Transcript The Reproductive System

The Reproductive System
The reproductive system consists of:
1- The primary sex organs, or gonads are the testes in
males and the ovaries in females. The gonads
produce sex cells, or gametes and secrete a variety of
steroid hormones commonly called sex hormones.
2- The accessory reproductive organs :ducts, glands,
and external genitalia .
-Although male and female reproductive organs are
quite different, their common purpose is to produce
offspring.
• A sperm and egg may fuse to form a
fertilized egg, the first cell of the new
individual, from which all body cells will arise.
Once fertilization has occurred, the uterus
provides the protective environment in which
the embryo develops until birth.
• Sex hormones–androgens in males and
estrogens and progesterone in females–play
vital roles both in the development and
function of the reproductive organs and in
sexual behavior and drives.
Anatomy of the Male Reproductive
System
• The sperm-producing testes or male gonads, lie
within the scrotum.
• From the testes, the sperm are delivered to the
body exterior through a system of ducts including
(in order) the epididymis, the ductus deferens,
the ejaculatory duct, and finally the urethra,
which opens to the outside at the tip of the
penis.
• The accessory sex glands, which empty their
secretions into the ducts during ejaculation, are
the seminal vesicles, prostate, and bulbourethral
glands.
• The Scrotum
The scrotum is a sac of skin ,covered with
sparse hairs, and contains paired oval
testes. A midline septum divides the
scrotum, providing a compartment for
each testis. Because viable sperm cannot
be produced in abundance at core body
temperature (37°C), the superficial
location of the scrotum, which provides a
temperature about 3°C lower, is an
essential adaptation.
• Furthermore, the scrotum responds to
temperature changes:
• When it is cold, the testes are pulled
closer to the warmth of the body wall,
and the scrotum becomes shorter and
heavily wrinkled, increasing its thickness
to reduce heat loss.
• When it is warm, the scrotal skin is flaccid
and loose to increase the surface area for
cooling (sweating) and the testes hang
lower, away from the body trunk.
The Testes
Each testis is approximately 4 cm (1.5 inches) long
and 2.5 cm (1 inch) in width and is surrounded by
the tunica albuginea ,the fibrous capsule of the
testis.
Septa extending from the tunica albuginea divide
the testis into 250 to 300 wedge-shaped lobules
,each containing one to four tightly coiled
seminiferous tubules, the actual “sperm factories.”
Surrounding each seminiferous tubule are smooth
muscle cells .By contracting rhythmically, these
muscles squeeze sperm and testicular fluids
through the tubules and out of the testes.
• The seminiferous tubules of testis converge
into the rete testis (re′te), a tubular network
on the posterior side of the testis. From the
rete testis, sperm leave the testis and enter
the epididymis.
• Lying in the soft connective tissue
surrounding the seminiferous tubules are the
interstitial cells, also called Leydig cells .These
cells produce androgens (testosterone).
• Thus, the sperm-producing and hormoneproducing functions of the testis are carried
out by completely different cell populations.
• testicular arteries, which branch from the abdominal
aorta superior to the pelvis supply the testes.
• The right testicular vein drains into the inferior vena
cava ,the left drains into the left renal vein superiorly.
• The testes are served by both divisions of the
autonomic nervous system.
• Associated sensory nerves transmit impulses that
result in agonizing pain and nausea when the testes
are hit forcefully.
• The nerve fibers are enclosed, along with the blood
vessels and lymphatics, in a connective tissue sheath
called the spermatic cord which passes through the
inguinal canal .
The Male Duct System
In order (proximal to distal), the accessory ducts
are :
1-The Epididymis
The cup-shaped epididymis is about 3.8 cm long .
Its head caps the superior aspect of the testis. Its
body and tail are on the posterolateral area of the
testis. It`s an uncoiled length is about 6 m.
• The immature, nearly nonmotile sperm as it
moves along its tortuous course (a trip that takes
about 20 days), the sperm gain the ability to swim.
• Sperm are ejaculated as the smooth
muscle in the epididymis walls contracts,
expelling sperm into the next segment of
the duct system, the ductus deferens.
• Sperm can be stored in the epididymis for
several months, but if held longer, they
are eventually phagocytized by epithelial
cells of the epididymis.
2-The Ductus Deferens and Ejaculatory Duct
The ductus deferens or vas deferens, is
about 45 cm long. It runs upward as part of
the spermatic cord through the inguinal
canal into the pelvis .It then descends
along the posterior bladder wall, then it
joins with the duct of the seminal vesicle (a
gland) to form the short ejaculatory duct.
Each ejaculatory duct enters the prostate;
there it empties into the urethra.
• Its muscular layer is extremely thick and the duct
feels like a hard wire when squeezed between the
fingertips. At the moment of ejaculation, the thick
layers of smooth muscle in its walls create strong
peristaltic waves that rapidly squeeze the sperm
forward along the tract and into the urethra.
• Part of the ductus deferens lies in the scrotal sac.
Some men opt to take full responsibility for birth
control by having a vasectomy. Sperm are still
produced, but they can no longer reach the body
exterior. Eventually, they deteriorate and are
phagocytized.
The Urethra
The urethra is the terminal portion of the male duct
system. It conveys both urine and semen (at
different times), so it serves both the urinary and
reproductive systems. Its three regions are
(1) the prostatic urethra, the portion surrounded by
the prostate;
(2) the membranous (or intermediate part of the)
urethra in the urogenital diaphragm; and
(3) the spongy (penile) urethra, which runs through
the penis and opens to the outside at the external
urethral orifice(about 15 cm long) .
