Reproductive System
Consists of gonads-reproductive organsmake gametes & hormones,
ducts-receive & transport gametes-comprise reproductive tract, accessory
glands & organs and external genitalia.
Male-principle structures-testes-gonads-paired, plum sized-produce sperm,
epididymus, ductus deferens, urethra & ejaculatory ducts-nourish, store,
transport & mature sperm. Accessory structures-seminal vesicles, prostate
gland and bulbourthral glands. External genitalia-scrotum-within which
testes are found & penis. Fetal development-form inside body cavity next
to kidneys. Connective tissue fibers extend from testes to posterior wall of
peritoneum-gubernaculums testes-do not grow in length as fetus grows-lock
testes into place. As body enlargesposition of testes changesgradually
move inferiorly and anteriorly toward anterior abdominal wall. 7th monthfetal growth-rapid. Hormones cause gubernaculums testes to contract.
During this time testes move through abdominal musculature bringing small
pockets of peritoneal cavity with themforms spermatic cord. Spermatic
cords extend between abdomino-pelvic cavity & testes. Enclose ductus
deferens, blood vessels, nerves and lymph vessels. Come to be suspended
outside abdominal cavity by scrotum-pouch of skin-keeps testes close or far
from body at optimal temperature for sperm development. Normal body
temperature-too hot-lethal to sperm-testes-outside abdominal cavity where
temperature is about 2° C (3.6° F) lower. Woman’s body temperature-lowest
around time of ovulation to help insure sperm live longer to reach egg.
Scrotum-divided internally into 2 chambers-partition between marked by
raphe-raised thickening. Surrounded by 2 tunics. Tunica vaginalis-lines
scrotal cavity & testes. Tunica albuginea-dense fibrous capsule-lots of
collagen fibers. Collagen fibers form septa-fibrous partitions-divide testis
into lobules. Each lobule has 1-4 convoluted seminiferous tubules-produce
sperm-spermatogenesis. Each tubule forms a loop that is connected to maze
of passageways-rete testis. 15-20 efferent ductules connect rete testes to
epididymus. Each tubule surrounded by capsule; areolar tissue fills spaces
between tubules. With in spaces-Leydig Cells-interstitial cells-make
androgens. Spermatozoa production begins at outermost layer of cells in
seminiferous tubules-proceeds toward lumen. Leads to an orderly
progression along tubule of stages of sperm development-called waveensures portion of seminiferous tubule releasing sperm at any given time.
Spermatogenesis
Activity of germ cells divide spermatogenesis-into 3 phases: 1spermatocytogenesis- proliferative phase, 2-meiosis-production of haploid
gamete & 3-spermiogenesis-metamorphosis of spermatids into spermatozoa.
First-spermatocytogenesis-stem cells-spermatogonia divide by mitosis
daughter cellsstem cells and primary spermatocytes. primary
spermatocytes-->Meiosis-special cell division (reduction division)germ
cells-gametes-produces sperm with half number of chromosomes (haploid1N) as somatic cells. Number of chromosomes in gametes = 23. Egg +
sperm = 46 chromosomes-2N-diploid number. Allows recombination of
haploid gametes at fertilization without increasing number of chromosomes
each generation. Involves-duplication & exchange of genetic material & 2
cell divisions-reduces chromosome number and yields 4 spermatidsimmature gametes. Spermiogenesis-differentiation of spherical spermatids
into mature spermatids which are released at luminal surface as
spermatozoa.
Spermatocytogenesis and Meiosis
Occurs in seminiferous tubules. Begins at puberty, continues through life.
400 X 106 made/day. Spermatozoa originate from precursor cellsspermatogonia-line basement membrane of seminiferous tubule. Divide
continually by mitosis. Until puberty all daughter cells-spermatogonia.
