Chapter 12 Reproductive System
The term “reproduction” usually means formation of
new living entities, which closely resemble to the
parent species.
In higher animals, reproduction is accomplished by
activities of both sexual organs of male and female.
The processes are very complex including production
of sperms and ova, sexual intercourse, fertilization
of ovum, pregnancy, development of fetus and
lactation.
Section 1 Reproductive and Hormonal Functions of
the Male
Functions of testes:
spermatogenesis in seminiferous tubules
secrete hormones (testosterone and inhibin).
I. Endocrine Function of Testes
1. Androgens
Testes secrete several male sex hormones, which are
collectively called androgens, including
testosterone, dihydrotesterone and androstenodione.
Testosterone is so much more abundant than the others
that one can consider it the significant testicular
hormone.
(1) Functions of testosterone
1) Maintenance of spermatogenesis
2) Stimulation of development of genitalia.
3) Development and maintenance of the male secondary
sexual characteristics.
4) Maintenance of libido.
5) Effect on metabolism.
Testosterone increases anabolism of protein, especially
that in muscle and genitalia, and causes growth of the
bones in the thickness, deposition of calcium salts as
well enhancement of production of red cells.
2. Inhibin
Inhibin, a glycoprotein with a molecular weight of
31,000-32,000, has a potent inhibitory effect on FSH
secretion by pituitary gland.
The effect of inhibin on hypothalamus is slightly
inhibitory in GnRH secretion.
II. Regulation of Testis Function
1. Hypothalamic-Pituitary-Testis
Axis
(1) GnRH (gonadotropin –
releasing hormone)
GnRH is secreted by
hypothalamus and transported
to the anterior pituitary gland
in portal blood.
The function of GnRH is to
stimulate the synthesis and
release the two gonadotropins,
LH and FSH.
critinism, no GnRH secreted during the puberty, no development of
the sexual organs, no reproductive ability
(2) LH (Luteinizing hormone or interstitial cell stimulating
hormone)
LH and FSH are secreted by cells in the anterior pituitary
gland. LH causes interstitial cell of Ldydig to synthesize
testosterone by activating the cAMP second message system
and to release testosterone.
(3) FSH (follicle stimulating hormone)
FSH stimulates Sertoil cells to produce ABP (androgen
binding protein). ABP binds with testosterone and enters the
seminiferous tubule, increasing the androgen concentration in
tubule and promoting spermatogenesis.
FSH stimulates Sertoil cell to produce and secrete inhibin,
which plays a negative feedback mechanism for control of
testis function.
2 Inhibin
Inhibin has a strong direct effect on the anterior
pituitary gland in inhibiting FSH secretion and a
light effect on the hypothalamus in inhibiting
GnRH secretion.
These automatic feedback mechanisms can maintain a
normal speed of spermatiogenesis, which is
required for male reproduction function.
Section 2 Reproductive and Hormonal Functions
of the Female
Organs and Function
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Ovaries –2 female sex glands
Produce egg cells
Produces the hormone, estrogen
Uterus or Womb
Muscular pear-shaped organ
Where baby develops
Fallopian Tubes
Egg travels through these 2 tubes
to the uterus
Cervix
Opening between the uterus & vagina
Vagina
Muscular tunnel from cervix to outside
I. Menstruation, Ovulation and Hormonal Regulation
The normal reproductive years of the female are characterized
by monthly rhythmical changes of secretion of the female
hormones and corresponding changes in the ovaries and
sexual organs.
This rhythmical pattern is called the female sexual cycle of
menstrual cycle, which may be regarded as periodic
preparation for fertilization and pregnancy.
Its most conspicuous feature is the periodic vaginal bleeding
that occurs with the shedding of the uterine mucosa,
which is known as menstruation.
The length of a menstrual cycle is about 28 days.
Diagrammatic section of the ovary showing various stages in the life of a
Graafian follicle: a primary oocyte developing into a fully mature follicle
about to ovvlate, then turning into a corpus luteum and finally ending up as
a corpus albicans.
1. Ovarian Cycle (1) Follicular phase; (2) Ovulation; (3) Luteal phase
Phases of endometrial growth and menstruation during each
monthly female sexual cycle.
2. Uterine cycle and menstruation
(1) Proliferative phase (estrogen phase)
(2) Secretory phase (progesterone phase)
(3) Menstruation
II. Endocrine Functions of Ovaries
1. Functions of Estrogen
(1) On sexual organs.
The principle function of estrogen is to cause cellular
proliferation and growth of the tissues of the sex
organs and other tissues related to production.
Estrogen stimulates the development of sexual organs
including ovaries, fallopian tubes, uterus, vagina
and the external genitalia.
(2) On secondary female sexual characteristics.
Estrogen stimulates development of the secondary sex
characteristics of female such as feminine figure, soft and
smooth skin.
It cause the breast to grow and to show mature appearance.
(3) On metabolism.
Estrogen causes a slight increase in total body protein, which
result in growth-promoting effect on the sexual organs, the
bones, the breasts and a few other tissues.
Estrogen has a slight effect on retention of water and sodium
by the kidney tubules.
2. Progesterone
Functions of progesterone
(1) Effect on uterus.
By far the most important function of progesterone is to
promote secretary changes in the uterine endometrium, thus
preparing the uterus for implantation of the fertilized ovum
and providing nutrition for the early dividing ovum.
