Physiology%20of%20Reproduction

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Physiology%20of%20Reproduction

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Title: Physiology%20of%20Reproduction

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Physiology of Reproduction
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Gonadal development

Both the testes and the ovaries are derived from
the same gonadal primordium.
There are two sets of ducts, the Wolfian duct and
the Mullarian duct.
Development of the primary sexual characteristics
depends directly on the endocrine environment
during development.
An individual can be forced into either a female
development or a male development by application
of the appropriate hormones, regardless of
genetic makeup.
In the absence of hormonal stimulation, the
gonadal primordium will develop into ovaries and
the Mullarian ducts will develop into the uterine
ducts, uterus and vagina.

Development
The sex organs themselves, along with all their
associated ducts and glands are referred to as
the Primary sexual characters
Secondary sexual characteristics are structures
which will enhance reproduction, but are not
necessarily required. For example, beard growth
in men.

Without hormonal stimulation, the Wolffian duct
regresses.
In males, the gonadal primordium begins to
secrete testosterone and Mullarian Inhibiting
Substance (MIS).
Testosterone stimulates the development of the
Wolffian ducts, which subsequently differentiate
into the vas deferens, epididymis and seminal
vesicles.

MIS causes the Mullarian ducts to degenerate
Estradiol can prevent MIS from stimulating
Mullarian duct regression.
Testosterone is converted into dihydrotestosterone
(DHT) by the enzyme 5a-reductase.
DHT influences the development of the external
genitalia.
The genital tubercle becomes the penis.
The genital folds become the shaft of the penis.
The genital swellings become the scrotum.

Without DHT, the external genitalia are
feminized.
The genital tubercle becomes the clitorus.
The genital folds become the labia minora.
The genital swelling becomes the labia majora.
Currently, the structure of MIS is not known, but
it appears to be a glycoprotein.
Circulating levels of androgens (and possibly
estrogens) also trigger differential development
in the brain. Animals exposed to androgens
during a specific critical window will develop
male reproductive behavior, regardless of the
genotype or the physical phenotype.

Development of secondary sexual characteristics
This usually coincides with the final maturation
of the gonads. In humans, this is referred to as
puberty.
Mechanism controlling onset is unclear, but
appears to involve the loss of inhibition of
gonadal development.
One potential candidate (at least in males) is
melatonin.
During childhood, melatonin is produced in the
pars intermedia of the pituitary gland.
However, after childhood the pars intermedia
stops producing melatonin. Melatonin synthesis
and secretion are taken over by the pineal gland,
but at a much reduced rate.

This drastic drop in melatonin secretion (gt75)
may trigger the secretion of sex steroids by the
adrenal glands and/or the testes.
In females, the situation may be different.
There is good evidence that the hormone leptin is
also involved.
Leptin is a hormone released by adipose tissue.
Circulating leptin levels may reflect total body
fat storage by the body.
In females, a certain minimum total-body fat
content is required for puberty to progress and
for maintenance of the menstrual cycle.

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Male reproductive system
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Spermatogenesis I

The immature germ cell in the male is referred to
as the spermatogonium.
These cells are located just under the basement
membrane of the seminiferous tubules, between
adjoining sustentacular (Sertoli) cells.
Since sperm production continues throughout adult
life and at the peak, 100-200 million sperm can
be produced daily, the spermatogonia are
constantly renewed.
The first step in spermatogenesis is a mitotic
division of the spermatogonium. One of the
daughter cells remains, to replace the original
spermatogonium, while the other cell (now called
a primary spermatocyte) undergoes meiosis.

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Spermatid migration
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Spermatogenesis II

The first meiotic division yields two secondary
spermatocytes. Usually, these secondary
spermatocytes do not fully separate during cell
division, leaving a direct cytoplasmic connection
between the cells.
Following the second meiotic division (again, an
incomplete division), the cells are known as
spermatids. As the germ cells are undergoing
meiosis, they also migrate towards the lumen of
the seminiferous tubule.
As they approach the lumen, they shed much of
their cytoplasm. They are attached to the
Sustentacular cells, via specialized junctions,
which provide nutrients.
When the spermatids reach the lumen, they remain
embedded within the sustentacular cells, where
they undergo tail development, acrosome formation
and nuclear condensation.
Finally, the fully-formed spermatozoa are shed
into the lumen of the seminiferous tubule, where
they are carried to the epididymus. This whole
process takes between 60 and 70 days.