• Accessory Glands
The accessory glands produce the bulk of semen
(sperm plus accessory gland secretions).
1-The Seminal Vesicles
Two fairly large, hollow glands , about the shape and
length (5–7 cm) of a little finger on the posterior
bladder surface.
• Its secretion is a yellowish viscous alkaline fluid( 60%
of the volume of semen) containing fructose sugar,
ascorbic acid and prostaglandins, as well as other
substances that enhance sperm motility or fertilizing
power.
• The duct of each seminal vesicle joins that of the
ductus deferens on the same side to form the
ejaculatory duct. Sperm and seminal fluid mix in the
ejaculatory duct and enter the prostatic urethra
together during ejaculation.
2-The Prostate
The prostate (pros′tāt) is a single doughnutshaped. It encircles the urethra just inferior to
the bladder. The prostatic gland secretion
enters the prostatic urethra via several ducts
when prostatic smooth muscle contracts
during ejaculation.
It plays a role in activating sperm and
accounts for up to one-third of the semen
volume. It is a milky, slightly acid fluid that
contains citrate (a nutrient source), several
enzymes and prostate-specific antigen (PSA).
3-The Bulbourethral Glands
The bulbourethral glands are pea-sized glands inferior
to the prostate gland .They produce a thick, clear
mucus, some of which drains into the spongy urethra
when a man becomes sexually excited and neutralizes
traces of acidic urine in the urethra.
• Semen is a milky white, somewhat sticky mixture of
sperm and accessory gland secretions. The liquid
provides a transport medium and nutrients. Mature
sperm contains little cytoplasm or stored nutrients.
Catabolism of the fructose in seminal vesicle secretion
provides nearly all the fuel needed for sperm ATP
synthesis.
-Prostaglandins in semen decrease the viscosity of
mucus guarding the entry (cervix) of the uterus and
facilitate sperm movement through the female
reproductive tract.
-The relative alkalinity of semen as a whole (pH 7.2–
7.6) helps neutralize the acid environment of the
male’s urethra and the female’s vagina, thereby
protecting the delicate sperm and enhancing their
motility.
-Sperms are very sluggish under acidic conditions
(below pH 6).
- Semen also contains substances that suppress the
immune response in the female’s reproductive tract
and an antibiotic chemical called seminalplasmin,
which destroys certain bacteria.
- Clotting factors found in semen coagulate it just
after it is ejaculated. Soon, its contained fibrinolysin
liquefies the sticky mass, enabling the sperm to
swim out and begin their journey through the
female duct system.
-The amount of semen propelled out of the male duct
system during ejaculation is 2–5 ml, but there are
between 20 and 150 million sperm per milliliter.
Spermatogenesis (sperm formation)
-The process begins around the age of 14 years in
males, and continues throughout life. Every day, a
healthy adult male makes about 400 million sperm.
The normal chromosome number in most body
cells is referred to as the diploid chromosomal
number of the organism, symbolized as 2n.
In humans, this number is 46, and such diploid cells
contain 23 pairs of similar chromosomes called
homologous chromosomes. One member of each
pair is from the male parent (the paternal
chromosome); the other is from the female parent
(the maternal chromosome).
• The number of chromosomes present in human
gametes is 23, referred to as the haploid chromosomal
number ,or n; gametes contain only one member of
each homologous pair. When sperm and egg fuse, they
form a fertilized egg that reestablishes the typical
diploid chromosomal number of human cells.
Gamete formation in both sexes involves meiosis, a
unique kind of nuclear division that occurs only in the
gonads.
• Meiosis consists of two consecutive nuclear divisions,
and its product is four daughter cells instead of two,
each with half as many chromosomes as typical body
cells. Thus, meiosis reduces the chromosomal number
by half (from 2n to n) in gametes.
Mitosis of Spermatogonia: Forming Spermatocytes
The outermost tubule cells are stem cells called
spermatogonia .
-The spermatogonia divide more or less continuously by
mitosis and, until puberty, all their daughter cells
become spermatogonia.
- Spermatogenesis begins during puberty, and from
then on, each mitotic division of a spermatogonium
results in two distinctive daughter cells–types A and B.
The type A daughter cell remains at the basement
membrane to maintain the germ cell line.
-The type B cell gets pushed toward the lumen, where it
becomes a primary spermatocyte destined to produce
four sperms.
Meiosis: Spermatocytes to Spermatids
Each primary spermatocyte generated during the first
phase undergoes meiosis I, forming two smaller
haploid cells called secondary spermatocytes. The
secondary spermatocytes continue on rapidly into
meiosis II, and their daughter cells, called spermatids.
Spermiogenesis: Spermatids to Sperm
Each spermatid has the correct chromosomal number
for fertilization (n), but is nonmotile. It still must
undergo a process called spermiogenesis, during which
it elongates, sheds its excess cytoplasmic baggage, and
forms a tail. The resulting sperm, or spermatozoon has
a head, a midpiece, and a tail.
-The head of a sperm consists almost entirely of its
flattened nucleus, which contains the compacted
DNA. Adhering to the top of the nucleus is a
helmetlike acrosome .
-Acrosome contains hydrolytic enzymes that enable
the sperm to penetrate and enter an egg.
- The sperm midpiece contains mitochondria spiraled
tightly around the contractile filaments of the tail.
-The long tail is a typical flagellum produced by the
centriole near the nucleus. The mitochondria provide
the metabolic energy (ATP) needed for the whiplike
movements of the tail that will propel the sperm
along its way in the female reproductive tract.