During puberty, each mitotic division2 cell types-types A & B. Type A
remain at basement membranemaintain germ cell line. Type Bpushed
toward lumenbecomes primary spermatocyte. Meiosis. One diploid
cell4 haploid cells. Occurs in 2 steps. Meiosis I-primary spermatocytes
two secondary spermatocytes. Meiosis II-2 secondary spermatocytes4
spermatids. During first phase duplicated sister chromatids come in close
contact with their homologous pairs. Portions of homologous chromosomes
exchanged in process-crossing over-at chiasma-mixes maternal & paternal
genes forming new combination of genes. 4 daughter cells formed-each 23
chromosomes-each different genetic composition-spermatids-spherical
cells-centrally located nuclei. Have correct number of chromosomes but not
motile. Must be transformed into functional spermatozoa-spermiogenesis.
Nuclear and cytoplasmic changes resulting in spermatozoa. Restructuring:
condensation of nuclear material-chromatin condensation helps streamline
cell by reducing volume. May serve protective function, reducing
susceptibility of DNA to mutation or physical damage, acrosome formation,
tail formation, mitochondrial spiral formation and removal of extraneous
cytoplasm. Steps: 1) golgi packages acrosomal enzymes, 2) acrosome
positioned at anterior end of nucleus & centrioles at opposite ends of
nucleus, 3) microtubules elaborated making-flagellum, 4) mitochondria
multiply & position themselves around proximal part of flagellum, 5)
cytoplasm sloughed off, 6) immature sperm produced. Sertoli cells,
sustentacular cells or nurse cells-found attached to tubular capsule-extend
toward lumen. Spermatocytes & spermatids-surrounded by cytoplasm of
these cells. As spermiogenesis proceeds-spermatids begin to look like
mature spermatozoa. At spermiation-lose attachment to sustentacular cells.
At appropriate time, cytoplasm of spermatid pinched off by Sertoli cells.
Spermiogenesis ends with release of spermatozoa from Sertoli cellinto
lumen of tubules. 64-72 days from primary spermatocyte formation. Move
into epididymismature and storage.
Sustentacular Cells
Several functions-1) maintains blood-testis barrier-seminiferous tubules
isolated from general circulation by blood testis barrier. Cells-large, form
tight junctionsforms layer that divides seminiferous tubule into outer
basal compartment-contains spermatogonia and inner lumenal or adlumenal
compartment. Basal compartment below tight junctions-from basal lamina
to tight junctions-has contact with circulatory system-spermatogonia develop
to primary spermatocytes here. Adluminal compartment-meiosis completed
& spermatid develop. Sertoli cells regulate environment that bathes germ
cells. Luminal compartment-conditions-stable. Fluid in lumen produced by
sustentacular cells-regulates fluids and composition. Fluids-different from
interstitial fluid. Barrier essential to preserve differences and prevent cell of
immune system from getting to spermatozoa which have antigens on their
cell membranes. 2) supports mitosis & meiosis-spermatogenesis depends on
stimulated sustentacular cells by FSH & testosteronepromotes division of
spermatogonia. 3) spermiogenesis-requires sustentacular cells-provide
nutrients and chemical stimuli which promote spermatid development. 4)
secrete inhibin-depresses pituitary production of FSH & hypothalamic
production of GnRH-feedback control of spermatogenesis. 5) secretes ABPandrogen binding protein-binds androgens in fluid of tubuleselevates
concentration of androgensstimulates spermiogenesis and 6) secretes
Mullerian-inhibiting factor-secreted during fetal development-causes
regression of mullerian ducts-will be oviducts in females.
Sperm Structure
3 major regions: 1) head-genetic,2) midpiece-metabolic & 3) taillocomotion. Head-flattened-almost entirely filled with nucleus-DNA. Tip of
nucleus-acrosome-contains hydrolytic enzymes-used to enter egg during
acrosome reaction. Midpiece-mitochondria-provide energy for movement.
Mitochondria arranged in spiral around microtubules which are continuous
with centrioles.Tail-flagellum-only one in humans; moves in corkscrew
motion. No ER, Golgi, lysosomesreduced cell size and mass. No
glycogen-must absorb nutrients.