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Mitosis vs. Meiosis
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Male reproductive ducts

The spermatozoa traverse the epididymus in 2 to 4
weeks.
During this time, they lose most of the remaining
cytoplasm, as well as increase in mobility.
The epithelial cells which line the epididymus
secrete proteins which bind to the sperm cell
membranes, to enhance their forward mobility and
ability to fertilize an ovum.
The sperm migrate into the ductus (or vas)
deferens, where they can be stored for several
months.
The vas deferens runs up through the spermatic
cord, conducting the sperm to the prostate
gland.

The end of each ductus deferens (two) enlarges
to form ampullae, where sperm are stored until
ejaculation.
The prostate contains the first part of the
urethra (prostatic urethra) which is where the
ejaculatory ducts merge with the urethra.
The urethra exits the prostate, penetrated the
urogenital diaphragm and runs the length of the
penis.

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Male sexual response

Erection
The first phase of the male sexual response is
erection of the penis, which allows it to
penetrate the female vagina.
This occurs when the erectile tissue of the penis
becomes engorged with blood.
When a male is not sexually aroused, the
arterioles supplying the erectile tissues are
constricted.

During sexual excitement, a parasympathetic
reflex is triggered that causes these arterioles
to dilate (NO2).
As a result, the vascular spaces of the penis
fill with blood causing the penis to become
enlarged and rigid.
Expansion of the penis also compresses the veins
retarding the outflow of blood and further
contributing to the swelling of the penis.
This reflex is initiated by a variety of stimuli
ranging from thought to touch.

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Ejaculation

A spinal reflex is initiated, producing a
sympathetic discharge to the genital organs.
As a result, the reproductive ducts and accessory
glands contract peristaltically discharging their
contents into the urethra.
The muscles of the penis undergo a rapid series
of contractions propelling semen from the
urethra.
This is followed by muscular and psychological
relaxation and vasoconstriction of the arterioles
serving the penis, allowing blood to drain out of
the erectile tissue, which subsequently causes
the penis to become flaccid again.

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Role of the Accessory Glands

The seminal vesicles are paired glands that
produce about 60 of the semen.
Their secretions contain fructose sugar, ascorbic
acid and prostaglandins.
These are sac shaped glands, approximately 5
centimeters long, which lie along side the
ampullae of the ductus deferens.
They each empty into a short duct, the
ejaculatory duct, which merges with the terminal
end of the ductus deferens.

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These, in turn, fuse with the prostatic urethra
which runs from the bladder through the prostate
gland.
The alkalinity of the fluid serves to neutralize
the normally acidic environment in the distal
urethra and in the vagina.
The fructose is supplied as an energy source for
the sperm, and the prostaglandins serve to
stimulate smooth muscle contractions in the
vagina and cervix.
This is thought to facilitate the uptake of sperm
into the uterus.

The bulbourethral glands are paired glands that
secrete a small amount of thick clear mucus. This
secretion is released prior to ejaculation and is
believed to neutralize traces of acidic urine in
the urethra.
The prostate gland is a single gland, which
secretes about one third of the semen volume. It
secretes a milky, slightly acidic fluid
containing citrate, acid phosphatase and several
proteolytic enzymes. These enzymes are probably
involved in breaking down the mucus plug in the
cervix. They also appear to contribute to the
motility and viability of the sperm

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Semen Production

Remember, Sperm seminal fluid semen.
Semen provides a transport medium for the sperm.
It also provides nutrients for the sperm and
chemicals that protect them, activate them and
facilitate their movement.
The amount of semen released during ejaculation
is relatively small, about 2-6 ml but it contains
50-100 million sperm per ml.

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Sperm capacitance

Freshly ejaculated sperm are incapable of
fertilizing an egg.
As the sperm travel up the female reproductive
tract, they lose cholesterol from their membranes
When the sperm reach the fallopian tubes, the
membranes around the acrosome are fragile enough
to allow the release of the acrosomal enzymes.