Hormonal Regulation of Male Reproductive Function
The Brain-Testicular Axis
Hormonal regulation of spermatogenesis and
testicular androgen production involves interactions
between the hypothalamus, anterior pituitary
gland, and testes, a relationship sometimes called
the brain-testicular axis.
The hypothalamus releases gonadotropin-releasing
hormone (GnRH), which controls the release of the
two anterior pituitary gonadotropins: folliclestimulating hormone (FSH) and luteinizing
hormone (LH). (Both FSH and LH were named for
their effects on the female gonad).
- FSH stimulates spermatogenesis.
- LH stimulates the interstitial cells, prodding
them to secrete testosterone (and a small
amount of estrogen).
- Locally, testosterone serves as the final
trigger for spermatogenesis. Testosterone
entering the bloodstream exerts a number of
effects at other body sites(secondary male
sex characters) .
- Male secondary sex characteristicsare the
features induced in the nonreproductive
organs by the male sex hormones (mainly
testosterone)–make their appearance at
puberty.
• These include the appearance of pubic, axillary,
and facial hair, enhanced hair growth on the chest
or other body areas in some men, and a
deepening of the voice as the larynx enlarges. The
skin thickens and becomes oilier (which
predisposes young men to acne), bones grow and
increase in density, and skeletal muscles increase
in size and mass.
-Testosterone is the basis of the sex drive (libido) in
males and to some extent in females.
-The testes are not the only source of androgens;
the adrenal glands of both sexes also release
androgens.
Female Reproductive System
The reproductive role of the female is far more
complex than that of a male. Not only must she
produce gametes, but her body must prepare to
nurture a developing embryo for a period of
approximately nine months.
Ovaries, the female gonads, are the primary
reproductive organs of a female, and like the male
testes, ovaries serve a dual purpose: They produce
the female gametes (ova) and sex hormones, the
estrogens and progesterone .
The female’s accessory ducts, from the ovary to the
body exterior, are the uterine tubes, the uterus,
and the vagina.
The Ovaries
The paired ovaries flank the uterus on each side
.Shaped like an almond and about twice as large,
each ovary is held in place within the peritoneal
cavity by several ligaments. The ovarian ligament
anchors the ovary medially to the uterus; the
suspensory ligament anchors it laterally to the
pelvic . In between , they are enclosed and held in
place by a fold of peritoneum, the broad ligament.
The ovaries are served by the ovarian arteries,
branches of the abdominal aorta and by the ovarian
branch of the uterine arteries.
• Embedded in the highly vascular connective tissue
of the ovary cortex are many tiny saclike structures
called ovarian follicles. Each follicle consists of an
immature egg, called an oocyte surrounded by one
or more layers of very different cells called follicle
cells. Follicles at different stages of maturation are
distinguished by their structure.
• A primordial follicle, one layer of squamouslike
follicle cells surrounds the oocyte.
• A primary follicle has two or more layers of
cuboidal or columnar-type cells enclosing the
oocyte; it becomes a secondary follicle when fluidfilled spaces appear and then coalesce to form a
central fluid-filled cavity called an antrum.
• At the mature vesicular follicle, or Graafian follicle
(graf′e-an), stage, the follicle bulges from the
surface of the ovary.
• Each month in adult women, one of the ripening
follicles ejects its oocyte from the ovary, an event
called ovulation .
• After ovulation, the ruptured follicle is transformed
into a very different looking glandular structure
called the corpus luteum (lu′te-um; plural: corpora
lutea), which eventually degenerates. As a rule,
most of these structures can be seen within the
same ovary.
• In older women, the surfaces of the ovaries are
scarred and pitted, revealing that many oocytes
have been released.
The Female Duct System
1-The Uterine Tubes
• The uterine tubes (u′ter-in), also called fallopian
tubes or oviducts, form the initial part of the female
duct system .They receive the ovulated oocyte and
are the site where fertilization generally occurs.
• Each uterine tube is about 10 cm (4 inches) long and
extends medially from the region of an ovary to
empty into the superolateral region of the uterus .
• The distal end of each uterine tube expands as
funnel-shaped infundibulum bearing ciliated,
fingerlike projections called fimbriae that partially
surround the ovary.
• Unlike the male duct system, which is continuous
with the tubules of the testes, the uterine tubes
have little or no actual contact with the ovaries. An
ovulated oocyte is cast into the peritoneal cavity,
and many oocytes are lost there.
• The uterine tube performs complex movements to
capture oocytes–it bends to drape over the ovary
while the fimbriae stiffen and sweep the ovarian
surface. The beating cilia on the fimbriae then
create currents in the peritoneal fluid that tend to
carry an oocyte into the uterine tube, where it
begins its journey toward the uterus.
HOMEOSTATIC IMBALANCE
• The fact that the uterine tubes are not
continuous with the ovaries places women at
risk for ectopic pregnancy in which a zygote,
fertilized in the peritoneal cavity or distal
portion of the uterine tube, begins developing
there. Because the tube lacks adequate mass
and vascularization to support the full term of
pregnancy, such pregnancies naturally abort,
often with substantial bleeding.
HOMEOSTATIC IMBALANCE
• Another potential problem is infection spreading
into the peritoneal cavity from other parts of the
reproductive tract causing an extremely severe
inflammation called pelvic inflammatory disease
(PID). Unless treated promptly with broad-spectrum
antibiotics, PID can cause scarring of the narrow
uterine tubes and of the ovaries, causing sterility.
