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