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Brain-testicular axis
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Female reproductive system
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OOGENESIS I

This process is the equivalent of spermatogenesis
in the male. However, the two processes are
vastly different.
In females, much of the process occurs during
fetal development.
The primitive germ cells undergo numerous rounds
of mitosis, which produces millions of oogonia
(2n).
Most of these oogonia are resorbed (through a
process called atresia).
However, a few hundred thousand begin meiosis and
enter prophase I. These are now referred to as
primary oocytes.

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OOGENESIS II

There are no oogonia present in the adult female.
The primary oocytes are arrested in prophase I
and become quiescent until puberty.
Cyclical changes in LH and FSH will trigger three
or four primary oocytes to finish meiosis each
uterine cycle.
During the two meiotic divisions, all the
cytoplasm will stay with a single daughter cell,
which is destined to become the ovum.
The other three daughter cells simply develop as
small polar bodies that are eventually degraded
and resorbed.

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Female Sexual Response

As with Males, arousal is controlled by
parasympathetic stimulation.
Involves engorgement of the erectile tissues.
Increased bloodflow to the external genitalia and
vaginal walls.
Stimulation of secretion of cervical mucous
glands and greater vestibular glands.

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Female Sexual Response (cont.)

Rhythmic contact of the clitoris and vaginal
walls, reinforced by touch sensations from the
breasts and other stimuli, can lead to orgasm.
As with male climax, female climax results in
rhythmic peristaltic contractions of the uterus
and vaginal walls and associated skeletal
muscles.
This is thought to enhance the migration of
sperm up the female reproductive tract.
Female climax is NOT required for fertilization.

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Fertilization and pregnancy
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Fertilization
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Blocks to polyspermy

If more than one sperm were to fertilize the egg,
then the genetic complement would be 3n.
In order to prevent multiple sperm penetrations,
two responses have evolved in the egg.
First, as soon as the first sperm head penetrates
the egg, it triggers a massive influx of Na.
This influx depolarizes the egg, making it
positive inside. This repels the positively
charged sperm, inhibiting penetration of more
sperm.

Second, the depolarization triggers an influx of
Ca 2 . This Ca 2 facilitates the exocytosis of
a number of secretory vesicles, known as cortical
vesicles.
The contents of these vesicles surrounds the egg,
swells with water and gels, pushing other sperm
away from the egg and blocking their entry.

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Implantation
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Chorionic villi
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Placental hormones

During early pregnancy, HCG is secreted by the
syncitial trophoblasts.
Later, the placenta secretes estradiol,
progesterone, relaxin and somato-mammotropin.

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Function of placental hormones

HCG is similar to LH and maintains the corpus
luteum in a functional state for 3-4 months.
This keeps progesterone levels high and they
maintain the functional endometrium.
Relaxin increases flexibility in the pelvic
joints, as well as suppressing release of
oxytocin.
Placental progesterone keeps the uterine wall
intact.
Somatomammotropin acts like prolactin and
triggers the mammary glands to develop.
Estrogen increases the sensitivity of the
myometrium to mechanical irritation, as well as
oxytocin stimulation.

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Labour

Towards the end of pregnancy, relaxin secretion
falls off, thus, the uterus becomes more
sensitive to oxytocin.
Initially, the fetus secretes oxytocin into the
maternal circulation.
The oxytocin stimulates contractions, which push
the head down against the cervix.

This pressure on the cervix stimulates the
release of oxytocin from the maternal pituitary
gland.
The maternal oxytocin causes more contractions of
the uterus, forcing the head of the fetus against
the cervix even harder.
This is a positive feedback system.

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Labour and delivery

As the head of the fetus is pressed down against
the cervix, it thins and then starts to dilate.
This stage is known as the Dilation Stage and can
last several hours, or days (usually around 8
hours).
Once the cervix has dilated, the fetus starts
moving through the birth canal. Contractions are
maximal and come about 2-3 minutes.
This is known as the Expulsion Stage.
If the vaginal wall has not stretched enough,
tearing may occur.

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Labour and delivery cont.

There is also a chance that the fetus will get
stuck in the birth canal (usually caused by
insufficient molding of the head.
In these cases, a cesarean section is performed.
Finally, after expulsion of the fetus, the
placenta detaches from the uterine wall and is
delivered through the birth canal.
This is known as the Placental Stage.