Indeed, scarring and closure of the uterine tubes,
which have an internal diameter as small as the
width of a human hair in some regions, is one of the
major causes of female infertility.
2-The Uterus
The uterus is located in the pelvis, anterior to the
rectum and posterosuperior to the bladder .It is a
hollow, thick-walled, muscular organ that functions
to receive, retain, and nourish a fertilized ovum.
- In a premenopausal woman who has never been
pregnant, the uterus is about the size and shape of
an inverted pear, but it is usually somewhat larger in
women who have borne children.
- Normally, the uterus flexes anteriorly where it joins
the vagina, causing the uterus as a whole to be
inclined forward, or anteverted. However, the organ
is frequently turned backward, or retroverted, in
older women.
-The major portion of the uterus is referred to as
the body .
-The rounded region superior to the entrance of the
uterine tubes is the fundus, and the slightly
narrowed region between the body and the cervix
is the isthmus.
-The cervix of the uterus is its narrow neck, or
outlet, which projects into the vagina inferiorly. The
cavity of the cervix, called the cervical canal,
communicates with the vagina via the external os
(os = mouth) and with the cavity of the uterine
body via the internal os.
The mucosa of the cervical canal contains cervical glands
that secrete a mucus that fills the cervical canal and
covers the external os, presumably to block the spread
of bacteria from the vagina into the uterus. Cervical
mucus also blocks the entry of sperm, except at
midcycle, when it becomes less viscous and allows
sperm to pass through.
HOMEOSTATIC IMBALANCE
• Cancer of the cervix strikes about 450,000 women
worldwide each year, killing about half.
• It is most common among women between the ages of
30 and 50.
• Risk factors include frequent cervical inflammations,
sexually transmitted diseases including genital warts,
and multiple pregnancies.
• A Pap smear is the most effective way to detect this
slow-growing cancer.
• Supports of the Uterus The uterus is supported
laterally by the broad ligament and anchord
anteriorly and posteriorly by the round and
uterosacral ligaments respectively. These ligaments
allow the uterus a good deal of mobility, and its
position changes as the rectum and bladder fill and
empty.
HOMEOSTATIC IMBALANCE Despite the many
anchoring ligaments, the principal support of the
uterus is provided by the muscles of the pelvic floor,
namely the muscles of the urogenital and pelvic
diaphragms .These muscles are sometimes torn
during childbirth. Subsequently, the unsupported
uterus may sink inferiorly, until the tip of the cervix
protrudes through the external vaginal opening. This
condition is called prolapse of the uterus.
The Uterine Wall
Is composed of three layers :
• The perimetrium, the incomplete outermost
serous layer, is the peritoneum.
• The myometrium (mi″o-me′tre-um; “muscle of the
uterus”) is the bulky middle layer, composed of
interlacing bundles of smooth muscle, that
contracts rhythmically during childbirth to expel
the baby from the mother’s body.
• The endometrium is the mucosal lining of the
uterine cavity. If fertilization occurs, the young
embryo burrows into the endometrium (implants)
and resides there for the rest of its development.
• The endometrium has two chief strata (layers):
-The functional layer, undergoes cyclic changes in
response to blood levels of ovarian hormones and
is shed during menstruation (approximately every
28 days).
-The thinner, deeper stratum basalis (ba-să′lis), or
basal layer, forms a new functional layer after
menstruation ends . It is unresponsive to ovarian
hormones.
-The endometrium has numerous uterine glands
that change in length as endometrial thickness
changes.
The Vagina
The vagina is a thin-walled tube, 8–10 cm long. It lies
between the bladder and the rectum and extends
from the cervix to the body exterior .The urethra is
embedded in its anterior wall.
• called the birth canal, the vagina provides a
passageway for delivery of an infant and for menstrual
flow.
• The mucosa is a stratified squamous epithelium
adapted to stand up to friction. The vaginal mucosa
has no glands; it is lubricated by the cervical mucous
glands.
• The pH of a woman’s vagina is normally quite acidic.
This acidity helps keep the vagina healthy and free of
infection, but it is also hostile to sperm.
• In virgins (those who have never participated in
sexual intercourse), the mucosa near the distal
vaginal orifice forms an incomplete partition called
the hymen (hi′men) .
• The hymen is very vascular and tends to bleed
when it is ruptured during the first coitus (sexual
intercourse). However, its durability varies. In some
females, it is ruptured during a sports activity,
tampon insertion, or pelvic examination.
• ccasionally, it is so tough that it must be breached
surgically if intercourse is to occur.
• The upper end of the vaginal canal loosely
surrounds the cervix of the uterus, producing a
vaginal recess called the vaginal fornix. The
posterior part of this recess, the posterior fornix,
is much deeper than the lateral and anterior
fornices .
• Generally, the lumen of the vagina is quite small
and, except where it is held open by the cervix, its
posterior and anterior walls are in contact with
one another. The vagina stretches considerably
during copulation and childbirth, but its lateral
distension is limited by the ischial spines .
The External Genitalia
The female reproductive structures that lie external to
the vagina are called the external genitalia .The
external genitalia, also called the vulva .
• The mons pubis is a fatty, rounded area overlying the
pubic symphysis. After puberty, this area is covered
with pubic hair.
• Running posteriorly from the mons pubis are two
elongated, hair-covered fatty skin folds, the labia
majora . The labia majora enclose the labia minora
,two thin, hair-free skin folds. The labia minora
enclose a recess called the vestibule (“entrance hall”),
which contains the external openings of the urethra
and the vagina.
