Post on 27-Apr-2020
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The Female Reproductive System
Chapter 20
Part II
The Female Reproductive System
Female Reproductive System
• Ovaries contain large number of follicles
– produce female gametes (eggs or ova) in ovarian cycle
• Fimbriae – extensions of fallopian tubes
– partially cover each ovary
– Its cilia draw in ovulated eggs
• Uterus – 3 layers:– Perimetrium – outer layer of connective tissue
– Myometrium – middle layer of smooth muscle
– Endometrium – hormonally-responsive inner epithelial layer • shed during menstruation
• Cervix – between the uterus and vagina
The Female Reproductive System
Organs of the Female Reproductive System
• Ovaries – female gonads
• Accessory sex organs: Vagina, uterus, and uterine tube
– Vaginal opening posterior to urethra, both covered by labia minora and majora
– Erectile organ: clitoris, anterior margin of labia minora
Production of Oocytes
• From the yolk sac germ cells migrate into ovaries
– 5 months gestation, ovaries contain 6-7 million oogonia
– Production of oogonia then ceases forever
• Toward end of gestation, oogonia now primary oocytesbegin meiosis I
– arrest in prophase I
• Loss of primary ocytes continues throughout life:
– At birth 2 million left
– At puberty 400,000
• 400 oocytes are ovulated during reproductive years
– rest undergo apoptosis
Follicles at Different Stages of Development
Follicles at Different Stages of Development
• Primary oocytes are contained in primary follicles
– oocyte + follicle cells
• In response to FSH some follicles will enter ovarian
cycle
– and grow, producing layers of granulosa cells
• Some primary follicles continue developing vesicles
– and becoming secondary follicles
• One follicle continues growth
– Vesicles fuse, forming fluid-filled cavity called an antrum
• Graafian follicle
Ovarian Cycle
• As graafian follicle develops, primary oocyte completes meiosis I
• One daughter cell (secondaryoocyte) receives cytoplasm– Other daughter, now a small
polar body degenerates
• Secondary oocyte arrests at metaphase II– Only fertilized ova complete
meiosis II
Ovarian Cycle
• Secondary oocyte is part of the graafian follicle
• Granulosa cells form a layer around outside of follicle
– Oocyte sits on a mound of this layer the cumulus oophorus
• Corona radiata – ring of granulosa cells enclosing the
secondary oocyte
• Zona pellucida – gelatinous layer between oocyte and
radiata
– forms barrier to sperm penetration
Ovulation
• 10-14 days after start of menstruation
– Only 1 follicle survives
– Others become atretic
follicles (degenerate)
• Surviving graafian follicle forms bulge on surface of ovary
– Secretes increasing
levels of Estrogen
• Graffian follicle releases secondary oocyte
• Into uterine tube at ovulation
Ovulation From a Human Ovary
Fertilization
• If a sperm passes through the corona radiata and zona pellucida and
– Enters the cytoplasm of the occyte now the
– Oocyte completes meiosis II
– With formation of another
polar body
• If not fertilized within 2 days secondary oocyte degenerates
Ovarian Cycle
• Following ovulation empty follicle under the influence of LH becomes a corpus luteum
– Which secretes progesterone and estradiol
– Nonfertile cycle, becomes corpus albicans (non- functional remnant)
Pituitary-Ovarian Axis
• Hormonal interactions between anterior pituitary and
ovaries
• Anterior pituitary secretes luteinizing hormone (LH)
and follicle-stimulating hormone (FSH)
– Both promote cyclic changes in the ovaries
• Both are controlled by GnRH from hypothalamus
– FSH secretion slightly greater during early phase of
menstrual cycle
– LH secretion greatly exceeds FSH secretion prior to ovulation
– Not clearly understood but believed to result from negative
feedback effects
Menstrual (Monthly) Cycle
• Approximately month-long cycle of ovarian activity
– Humans, apes, Old_world monkeys
• Menstration – characterized by shedding of endometrial lining accompanied by bleeding