Progesterone decreases excitability and sensitivity to oxytocin
of myometrium.
It decreases the frequency and intensity of uterine
contractions, thereby helping to prevent expulsion of the
implanted ovum.
(2) Effect on breasts.
Progesterone promotes development of the tubules and
alveoli of the breasts, causing the alveolar cells to
proliferate, enlarge, and become secretary nature
but not to secrete milk.
(3) Thermogenic effect.
Progesterone causes thermogenic effect and is
responsible for the rise in basal body temperature at
the time of ovulation.
(4) Progesterone relaxes the smooth muscle of the
blood vessels and digestive tracts.
III. Hormonal control of the menstrual cycle
Menstrual Cycle
 A woman is born with all the follicles she will
have (~400,000).
 Most will not mature, ~ 400 will
 LH and FSH influence developing follicle, which
produces estrogen (in particular, estradiol).
 An increase in estrogen results in peak levels of FSH
and LH which trigger ovulation (positive feedback
loop).
 Ovulation occurs – ovum ruptures from dominant
follicle.
 High LH levels 12-36 hrs before ovulation mark
division of follicular and luteal phases.
Menstrual Cycle
Luteal phase: follicle changes into a
temporary endocrine organ known as corpus
luteum
Secretes several hormones, including
progesterone, which helps prepare the lining of
the uterus for implantation should the egg be
fertilized
In absence of fertilization, the endometrial
lining sheds during menstruation.
Menstrual Cycle
If fertilization occurs, the corpus luteum
continues to synthesize and secrete steroid
hormones that maintain pregnancy for the
first 4-6 weeks.
Aided by another hormone, human chorionic
gonadotropin (hCG), that is biologically similar
to LH.
hCG is secreted by cells in the conceptus
destined to become the placenta
menstrual cycle
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proliferative phase
1. stimulated by estrogen
2. rebuilds endometrium
LH
FSH
secretory phase
1. stimulated by estrogen and
estrogen
progesterone
2. endometrium prepares forprogesterone
implantation
endometrium
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menstrual phase
1. lack of estrogen/progesterone
proliferative
2. removes endometrium
uterus
secretory uterus
Fig. 34-1: Events of the Menstrual Cycle
Hypothalamus secretes GnRH
Proliferative phase
Follicular stage
Anterior pituitary secretes LH and FSH
Graafian follicle stimulated
Ovaries release estrogen
High estrogen levels inhibit FSH secretion, stimulate LH production
Luteal stage
Secretory phase
LH makes mature follicle burst: ovulation
LH makes corpus luteum secrete progesterone
Progesterone inhibits LH secretion
Menstrual
phase
Decreased LH and FSH levels
Corpus luteum atrophies, stops making progesterone
Decreased estrogen and progesterone levels stimulate GnRH secretion
Cyclic changes in ovarian hormones secretion also cause
cyclic changes in basal temperature.
On the ay of the LH peak, when estrogen secretion begins to
decline, there is a slight drop in basal body temperature.
Starting about one day after the LH peak, the basal body
temperature sharply rises as a result of progesterone secretion
and remains elevated throughout the luteal phase of the cycle.
The day of ovulation can be accurately determined by this
method, making the method useful in increasing fertility if
conception is desired.
Since the day of the cycle in which ovulation occurs is quite
variable in many women, however, the rhythm method is not
reliable for contraception by predicting when the next
ovulation occur.
Section 4 Pregnancy
I. Fertilization and Implantation
II. Hormones of Placenta
The placenta forms large quantities of human
chorionic gonadotropin, estrogen, progesterone and
human chorionic somatomammotropin, which are
all essential to a normal pregnancy
1. Human Chorionic Gonadotropin (HCG)
HCG is a glycoprotein with a molecular weight of 39,000.
It is secreted by the syncytial trophoblast cells and can be measured in
the blood 8 to 9 days after ovulation.
The rate of secretion rises rapidly to reach maximum bout 10 to 12
weeks after ovulation and decreases to much lower value by 16 to 20
weeks after ovulation.
It continues at this level for the remainder of pregnancy.
This hormone is identical to LH in its effect and therefore is able to
maintain the corpus letum past the time when it would otherwise
regress.
The secretion of estradiol and progesterone is thus maintained and
menstruation is normally prevented.
Diagnosis of the early pregnancy
2. Progesterone and
Estrogen
At the fifth to sixth
week the mother’s
corpus luteum begins
to regress (even in the
presence of hCG), but
by this time the
placenta is secreting
more than sufficient
amounts of steroids
(estrogen and
progesterone) to
maintain the
endometrium and
prevent menstruation.
During pregnancy, the extreme quantities of estrogen
cause enlargement of uterus, breasts and mother’s
female external genitalia, which are convenient t
developing of fetus.
Progesterone has a specific effect o decreasing the
contractility of pregnant uterus, thus preventing
uterine contractions from causing abortion.
3. Human Chorionic Somatomammotropin (HCS)
HCS is a protein with molecular weight of 38,000.
It begins to be secreted by the placenta at the 5th week of
pregnancy.
Secretion of HCS increases progressively throughout the
remainder of pregnancy in direct proportion t the weight of
placenta.
HCS mainly stimulates growth of fetus and regulates
metabolism of carbohydrate, fat and protein in both the fetus
and the mother