• anterior to the vestibule is the clitoris ,a small,
protruding structure composed largely of erectile
tissue, which is homologous to the penis of the male.
• Flanking the vaginal opening are the pea-size
greater vestibular glands (Bartholin`s glands),
homologous to the bulbourethral glands of males.
These glands release mucus into the vestibule and
help to keep it moist and lubricated, facilitating
intercourse.
• The female perineum is a diamond-shaped region
located between the pubic arch anteriorly, the
coccyx posteriorly, and the ischial tuberosities
laterally.
The Mammary Glands
They are present in both sexes, but they normally
function only in females .
• They are modified sweat glands that are really
part of the skin, or integumentary system. Each
mammary gland is contained within a rounded
skin-covered breast within the hypodermis
(superficial fascia), anterior to the pectoral muscles
of the thorax. Slightly below the center of each
breast is a ring of pigmented skin, the areola (ahre′o-lah), which surrounds a central protruding
nipple.
• Internally, each mammary gland consists of 15 to 25
lobes that open at the nipple. Within the lobes are
smaller units called lobules, which contain glandular
alveoli that produce milk when a woman is
lactating. These compound alveolar glands pass the
milk into the lactiferous ducts which open to the
outside at the nipple. Just deep to the areola, each
duct has a dilated region called a lactiferous sinus
where milk accumulates during nursing.
• In nonpregnant women, the glandular structure of
the breast is largely undeveloped and the duct
system is rudimentary; hence breast size is largely
due to the amount of fat deposits.
Breast Cancer
Invasive breast cancer is the most common malignancy and
the second most common cause of cancer death of U.S.
women. Thirteen percent of women in the general population
(132 out of 1000 individuals) will develop this condition.
Breast cancer usually arises from the epithelial cells of the
smallest ducts, not from the alveoli. A small cluster of cancer
cells grows into a lump in the breast from which cells
eventually metastasize.
Known risk factors for developing breast cancer include:
(1) early onset menses and late menopause;
(2) no pregnancies or first pregnancy later in life and no or short
periods of breast feeding.
(3) family history of breast cancer (especially in a sister or
mother), and
(4) postmenopausal estrogen-progesterone replacement.
MAMMOGRAPHY
Mammography is an x-ray technique that is used to
evaluate breast tissue for abnormalities. By far the
most frequent usage is to detect breast cancer,
which is one of the most common malignancies in
women.
If detected early, breast cancer may be cured through
a combination of surgery, radiation, and
chemotherapy.
Women should practice breast self-examination
monthly , but mammography can detect lumps that
are too small to be felt manually.
Women in their 30s may have a mammogram done to
serve as a comparison for mammograms later in
life.
Physiology of the Female Reproductive System
Oogenesis
• Gamete production in males begins at puberty and
continues throughout life, but the situation is quite
different in females.
• It has been assumed that a female’s total supply of
eggs is already determined by the time she is born,
and the time span during which she releases them
extends only from puberty to menopause (about the
age of 50).
• Meiosis, the specialized nuclear division that occurs in
the testes to produce sperm, also occurs in the
ovaries. In this case, female sex cells are produced,
and the process is called oogenesis .The process of
oogenesis takes years to complete.
• First, in the fetal period the oogonia, the diploid
stem cells of the ovaries, multiply rapidly by mitosis.
Gradually, primordial follicles begin to appear as the
oogonia are transformed into primary oocytes and
become surrounded by a single layer of flattened
follicle cells.
• By birth, a female has been presumed to have her
lifetime supply of primary oocytes; of the original 7
million oocytes approximately 2 million of them
escape programmed death and are already in place
in the immature ovary. The wait is a long one–10 to
14 years at the very least.
• At puberty, perhaps 250,000 oocytes remain and
beginning at this time a small number of primary
oocytes are activated each month in response to
the LH surge midcycle. However, only one is
“selected” each time to continue meiosis I,
ultimately producing two haploid cells (each with
23 replicated chromosomes) that are quite
dissimilar in size.
• The smaller cell is called the first polar body. The
larger cell, which contains nearly all the cytoplasm
of the primary oocyte, is the secondary oocyte.
• The first polar body may continue its development and
undergo meiosis II, producing two even smaller polar
bodies.
• The secondary oocyte (not a functional ovum) is
ovulated. If an ovulated secondary oocyte is not
penetrated by a sperm, it simply deteriorates. But, if
sperm penetration does occur, it quickly completes
meiosis II, yielding one large ovum and a tiny second
polar body .
• The union of the egg and sperm nuclei, constitutes
fertilization.
• The potential end products of oogenesis are three tiny
polar bodies, nearly devoid of cytoplasm, and one
large ovum. All of these cells are haploid, but only the
ovum is a functional gamete.
• This is quite different from spermatogenesis, where
the product is four viable gametes–spermatozoa.
• The unequal cytoplasmic divisions that occur during
oogenesis ensure that a fertilized egg has ample
nutrients for its six- to seven-day journey to the
uterus. Without nutrient-containing cytoplasm the
polar bodies degenerate and die.
• Since the reproductive life of a female is at most
about 40 years (from the age of 11 to approximately
51) and typically only one ovulation occurs each
month, fewer than 500 oocytes out of her
estimated pubertal potential of 250,000 are
released during a woman’s lifetime.
The Ovarian Cycle
The monthly series of events associated with the maturation
of an egg is called the ovarian cycle. The ovarian cycle is best
described in terms of two consecutive phases.