– And sexual receptivity anytime throughout the cycle
• Nonprimate female mammals have estrous cycles
– no shedding of endometrium and receptivity is limited
– estrous animals that bleed (dogs and cats) is due to high estrogen that accompanies receptive period
Menstrual Cycle
• In humans is about 28 days
• Day 1 is taken to be the first day of menstruation
• Days 1 thru ovulation constitutes the follicular phase
• Time from ovulation to menstruation is luteal phase
• Endometrial changes are called: menstrual,
proliferative, and secretory phases
Follicular Phase
• Lasts from day 1 to ~13
• Dominated by growth and death of a cohort of primary follicles into secondary follicles
– With one survivor becoming
a graafian follicle
– Which will undergo
ovulation
• As follicles grow granulosa cells secrete increasing amounts of estradiol
– Reaching peak about day 12
Follicular Phase
• Follicular growth and estradial secretion dependent on FSH
• FSH and estradiol induce formation of FSH receptors in granulosa cells
– Follicles increasingly sensitive to
the same level of FSH
• At same time – FSH and estradiol recruit LH receptors in graafian follicle
Follicular Phase
• Rapidly rising estradiol secretion:
– Hypothalamus increases pulses
of GnRH
– Anterior pituitary sensitivity to
GnRH increases:
– results in greater LH secretion
• Positive feedback between Estrogen and anterior pituitary
– Results in LH surge, peaks 16 hrs before ovulation
• And causes ovulation
The Cycle of Ovulation and Menstruation
The Luteal Phase
• After ovulation – LH causes empty follicle to become
corpus luteum
– which secretes Estrogen and Progesterone
• Progesterone levels rise and peak about a week after ovulation
• Development of new follicles and another ovulation inhibited by:
– High progesterone and estrogen exert strong negative feedback on LH and FSH
– Inhibin from corpus luteum further suppresses FSH
• No fertilization
– corpus luteum
regresses
– Estrogen and
Progesterone levels decline
– With menstruation and new cycle of
follicle development
Luteal Phase
20-69
Cyclic Changes in the Endometrium
• Driven by cyclic changes in estrogen and progesterone
levels
• Proliferative phase (menstration cycle) occurs during
follicular phase – increased levels of estrogen
– Stimulates growth of endometrial lining and development of
spiral arteries
– Causes cervical mucus to become thin and watery to allow
sperm penetration
• Secretory phase occurs during luteal phase – endometrium becomes ready for implantation
– Progesterone stimulates development of uterine glands
– Progesterone and Estrogen cause endometrium to become thick, vascular,
and spongy
– Progesterone causes cervical mucus to thicken and become sticky
Cyclic Changes in the Endometrium
• Menstrual phase results from drop in Progesterone and Estrogen following Corpus Luteum degeneration
– Low progesterone: constriction of spiral arteries
– Blood flow stops followed by necrosis and sloughing of endometrium
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” in which cycles of
roommates become synchronized
Factors Affecting Menstrual Cycle
• Limbic system input to the hypothalamus:
– In times of stress can cause functional amenorrhea (cessation
of menstruation)
– Also occurs in very thin or athletic females with low body
weight
• May be related to reduced leptin secretion by small
adipocytes
Contraceptive Methods
• Oral contraceptive pills – synthetic estrogen and
progesterone
– Taken daily for 3 weeks after menstrual period
– Mimic Corpus Luteum so that negative feedback inhibits
ovulation
– Placebo pills taken in 4th wk to permit menstruation
Rhythm Method
• Involves daily measurement of oral basal body temperature (BT)
upon awakening because:
– Oovarian steroids cause BT changes
– Declining Estrogen on day of LH surge causes a slight drop in BT
– Rising Progesterone on day after LH peak causes elevated BT
Menopause