Reproductive Tract
Epididymus-spermatozoa in lumen of seminiferous tubules-functionally
immature-incapable of locomotion or fertilization. Move into
epididymismature and storage. Efferent ductules lined with cilia
movefluid currenttransports gametes into epididymus-coiled tube bound
to posterior border of testes-start of male reproductive tract. Has headreceives spermatozoa, body-extends along posterior margin of testes and
tail-connects with ductus deferens & stores sperm. Functions: 1) monitors &
adjusts composition of fluid made by seminiferous tubules, 2) recycles
damaged spermatozoa-absorbs cellular debris, 3) stores & protects sperm &
facilitates their maturation. Pass through in 2 weeks-complete functional
maturation. Leave epididymus-mature but immobile-incapable of fertilizing
eggs. To become mobile must undergo capacitation. Occurs in 2 steps: 1)
mixed with secretions of seminal vesicles and 2) exposure to female tract.
Epididymus secrets unidentified substance preventing premature
capacitation. From epididymus tailductus deferens-has pseudostratified,
ciliated columnar epithelium which along with peristaltic contractions
moves sperm along duct. Can also store sperm for several months. Just
before reaches prostate and seminal vesicles-lumen enlarges-ampulla.
Where ampulla joins with seminal vesicle-ejaculatory duct begins
(short)penetrates prostateempties into urethra-passage for urinary &
reproductive systemtip of penis.
Accessory Glands
Seminal vesicles, prostate and bulbourethal glands function to 1) activate
sperm, 2) nourish sperm, 3) propel along reproductive tract and 4) buffer
acidity. Seminal vesicles-behind bladder, drained by ductus deferens.
Contributes about 60% total volume of semen= sperm + associated fluid.
Seminial fluid contains 1. fructose-main energy source-easily metabolized
by sperm, 2. prostaglandins-stimulates female uterine contractions to move
semen into uterus and 3. fibrinogen-after ejaculation forms temporary clot in
vagina. Secretions-slightly alkaline-helps neutralize acid from prostate &
vagina. When mixed with seminal secretionsspermundergo first step in
capacitationbegin beating flagella. Seminal vesicles empty into
ejaculatory ducts which empty into urethra. Initial segment of urethra
surrounded by prostate gland-largest accessory gland. Makes-prostatic
fluid-slightly acid. Contributes 20-30% semen volume. Contains seminal
plasmin-antibiotic-may help prevent urinary track infections in males.
Prostate needs lot of zinc-insufficient dietary zinc can lead to enlargement
which can constrict urethra to point of interferring with urination.
Bulbourethral glands or Cowper’s glands -small pair of glands along
urethra below prostate. Fluid-thick, alkaline mucus may serve as lubricant
and help neutralize urinary acids. Semen contains: spermatozoa-normal
sperm count-20-100 X 106/ml; seminal fluid-combined secretions ofprostate-30%, sustentacular cells & epididymus secretions-5% ,
bulbourethral-less than 5% and seminal veasciles-60%. Has-enzymesprotease-may help dissolve mucous secretions in vagina, prostatic enzymeconverts fibrinogenfibrin and fibrinolysin-liquifies clotted semen.
Urethra goes through penis-contains 3 cylinders of spongy, erectile tissue.
During arousal-become filled with blood-pressure seals off veins that drain
causing-erection. Head of penis-glans penis-very sensitive to stimulationglans covered by foreskin or prepuce-may be removed by circumcision.
Medically, circumcision is not necessity, but cultural tradition.