1-follicular phase is the period of follicle growth, typically
indicated as lasting from the first to the fourteenth day of
the cycle.
2-The luteal phase is the period of corpus luteum activity, days
14–28. The so-called typical ovarian cycle repeats at intervals
of 28 days, with ovulation occurring midcycle.
However, only 10–15% of women naturally have 28-day cycles;
cycles as long as 40 days or as short as 21 days are fairly
common. In such cases, the length of the follicular phase and
timing of ovulation vary, : It is 14 days between the time of
ovulation and the end of the cycle.
The Follicular Phase
It occupies the first half of the cycle and involves several events.
• Primordial Follicle Becomes a Primary Follicle When a
primordial follicle is activated ,the squamouslike cells
surrounding the primary oocyte grow, becoming cuboidal
cells, and the oocyte enlarges. The follicle is now called a
primary follicle
• A Primary Follicle Becomes a Secondary Follicle Next,
follicular cells proliferate forming a stratified epithelium
around the oocyte .
• In the next stage , a layer of connective tissue condenses
around the follicle, forming the theca folliculi. As the follicle
grows, the surrounding cells cooperate to produce estrogens .
• At the same time, the oocyte secretes a glycoprotein-rich
substance that forms a thick transparent membrane, called
the zona pellucida that encapsulates it .
• Clear liquid accumulates between the cells and
eventually coalesces to form a fluid-filled cavity called
the antrum .The presence of an antrum distinguishes
the new secondary follicle from the primary follicle.
• A Secondary Follicle Becomes a Vesicular Follicle. The
antrum continues to expand with fluid until it isolates
the oocyte, along with its surrounding capsule of cells
called a corona radiata. When a follicle is full size
(about 2.5 cm, or 1 inch, in diameter), it becomes a
vesicular follicle and bulges from the external ovarian
surface .This usually occurs by day 14.
As one of the final events of follicle maturation, the
primary oocyte completes meiosis I to form the
secondary oocyte and first polar body. Once this has
occurred , the stage is set for ovulation.
• Ovulation
Ovulation occurs when the ballooning ovary wall
ruptures and expels the secondary oocyte (still
surrounded by its corona radiata) into the
peritoneal cavity . Some women experience a
middle pain caused by the intense stretching of the
ovarian wall during ovulation.
In the ovaries of adult females, there are always
several follicles at different stages of maturation. As
a rule, one follicle outstrips the others The others
degenerate (undergo programmed cell death, or
apoptosis) and are reabsorbed.
• In 1–2% of all ovulations, more than one oocyte is
ovulated. This phenomenon, which increases with age,
can result in multiple births. Since, in such cases,
different oocytes are fertilized by different sperm, the
siblings are fraternal, or nonidentical, twins.
• Identical twins result from the fertilization of a single
oocyte by a single sperm, followed by separation of the
fertilized egg’s daughter cells in early development.
N.,B. in some women, oocytes may be released at times
unrelated to the woman’s hormone levels, which may
help to explain why a rhythm method of contraception
sometimes fails and why some fraternal twins have
different conception dates.
The Luteal Phase
After ovulation, the ruptured follicle collapses, and
form a quite different endocrine gland, the corpus
luteum (“yellow body”), that begins to secrete
progesterone and some estrogen.
• If pregnancy does not occur, the corpus luteum
starts degenerating in about ten days and its
hormonal output ends. In this case, all that
ultimately remains is a scar called the corpus
albicans .
• On the other hand, if the oocyte is fertilized and
pregnancy ensues, the corpus luteum persists until
the placenta is ready to take over its hormoneproducing duties in about three months.
Hormonal Regulation of the Ovarian Cycle
Establishing the Ovarian Cycle
As puberty nears, the hypothalamus begins to release
GnRH in a rhythmic manner which stimulates the
anterior pituitary to release FSH and LH, which
prompt the ovaries to secrete hormones (primarily
estrogens).
Gonadotropin levels continue to increase for about
four years and, during this time, pubertal girls are still
not ovulating and thus are incapable of getting
pregnant. The young woman’s first menstrual period
is referred to as menarche.
• Usually, it is not until the third year postmenarche
that the cycles become regular and all are ovulatory.
• As the corpus luteum degenerates , blood estrogen
and progesterone levels drop sharply. The marked
decline in ovarian hormones at the end of the cycle
(days 26–28) ends their blockade of FSH and LH
secretion, and the cycle starts anew.
Although the ovarian events are described as if we
are following one follicle through the 28-day cycle,
this is not really the case. What is happening is that
the increase of FSH at the beginning of each cycle
activates several follicles to mature. Then, with the
midcycle LH surge, one (or more) Graafian follicles
undergo ovulation.
Uterine (Menstrual) Cycle
Although the uterus is where the young embryo
implants and develops, it is receptive to
implantation for only a short period each month
when a developing embryo would normally begin
implanting, six to seven days after ovulation.
The uterine, or menstrual cycle is a series of cyclic
changes that the uterine endometrium goes
through each month as it responds to ovarian
hormones in the blood. These endometrial changes
are coordinated with the phases of the ovarian
cycle, which are dictated by gonadotropins released
by the anterior pituitary.
The events of the uterine cycle, are as follows:
1. Days 1–5, Menstrual phase: In this phase,
menstruation ,the uterus sheds all but the deepest
part of its endometrium.
-At the beginning of this stage, ovarian hormones are
at their lowest normal levels and gonadotropins are
beginning to rise.