• Cessation of ovarian activity and menstruation roughly
~50 years
• Ovaries depleted of follicles produce no estrogen
– LH and FSH are high because of no negative feedback
• Lack of Estrogen from ovaries most responsible for:
– Hot flashes, osteoporosis, and increased risk of atherosclerosis
Fertilization and Changes in the Oocyte
Fertilization, Pregnancy, and Parturition
• Once fertilization has occurred, secondary oocyte
completes meiosis
• Then undergoes mitosis resulting in a structure called a
blastocyst
• Blastocyst secretes human chorionic gonadotropin
(HCG)
— Maintains corpus luteum of mother and prevents
menstruation
• Parturition (childbirth)– dependent upon strong
contractions of the uterus, stimulated by oxytocin
Fertilization, Cleavage, and Formation of a Blastocyst
Fertilization
• During intercourse a male ejaculates ~ 300 million
sperm into vagina of the female
• During their passage through the female reproductive
tract,
— 10% gain ability to fertilize an oocyte (capacitation)
• In order to become capacitated the sperm must be in
the female tract for at least 7 hours
― Capacitated sperm are guided up to the uterine tube
toward the oocyte by chemotaxis and thermotaxis
Fertilization
• Normally occurs in the uterine tube
• Acrosome – sperm’s large, enzyme filled vesicle (above
the nucleus)
• Interaction of sperm with the zona pellucida triggers an
acrosomal reaction
― stimulates fusion of acrosome with cell membrane
• Within 12 hrs. after fertilization
― nuclear membrane of ovum disappears and
― ovum chromosomes joins the sperm chromosomes
• Fertilized ovum now a zygote, 23 pairs of chromosomes
Fertilization and the Acrosome Reaction
Cleavage, Blastocyst, and Implantation
• Cleavage – diploid zygote undergoes mitotic divisions
– becomes a morula (ball of cells) and
– then a blastocyst (hollow ball of cells)
• Implantation of the blastocyst in the endometrium
begins between the 5th and 7th day
• Trophoblast cells of the blastocyst secrete HCG
– functions like LH and maintains mother’s corpus luteum for
the first 10 weeks
– provides fetal contribution to placenta
The Placenta
• Placenta is formed from the trophoblast cells of the
fetus and
– the adjacent maternal tissue in the endometrium
• Oxygen, nutrients, and wastes – exchanged by diffusion
between the fetal and maternal blood
• Umbilical cord contains the fetal blood
– carries it to and from the fetus and the placenta
• Placenta secretes hormones: human chorionic
somatomammotropin (hCS) and estradiol
Labor and Parturition
• Contractions of the uterus – stimulated by oxytocin
from the posterior pituitary and
– by prostaglandinsproduced within the uterus
• Androgens (primarily DHEAS) secreted by fetal adrenal
cortex are converted into estrogen by the placenta
• Estrogen secretion by the placenta induces oxytocin
synthesis
– enhances uterine sensitivity to oxytocin, and promotes
prostaglandin synthesis in the uterus
• Events culminate in labor and parturition
Lactation
• Prolactin – secreted by anterior pituitary stimulates
mammary glands to produce milk
– Controlled by prolactin-inhibiting hormone (PIH) and
stimulated by estrogen
– High estrogen levels prepare mammary glands for lactation
– But PIH prevents prolactin from stimulating milk production
– After parturition declining levels of estrogen allow increase
in secretion of prolactin
• Milk production is
prevented during
pregnancy by estrogen
stimulation of PIH
• Milk production stimulated by prolactin
• Milk ejection stimulated by oxytocin
• Stimulus of sucking triggers a neuroendocrine milk-ejection
reflex with an increased secretion of oxytocin and prolactin
Maternal Antibodies Protect the Baby
In Vitro Fertilization
• Intracytoplasmic sperm injection (ICSI)
– A single capacitated sperm is injected through the zona pellucida and
into cytoplasm of a secondary oocyte
• Grown in vitro for 3 days ( 8-cell stage) or 5 days (blastocyst)
– Transferred to the woman’s uterus