Hormones and Male Reproductive Function
Control of Spermatogenesis
Overall coordination of spermatogenesis orchestrated by endocrine
interactions between hypothalamus, pituitary gland and somatic cells of
testis-brain testicular axis. Hypothalamusgonadotropin-releasing
hormone-GnRHanterior pituitaryLH-luteinizing hormone & FSHfollicle stimulating hormone. Without GnRHtestes atrophy and sperm
production ceases. GnRH released in pulses-not continuously-60-90 minute
intervals. Pulse frequency remains relatively steady: hr-hr, day-day and yearyear-insures plasma levels of FSH, LH and testosterone remain within
narrow range through out adult life. FSHsustentacular cells of
seminiferous tubulespromotes spermatogenesis & spermiogenesis
secretes androgen binding protein. ABP-binds to androgensprompts
spermatogenic cells to bind & concentrate testosteronestimulates
spermatogenesis. Synthesis of ABP depends on FSH but only after cell has
been under androgen influence. Rate of spermatogenesis regulated by
negative feedback mechanism involving GnRH, FSH & inhibin.
GnRHFSHspermatogenesisspermatogenesis acceleratesinhibin
secreted by sustentacular cellsinhibits GnRH and FSH. Sperm count
highinhibin highhypothalamusinhibits GnRH and at anterior
pituitaryinhibits FSH. LH or ICSH-interstitial cell stimulating
hormoneinterstitial cellstestosterone & small amounts of estrogen.
Androgen-made from cholesterol-steroid. Diffuses across cell
membranebinds to intracellular receptor-steroid-hormone-receptor
complex binds to DNA in nucleus activates genesenhances synthesis of
proteins in target cells. In prostate converted to DHT-dihydrotesterone
before having effect and in brain to estrogen to become stimulatory.
Functions-1) stimulation of spermatogenesis & formation of functional
sperm, 2) affects CNSlibido, 3) stimulation of metabolism, especially
protein synthesis & muscle growth, 4) establishment and maintenance of
secondary sex characteristics. 5) maintenance of accessory glands and
organs.
Testosterone
Production begins about 7th week of fetal development; peaks after 6
months. Sustentacular cells secretes-Mulerian Inhibiting Factor causes
Mullerian ducts to regress & stimulates differentiation of male duct system.
Promotes development of undifferentiated gonad into testes-and accessory
organs. Effects CNS development-masculinizes brain-especially in
hypothalamus. Programs hypothalamic centers involved with 1) GnRH
production-regulates LH & FSH secretion from pituitary, 2) sexual behavior,
3) sex drive. As result of prenatal exposure to testosteronehypothalamic
centers will respond appropriately when individual becomes sexually
mature. Levels-low at birth. Stays low until puberty when secretion
acceleratesinitiates sexual maturation & appearance of secondary sex
characteristics-hair-facial, axillary, chest, pubic, voice-deepens due to
growth of larynx, skin-thickens, gets oilier, bones increase in density,
muscles increase in mass, BMR increases, behaviors change. Has anabolic
effects-causes increased size in target ducts, glands and muscles.
Withoutatrophy.
Female Reproductive System
Much more complicated than male. Sexual organs-almost entirely hidden.
Makes sex hormones, functional gametes and protects & supports
developing fetus. Principle organs-gonads-ovaries-located lower abdominal
cavity, uterine tubes, uterus, vagina and external genitalia. Ovaries, uterine
tubes and uterus enclosed in extensive mesentery-broad ligament-limits side
to side movement & rotation.
Ovaries
Paired, small, lumpy, almond shaped near lateral walls-pelvic cavity.
Position stabilized by mesovarium and ovarian & suspensory ligamentscontains major blood vessels. Functions: 1) produce immature gametesoocytes. 2) secrete hormones-estrogens and progestins. 3) secrete inhibinimportant in feedback control of FSH production. Visceral peritoneum or
germinal epithelium cover each ovary. Beneath germinal epithelium-tunica
albuginea-dense connective tissue layer. Internal tissues-stroma can be
divided into cortex and medulla. Gametes-made in cortex.
Oogenesis
Ovum production-begins before birth, accelerates-puberty; ends-menopause.
Between puberty and menopause occurs on monthly basis as part of ovarian
cycle. Oogonia-stem cells-complete mitotic divisions before birth-between
3rd & 7th month of development. Germ cells develop in fetus and before birth
begin meiosis-freeze right at start of first meiotic division. Stopped in
prophase I of
meiosis I-primary
oocytes. Remain in
suspended
development until
puberty. At pubertyas FSH levels rise
ovarian cycle begins.
