the vagina, not within it, so the normal use of tampons during menstruation will not rupture the hymen; it can be stretched by normal activities and is not an accurate indicator of a woman’s virginity  Sperm must pass through the narrow opening of the cervix, the neck of the uterus that protrudes slightly into the upper end of the vagina o the cervical canal is lined with mucous glands whose secretions create a barrier between the vagina and uterus  Sperm that make it through the cervical canal arrive in the lumen of the uterus (or womb), a hollow, muscular organ slightly smaller than a woman’s clenched fist  the uterus is the structure in which fertilized eggs implant and develop during pregnancy; it is composed of 3 layers o a thin outer connective tissue covering o a thick middle layer of smooth muscle known as the myometrium o an inner layer known as the endometrium  consists of an epithelium with glands that dip into a connective tissue layer below  the thickness and character of the endometrium vary during the menstrual cycle  cells of the epithelial lining alternately proliferate and slough off, accompanied by a small amount of bleeding in the process known as menstruation  sperm leave the uterus cavity through openings into the 2 fallopian tubes o fallopian tubes are 20-25cm long and about the diameter of a drinking straw o their walls have 2 layers of smooth muscle, longitudinal and circular, similar to the walls of the intestine o a ciliated epithelium lines the inside of the tubes  fluid movement created by the cilia and aided by muscular contractions transports an egg along with the fallopian tube toward the uterus o the sperm moving up the tube encounter an egg moving down the tube, fertilization may occur o pathological conditions in which ciliary function is absent are associated with female infertility and with pregnancies in which the embryo implants in the fallopian tube rather than the uterus  the open end of the fallopian tube divides into finger-like projections called fimbriae o frimbriae are held close to the adjacent ovary by connective tissue, which helps ensure that eggs released from the surface of the ovary will be swept into the tube rather than floating off into the abdominal cavity  the ovary is an elliptical structure, about 2-4 cm long o it has an outer connective tissue layer and an inner connective tissue framework known as the stroma o most of the ovary consists of a thick outer cortex filled with ovarian follicles in various stages of development or decline o the small central medulla contains nerves and blood vessels o produces both gametes and hormones o ~7 million oögonia in the embryonic ovary develop into half a million primary oocytes  each primary oocyte is enclosed in a primary follicle with a single layer of granulosa cells separated by a basement membrane from an outer layer of cells known as the theca  females produce gametes in monthly cycles (~28 days) o these cycles are commonly called menstrual cycles because they are marked by a 3-7 day period of bloody uterine discharge known as the menses or menstruation o the menstrual cycle can be described by following changes that occur in follicles of the ovary, the ovarian cycle, or by following changes in the endometrial lining of the uterus, the uterine cycle [figure26.10] o the ovarian cycle is divided into 3 phases: 1. Follicular phase: the first part of the ovarian cycle; a period of follicular growth in the ovary; most variable in length and lasts from 10 days to 3 weeks 2. Ovulation: once one or more follicles have ripened, the ovary releases the oocyte(s) 3. Luteal phase (aka postovulatory): the phase of the ovarian cycle following ovulation; the name comes from the transformation of a ruptured follicle into a corpus luteum, named for its yellow pigment and lipid deposits  The corpus luteum secretes hormones that continue that preparations for pregnancy  if pregnancy does not occur, the corpus lutem ceases to function after ~2 weeks, and the ovarian cycle begins again o the endometrial lining of the uterus goes through the uterine cycle in 3 phases:  Menses: the beginning of the follicular phase in the ovary corresponds to menstrual bleeding from the uterus  Proliferative phase: the latter part of the ovary’s follicular phase corresponds to this phase in the uterus, during which the endometrium adds a new layer of cells in anticipation of pregnancy  Secretory phase: after ovulation, hormones from the corpus luteum convert the thickened endometrium into secretory structure  the luteal phase of the ovarian cycle corresponds to this phase  if no pregnancy occurs, the superficial layers of the secretory endometrium are lost during menstruation as the uterine cycle begins again  the ovarian and uterine cycles are under the primary control of various hormones: o GnRH from the hypothalamus o FSH and LH from the anterior pituitary o Estrogen, Progesterone, Inhibin, and AMH from the ovary  During the follicular phase, the dominant steroid hormone is estrogen  Ovulation is triggered by surges in LH and FSH  In the luteal phase, the dominant steroid hormone is progesterone, although estrogen is still present  AMH is also produced by ovarian follicles in the first part of the ovarian cycle; AMH apparently acts as a brake to keep too many follicles from developing at one time  Early Follicular Phase o Just before the beginning of each cycle, gonadotropin secretion from the anterior pituitary increases; under the influence of FSH, several follicles in the ovaries begin to mature o As the follicles grow, their granulosa cells (under the influence of FSH) and their thecal cells (under the influence of LH) start to produce steroid hormones; granulosa cells also begin to secrete AMH  This AMH decreases follicle sensitivity to FSH, which apparently prevents recruitment of additional primary follicles once one group has started developing  Physicians now use blood AMH levels as an indicator of how many follicles are developing early in a cycle as a marker for the condition known as polycystic ovary syndrome (PCOS), in which ovarian follicles form fluid-filled cysts o Thecal cells synthesize androgens that diffuse into the neighboring granulosa cells, where aromatase converts them to estrogens > estrogen exerts negative feedback on pituitary FSH and LH secretion, which prevents the development of additional follicles in the same cycle > estrogen also stimulates additional estrogen production by the granulosa cells
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