-The thick, hormone-dependent functional layer of the
endometrium detaches from the uterine wall, a process
that is accompanied by bleeding for 3–5 days with
about 150 ml of blood. The detached tissue and blood
pass out through the vagina as the menstrual flow.
-By day 5, the growing ovarian follicles are starting to
produce more estrogen .
2. Days 6–14, Proliferative (preovulatory) phase
-Under the influence of rising blood levels of estrogens,
the basal layer of the endometrium generates a new
functional layer. As this new layer thickens, its glands
enlarge and its spiral arteries increase in number .
Consequently, the endometrium once again becomes
velvety, thick, and well vascularized.
-Normally, cervical mucus is thick and sticky, but rising
estrogen levels cause it to thin to facilitate the passage
of sperm into the uterus.
-Ovulation, which takes less than five minutes, occurs
in the ovary at the end of the proliferative stage (day
14) in response to the sudden release of LH from the
anterior pituitary. LH also converts the ruptured follicle
to a corpus luteum.
3. Days 15–28, Secretory (postovulatory) phase:
• This 14-day phase is the most constant timewise.
Rising levels of progesterone convert the functional
layer to a secretory mucosa. The uterine glands
enlarge, coil, and begin secreting nutritious
glycogen into the uterine cavity. These nutrients
sustain the embryo until it has implanted in the
blood-rich endometrial lining.
• Increasing progesterone levels also cause the
cervical mucus to become viscous again, forming
the cervical plug, which blocks sperm entry.
• Rising progesterone (and estrogen) levels inhibit LH
release by the anterior pituitary.
• If fertilization has not occurred, the corpus
luteum begins to degenerate toward the end
of the secretory phase as LH blood levels
decline. Progesterone levels fall, and the spiral
arteries kink and go into spasms so
endometrial cells die, setting the stage for
menstruation to begin on day 28.
• The spiral arteries then suddenly relax and
open wide. As blood gushes into the
weakened capillary beds, they fragment,
causing the functional layer to slough off.
Developmental Aspects of the Reproductive System
• Sex is determined (at the time of fertilization), by
the sex chromosomes: an X from the mother, an X
or a Y from the father. If the fertilized egg contains
XX, it is a female and develops ovaries; if it
contains XY, it is a male and develops testes.
• The development of male accessory structures and
external genitalia depends on the presence of
testosterone produced by the embryonic testes. In
its absence, female structures develop.
• The testes form in the abdominal cavity and
descend into the scrotum.
• Puberty is the interval when reproductive
organs mature and become functional. It
begins in males with penile and scrotal growth
and in females with breast development.
• During menopause, ovarian function
declines, and ovulation and menstruation
cease. Hot flashes and mood changes may
occur. Postmenopausal events include atrophy
of the reproductive organs, bone mass loss,
and increasing risk for cardiovascular disease.
Pregnancy and Human Development
• The term pregnancy refers to events that occur from
the time of fertilization (conception) until the infant is
born.
• The time during which development occurs is referred
to as the gestation period and extends from the last
menstrual period (a date the woman is likely to
remember) until birth, approximately 280 days. Thus,
at the moment of fertilization, the mother is officially
two weeks pregnant!
• From fertilization through week 8, the embryonic
period, the conceptus is called an embryo, and from
week 9 through birth, the fetal period, the conceptus is
called a fetus .
• At birth, it is an infant.
Accomplishing Fertilization
Before fertilization can occur, sperm must reach the
ovulated secondary oocyte.
• The oocyte is viable for 12 to 24 hours after it is cast
out of the ovary.
• Most sperm retain their fertilizing power for 24 to 48
hours after ejaculation.
• Consequently, for successful fertilization to occur,
coitus must occur no more than two days before
ovulation and no later than 24 hours after, at which
point the oocyte is approximately one-third of the way
down the length of the uterine tube.
• Fertilization occurs when a sperm nucleus fuses with
that of the egg (actually a secondary oocyte) to form a
fertilized egg, or zygote ,the first cell of the new
individual.
• Sperm freshly deposited must first be capacitated over the
next 6 to 8 hours; that is, their mobility is enhanced and their
membranes must become fragile so that the hydrolytic
enzymes in their acrosomes can be released. Thus, even
though the sperm may reach the oocyte within a few
minutes, they must “wait around” for capacitation to occur.
Hundreds of sperms must release their acrosomal enzymes to
break down the egg’s corona radiata and zona pellucida.
When one sperm binds to receptors on the egg, it triggers the
slow block to polyspermy.
• Polyspermy (entry of several sperm into an egg) occurs in
some animals, but in humans only one sperm is allowed to
penetrate the oocyte, ensuring monospermy
• Following sperm penetration, the secondary oocyte
completes meiosis II.
• Then the ovum and sperm pronuclei fuse (fertilization),
forming a zygote.
Results of fertilization
1- Determination of the sex of the embryo
2-Restoration of chromosomal number (2n)
characteristic of the human embryo which is 46.
3-Starting of cleavage.
Events of Embryonic Development: Zygote to Blastocyst
• Cleavage, a rapid series of mitotic divisions begins
with the zygote and ends with a blastocyst. The
blastocyst consists of the trophoblast and an inner cell
mass. Cleavage produces a large number of cells.
• Implantation The trophoblast adheres to, digests, and
implants in the endometrium. Implantation is
completed when the blastocyst is entirely surrounded
by endometrial tissue, about 12 days after ovulation.
• hCG released by the blastocyst maintains hormone
production by the corpus luteum, preventing menses.
hCG levels decline after four months.