Each month there
after-some primary
oocytes stimulated to
undergo further
development. Only
when signaled by
hormones will one
primary oocyte pick up meiosis where left off and finish first meiotic
division. Not all primary oocytes survive until puberty. At birth there are 2
X 106 primordial follicles-each with a primary oocyte. Primary oocyte +
follicular cells = primary or primordial follicle. By puberty-400,000 leftdegenerated-undergone atresia. Nuclear events in meiosis same as in males2 differences. Divides genetic material appropriately for meiosis I-undergoes
cytokinesis differently-physical separation of daughter cells into 2
completely unattached cells-occurs via cleavage furrow. 1-cytoplasm of
primary oocyte unevenly distributed during meiotic divisions1 ovum-has
most of original cytoplasm + 3 smaller, non-functional polar bodies-later
degenerate. Would not be good for females to make millions of ova-female
cannot carry millions of fetuses. Females do not need to have constant
mitosis of germ cells-OK that once germ cell used it is not replenished. Ova
need lot of nutrients-to get embryo through first set of divisions-cells need to
be big. 2-ovary releases secondary oocyte-product of first meiotic divisionparent cell was primary oocyte-not mature ovum. Oocyte and ovum-NOT
interchangeable.Suspended in metaphase of meiosis II-will not complete
meiosis until fertilization. Meiosis-only completed when producing
fertilized egg. If secondary oocyte comes in contact with spermatozoan
fertilization beginsundergoes second meiotic divisionovum +
another polar body. Now ovum-ready to fuse with spermatozoan. If does
not encounter spermatozoan, never undergoes second meiotic division.
Ovarian Cycle
Primordial or ovarian follicle-specialized structure in cortex near tunica
albuginia-found in clusters-egg nests. Follicle-consists of developing eggprimary oocyte surrounded by one outer layer of follicle cells. Primary
oocyte + follicle cells surrounding = primordial follicle. At birth each
female carries lifetime supply of oocytes-each in Prophase I. Every 28 days,
after puberty until menopause-one primordial follicle activated-stimulated to
begin to enlarge and complete first meiotic division-400-500 eggs. As
enlarges, entire follicle enlarges. Oocytesecondary oocyte + polar body
degenerates. Monthly process-ovarian cycle. 2 phases: 1) follicular or
preovulatory and 2) luteal or post ovulatory. Steps: 1) Formation of
Primary Follicles. FSHstimulates primordial follicleprimary follicle.
In primary follicle-follicular cells enlarge & divide repeatedlycreating
several layers of follicular cells around oocyte-now called granulosa cells.
Microvilli from granulosa cells intermingle with microvilli of cells of
primary oocyte. Microvilli surroundedb y glycoprotein layer-entire regionzona pellucida. As granulosa cells enlarge & multiply; adjacent cells in
stroma form thecal cell layer around follicle. Thecal cells + granulose cells
make sex hormones. 2) Formation of Secondary Follicles. Only few
primordial follicles go to this step. Wall of follicle thickens and granulosa
cells secrete follicular fluid or liquor folliculi-accumulates in small pockets
gradually expandsseparates inner & outer follicle layerssecondary
follicle. Follicle continues to enlarge as fluid accumulates. 3) Formation of
Tertiary Follicle or mature Graafian Follicle. By 10-14th day of cycle.
Spans entire width of cortex, distorts capsule creating bulge. Oocyte projects
into antrum-expanded chamber of follicle. Surrounded by many granulosa
cells. Primary oocyte has been suspended in prophase of meiosis I. As
tertiary follicle developsLH risesprompts primary oocyte to complete
meiosis Isecondary oocyte + polar bodyenters meiosis II-stops at
metaphase. Day 14-secondary oocyte and surrounding granulosa cells lose
connection to follicle wall. Granulosa cells-drift freecorona radiate. 4)
Ovulation- \due to stimulation of LH-tertiary follicle releases secondary
oocytes & corona radiata into pelvic cavity. Follicular fluid keeps corona
attached to oocyte. Egg rreleased into abdominal cavity near opening of
oviduct or Fallopian tube. Cilia in oviduct set up currentsdraw egg in.
Sperm presentegg fertilized near far end of tubequickly finishes
meiosisembryo starts to divide and grow as travels to uterus. 5)
Formation of Corpus Luteum. Tertiary follicle-now emptycollapses;
ruptured vessels bleed into antrum. Remaining granulosa cells invade area
and proliferatecorpus luteum-stimulated by LH. Lipids in corpus luteum
used to make progestins-primarily progesterone-principle hormone after
ovulationstimulates uterine lining maturation- endometrium & secretion
of uterine glands in preparation for fertilized egg. Trip down Fallopian tube
takes about week as cilia propel unfertilized egg or embryo to uterus.
Growing embryo reaches uterus-will implant in nutritious environment and
begin to secrete its own hormones to maintain endometrium. Remains first 6
weeks gestation. 6) Corpus Luteum Degeneration-leads to menses. Egg not
fertilized-dies & disintegrates-as corpus luteum also disintegrates
progesterone production fallsunneeded, built-up endometrium shed.
Degenerates 12 days post ovulation if no fertilization. Progesterone and
estrogen levels fallfibroblasts invade nonfunctional corpusknot of scar
tissue-corpus albicans. Involution of corpus-end of ovarian cycle. New
cycle begins with activation of another group of primordial follicles.
Uterine Tubes-Fallopian tubes or Oviducts
Secondary oocyte released from ovary. Once secondary oocyte exits ovary,
begins to travel down uterine (fallopian) tube. Hollow, muscular-3
segments. 1-Infundibulum-close to ovary-expanded funnel with many
finger like projections-fimbriae that extend into pelvic cavity. Lined with
cilia-beat toward middle part of tube-the ampulla. 2. Ampulla-has sooth
muscle layers that increase in thickness as tube nears uterus. 3. Isthmusshort segment; connected to uterine wall. Histology-epithelium-ciliated,
columnar with scattered mucin secreting cells. Oocyte transported with
ciliary movements & peristaltic contractions. Few hours before ovulationsympathetic and parasympathetic fibers turn beating & peristalsis ontransport secondary oocyte for final maturation and fertilization. Takes 3-4
days to go from infundibulum to uterine cavity. For fertilization-secondary
oocyte must meet sperm during 1st 12-24 hours of passage-occurs near
boundary of ampulla & isthmus. Uterine tube also nourishes oocyte and
sperm as well as pre-embryo (initial divisions after fertilization).
Unfertilized oocytes degenerate.
Uterus
Has thick, muscular walls-myometrium-90% uterine mass, pear shaped, very
small. Provides mechanical protection, nutritional support and waste
removal for developing embryo & fetus. Contraction of wallsdelivers
fetus. Broad ligament + 3 pairs of suspensory ligaments stabilize position
and limit its range of movement. Lining-endometrium-inner, glandular
lining-10% uterine mass-rich capillary supply to bring food to embryo. Vast
number of uterine glands open onto endometrial surface. Estrogenuterine
glands & blood vessels to change. Divided into 2 anatomical regions 1) body
or corpus-largest region. Fundus-rounded part of body-superior to uterine
tube attachment-ends at constriction-isthmus. 2) Cervix-second anatomical
division-inferior from isthmus to vagina. Histology-endometrium divided
into 1) functional zone-closest to uterine cavity and 2) basilar zone-outer
zone-adjacent to myometrium. Functional zone-undergoes cyclic changeshas most of uterine glands; contributes most to thickness. Basilar zone
attaches endometrium to myometrium, contains terminal branches of
endometrial glands-structure remains constant
Uterine Cycle-Menstrual Cycle
Repeating series of changes in endometrial structure. Average 28 days;
range 21-35. first-menarche; last-menopause. 3 phases: 1) menses, 2)
proliferative & 3) secretory. Occurs in response to hormones associated with
ovarian cycle. Ovarian & uterine cycles must coordinate to have proper
reproductive function. Menses & proliferation occur during follicular phase
of ovarian cycle. Secretory corresponds to luteal phase. Menses-begins
uterine cycle-degeneration of functional zone. Spiral arteries constrict
blood flow to endometrium slowsno oxygen or nutrientssecretory
glands & other tissues deterioratearterial walls weakenedrupture
blood pours into connective tissue of functional zoneblood cells &
degenerating tissues break awayenter uterine lumen. Straight arteries feed
basilar zone-unaffected. Tissue sloughing gradual-repairs begin
immediately. Before end-all functional zone is lost. Menstruation-1-7 days.