• Placentation The placenta acts as the respiratory,
nutritive, and excretory organ of the fetus and
produces the hormones of pregnancy. It is formed from
embryonic and maternal tissues. Typically, the placenta
is functional as an endocrine organ by the third month.
Gastrulation: formation of Germ Layers
ultimately the inner cell mass is transformed into a
three-layered embryo (gastrula) containing
ectoderm, mesoderm, and endoderm.
• Ectoderm forms the nervous system and the
epidermis of the skin and its derivatives.
• Endoderm forms the mucosa of the digestive and
respiratory systems, and the epithelial cells of all
associated glands (thyroid, parathyroids, thymus,
liver, pancreas). It becomes a continuous tube
when the embryonic body fuses ventrally.
• Mesoderm forms all other organ systems and
tissues.
Effects of Pregnancy on the Mother
• Anatomical Changes
1.Maternal reproductive organs and breasts become
increasingly vascularized during pregnancy, and the
breasts enlarge.
2. The uterus eventually occupies nearly the entire
abdominopelvic cavity. Abdominal organs are pushed
superiorly and encroach on the thoracic cavity, causing
the ribs to flare.
3. The increased abdominal mass changes the woman’s
center of gravity; lordosis and backache are common. A
waddling gait occurs as pelvic ligaments and joints are
loosened by placental relaxin.
4. A typical weight gain during pregnancy in a woman of
normal weight is 28 pounds.
• Physiological Changes
1. Many women suffer morning sickness,
heartburn, and constipation during pregnancy.
2. The kidneys produce more urine, and pressure
on the bladder may cause frequency, urgency, and
stress incontinence.
3. Tidal volume and respiratory rate increase, but
residual volume decreases. Dyspnea is common.
4. Total body water and blood volume increase
dramatically. Heart rate and blood pressure rise,
resulting in enhancement of cardiac output in the
mother.
Parturition (Birth) is giving birth to the baby. It
usually occurs within 15 days of the calculated due
date (280 days from the last menstrual period).
The series of events that expel the infant from the
uterus are collectively called labor.
Initiation of Labor
Fetal cells produce oxytocin, which stimulates
prostaglandin production by the placenta. Both
hormones stimulate contraction of uterine muscle.
Increasing stress activates the hypothalamus,
causing oxytocin release from the posterior
pituitary; this sets up a positive feedback loop
resulting in true labor.
Stages of Labor
• The dilation stage is from the onset of
rhythmic, strong contractions until the cervix
is fully dilated. The head of the fetus rotates
as it descends through the pelvic outlet.
• The expulsion stage extends from full cervical
dilation until birth of the infant.
• The placental stage is the delivery of the
afterbirth (the placenta and attached fetal
membranes).
Adjustments of the Infant to Extrauterine Life
1. After the umbilical cord is clamped, carbon dioxide
accumulates in the infant’s blood, causing respiratory
centers in the brain to trigger the first inspiration.
2. Once the lungs are inflated, breathing is eased by
the presence of surfactant, which decreases the surface
tension of the alveolar fluid.
3. During the first 8 hours after birth, the infant is
physiologically unstable and adjusting. After stabilizing,
the infant wakes approximately every 3–4 hours in
response to hunger.
4. Inflation of the lungs causes pressure changes in
the circulation; as a result, the umbilical arteries and
vein, ductus venosus, and ductus arteriosus collapse,
and the foramen ovale closes. The occluded blood
vessels are converted to fibrous cords; the site of the
foramen ovale becomes the fossa ovalis.
Lactation
1. The breasts are prepared for lactation during
pregnancy by high blood levels of estrogen,
progesterone, and placental lactogen.
2. Colostrum, a premilk fluid, is a fat-poor fluid that
contains more protein, vitamin A, and minerals than
true milk. It is produced toward the end of pregnancy
and for the first two to three days after birth.
3. True milk is produced around day 3 in response to
suckling, which stimulates the hypothalamus to prompt
anterior pituitary release of prolactin and posterior
pituitary release of oxytocin. Prolactin stimulates milk
production and oxytocin triggers milk let-down.
Continued breast-feeding is required for continued milk
production.
4. At first, ovulation and menses are absent or irregular
during nursing, but in most women the ovarian cycle is
eventually reestablished while still nursing.
•
•
•
•
Related Clinical Terms
Abortion Premature removal of the embryo or fetus
from the uterus; may be spontaneous or induced.
Ectopic pregnancy A pregnancy in which the embryo
implants in any site other than the uterus; most often
the site is a uterine tube (tubal pregnancy).
Hydatid (hydatidiform) mole Developmental
abnormality of the placenta; the conceptus
degenerates and the chorionic villi convert into a mass
of vesicles. Signs include vaginal bleeding, which
contains some of the grapelike vesicles.
Ultrasonography Noninvasive technique that uses
sound waves to visualize the position and size of the
fetus and placenta .
• Physiological jaundice Jaundice sometimes occurring
in normal newborns within three to four days after
birth. Fetal erythrocytes are short-lived, and they
break down rapidly after birth; the infant’s liver may
be unable to process the bilirubin (breakdown
product of hemoglobin pigment) fast enough to
prevent its accumulation in blood and subsequent
deposit in body tissues.
• Placenta abruptio Premature separation of the
placenta from the uterine wall; if this occurs before
labor, it can result in fetal death due to anoxia.
• Placenta previa Placental formation adjacent to or
across the internal os of the uterus. Represents a
problem because as the uterus and cervix stretch,
tearing of the placenta may occur. Additionally, the
placenta precedes the infant during labor.