Proliferative Phase-after menses-epithelial cells of uterine glands in basilar
zone-multiply-spread across endometrial surfacerestores integrity of
lining. Occurs as primary & secondary follicles enlarge. Phase-stimulated &
sustained by estrogen secreted by follicles. Secretory Phase-endometrial
glands enlargesecretions accelerate. Arteries supplying wall elongate &
spread through functional zone. Occurs under stimulation by progestins &
estrogens from corpus luteum. Persists as long as corpus luteum-intact.
Peaks 12 days post ovulation. Over next 2 days-glandular activity
decreases uterine cycle comes to endCL stops making hormonesnew
cycle begins with menses. Lasts 14 days.
Vagina
Relatively-thin-walled, elastic muscular tube. Extends between cervix &
vestibule-space bounded by external genitalia. Repository for sperm and
birth canal. Histology-lumen lined by nonkeratinized, stratified squamous
epithelium; relaxed-thrown into folds-rugae. Contains resident bacteriausually harmless; metabolic activityacidic environment restricts
pathogen growth.
Hormones and Female Reproductive Cycle
More complicated than males-coordinates ovarian & uterine cycles to ensure
proper reproductive function. Ovarian cycle covers events in ovary; uterine
cycle occurs in uterus. Hormones from hypothalamus & anterior pituitary
control ovarian cycle. HypothalamusGnRH-pulse frequency & amplitude
change through ovarian cycle. Changes in pulse frequency essential to
normal FSH & LH production & therefore to control ovulation. If absent or
constant (no pulses)FSH & LH production stops within hours.
Gonadotropes make FSH & LH. GnRH pulse frequency causes these cells to
change their pattern of production. Changes in pulse frequency are
controlled by circulating levels of estrogen and progestins. Estrogens
increase GnRH pulss frequency; progestinsdecrease pulse frequency.
Hormones and the follicular phase: FSHfollicle development. As
follicle enlarge-thecal cellsandrostenedione-intermediate in synthesis of
sex hormones. Androstenedione-absorbed by granulose cells3 types of
estrogens: 1) estradiol, 2) estrone & 3) estriol. All have similar effects.
Estradiol-most abundant; most pronounced effects;. Estrogen dominant
hormone before ovulation. Functions: 1) stimulates bone & muscle growth,
2) maintains secondary sex characteristics, 3) affects CNS activity-especially
hypothalamus-libido, 4) maintains functional accessory reproductive glands
& organs and 5) initiates repair & growth of endometrium. Early in follicular
phase-estrogens low; GnRH pulse frequency-16-24/dayFSH released by
anterior pituitarypromotes follicle cell developmentestrogeninhibits
LH production. As secondary follicle developsFSH decreases due to
negative feedback of inhibin. Follicular development continues supported
by estrogens, FSH & LH. As tertiary follicle formsestrogen increases
sharply sue to GnRH pulse frequency increasing to 36/daystimulates LH
production. Day 10 of cycleeffect of estrogen on LH production changes
from inhibition to stimulation. Occurs only after rising estrogen levels have
exceeded a threshold value for 36 hours. 14 daysestrogen levels peak
gonadotrope sare at a maximum sensitivity & GnRH pulses every 30
minutesmassive release of LH-sudden surge1)completion of meiosis I
by primary oocyte, 2) rupture of follicle wall and 3) ovulation-34-38 hours
after LH surge begins; 9 hours after LH peak.
Luteal Phase-high LHprogesterone secretion-main hormone of luteal
phase & corpus luteum formation. As progesterone increasesestrogen
decreases; GnRH pulse frequency declines sharply-1-4 pulses/day. This
frequency stimulates LH secretion more than FSH secretion. LH-maintains
CL. Continues preparation of uterus for pregnancy-enhances blood supply to
functional zone & stimulates secretion by endometrial glands. Progesterone
remains high for week-no pregnancyCL degenerates. 12 days post
ovulationCL-nonfunctionalprogesterone & estrogen fallblood supply
to functional zone restrictedendometrial tissues deteriorate. As
progesterone & estrogen fallGnRH pulses increasestimulates FSH
productionovarian cycle begins again.
Uterine Cycle-sloughing of endometrial tissues. Progesterone & estrogen
levels fall with degeneration of CLmenses-sloughing continues for several
days until increasing estrogen stimulates repair & regeneration of functional
zone. Proliferation phase continues until increasing progesterone makes
arrival of secretory phase. Estrogen + progesteroneendometrial glands
enlargesecretory activity increases.
Summary
First day of cycle-first day of blood flow (day 0)-menstruation-uterine lining
broken down & shed as menstrual flow. FSH & LH-secreted on day 0,
begins menstrual & ovarian cycle. Both FSH & LH stimulate maturation of
single follicle in one of ovaries & secretion of estrogen. Rising levels of
estrogen trigger secretion of LHstimulates follicle maturation & ovulation
(day 14-midcycle). LH stimulates remaining follicle cells to form corpus
luteum-produces both estrogen & progesteronestimulate development of
endometrium & preparation of uterine lining for implantation of zygote. If
pregnancy does not occur, FSH & LH dropcorpus luteum disintegrates.
Drop in hormones also causes sloughing of inner lining of uterus.
Days 1-5: Estrogen Falls, FSH Rises. Menstrual bleeding begins-Day 1 of
cycle; lasts approximately 5 days. During last few days, sharp fall in levels
of estrogen & progesterone signals uterus that pregnancy has not occurred.
Signal results in shedding of endometrial lining. Since high levels of
estrogen suppress secretion of FSH, drop in estrogen permits level of FSH
to rise. FSH stimulates follicle development. By Day 5 - 7 of cycle, one of
follicles responds to FSH stimulation more than others-becomes dominant
begins secreting large amounts of estrogen.
Days 6-14: Estrogen Secreted, FSH Falls. Large amount of estrogen
secreted by follicle during this phase. This estrogen does several things:
stimulates endometrial lining of uterus-become thicker and enriched so can
receive fertilized egg; suppresses further secretion of FSH. At about midcycle (Day 14), estrogen helps stimulate large and sudden release of (LH).
LH surge-accompanied by transient rise in body temperature, sign that
ovulation is about to happen. LH surge causes follicle to rupture and expel
egg into Fallopian tube.
Days 14-28: Estrogen And Progesterone Secretion First Rise, Then Fall.
After follicle rupturedwalls collapse-now known as corpus luteum.
Immediately after ovulation, corpus luteum begins secreting large amounts
of progesterone,-helps prepare endometrial lining for implantation of
fertilized egg. If egg is fertilized, small amount of hormone-human chorionic
gonadotrophin (HCG) released-can be detected as early as 7 days after
fertilization. HCG keeps corpus luteum viable, so can continue pumping out
estrogen and progesterone, which, keep endometrial lining intact. By about
Week 6 to 8 of gestation, newly formed placenta takes over secretion of
progesterone. If egg not fertilized, corpus luteum starts to crumble, causing
levels of estrogen and progesterone to drop. Without these to support it,
uterus soon sheds liningmenstruation begins. With no estrogen to suppress
it, FSH levels again start to rise. One cycle ends and another begins.