Physiology of
Reproduction
The following is an actual question given on a University of Washington chemistry mid-term.
The answer by one student was so "profound" that the professor shared it with colleagues:
Bonus Question: Is Hell exothermic (gives off heat) or endothermic (absorbs heat)?
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First, we need to know how the mass of Hell is changing in time. So we need to know the rate at which souls
are moving into Hell and the rate at which they are leaving. I think that we can safely assume that once a soul
gets to Hell, it will not leave. Therefore, no souls are leaving. As for how many souls are entering Hell, let's
look at the different religions that exist in the world today. Most of these religions state that if you are not a
member of their religion, you will go to Hell. Since there is more than one of these religions and since people
do not belong to more than one religion, we can project that all souls go to Hell.
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With birth and death rates as they are, we can expect the number of souls in Hell to increase exponentially.
Now, we look at the rate of change of the volume in Hell because Boyle's Law states that in order for the
temperature and pressure in Hell to stay the same, the volume of Hell has to expand proportionately as souls
are added.
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This gives two possibilities: 1. If Hell is expanding at a slower rate than the rate at which souls enter Hell,
then the temperature and pressure in Hell will increase until all Hell breaks loose. 2. If Hell is expanding at a
rate faster than the increase of souls in Hell, then the temperature and pressure will drop until Hell freezes
over.
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So which is it? If we accept the postulate given to me by Teresa during my Freshman year that, "It will be a
cold day in Hell before I sleep with you," and take into account the fact that I slept with her last night, then
number two must be true, and thus I am sure that Hell is exothermic and has already frozen over. The
corollary of this theory is that since Hell has frozen over, it follows that it is not accepting any more souls and
is therefore, extinct......leaving only Heaven, thereby proving the existence of a divine being which explains
why, last night, Teresa kept shouting "Oh my God.“
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THIS STUDENT RECEIVED THE ONLY "A"
Gonadal development:
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Both the testes and the ovaries are derived from the same
gonadal primordium.
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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.
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Development:
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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.
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Without hormonal stimulation, the Wolffian
duct regresses.
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In males, the gonadal primordium begins to
secrete testosterone and Mullarian Inhibiting
Substance (MIS).
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Testosterone stimulates the development of the
Wolffian ducts, which subsequently differentiate
into the vas deferens, epididymis and seminal
vesicles.
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MIS causes the Mullarian ducts to degenerate
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Estradiol can prevent MIS from stimulating
Mullarian duct regression.
Testosterone is converted into
dihydrotestosterone (DHT) by the enzyme 5αreductase.
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.
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Without DHT, the external genitalia are feminized.
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The genital tubercle becomes the clitorus.
The genital folds become the labia minora.
The genital swelling becomes the labia majora.
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Currently, the structure of MIS is not known, but it
appears to be a glycoprotein.
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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.
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Development of secondary sexual characteristics:
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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.
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This drastic drop in melatonin secretion (>75%)
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:
Spermatogenesis I:
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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.
Spermatid migration:
Spermatogenesis II:
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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.
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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.
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As they approach the lumen, they shed much of their cytoplasm. They are attached to the
Sustentacular cells, via specialized junctions, which provide nutrients.
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When the spermatids reach the lumen, they remain embedded within the sustentacular cells,
where they undergo tail development, acrosome formation and nuclear condensation.
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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.
Mitosis vs. Meiosis:
Male reproductive ducts:
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The spermatozoa traverse the epididymus in 2 to 4 weeks.
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During this time, they lose most of the remaining cytoplasm, as well as
increase in mobility.
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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.
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The sperm migrate into the ductus (or vas) deferens, where they
can be stored for several months.
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The vas deferens runs up through the spermatic cord,
conducting the sperm to the prostate gland.
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The end of each ductus deferens (two) enlarges
to form ampullae, where sperm are stored until
ejaculation.
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The prostate contains the first part of the
urethra (prostatic urethra) which is where the
ejaculatory ducts merge with the urethra.
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The urethra exits the prostate, penetrated the
urogenital diaphragm and runs the length of the
penis.
Male sexual response:
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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.
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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.
Ejaculation
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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.
Role of the Accessory Glands:
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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.
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The alkalinity of the fluid serves to neutralize the
normally acidic environment in the distal urethra and in
the vagina.
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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.
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This is thought to facilitate the uptake of sperm into the
uterus.
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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.
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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, fibronectin, prostate
specific antigen (PSA), and semenogelins I and
II. and several proteolytic enzymes.
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The proteolytic 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.
After ejaculation, SgI, SgII and fibronectin
aggregate to form a gelatinous mass, which is
believed to trap the spermatozoa within the
vagina.
Liquefaction occurs 5-20 minutes later, through
cleavage of the semenogelins by PSA prostatespecific antigen).
Semen Production
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Remember, Sperm + seminal fluid = semen.
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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.
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The amount of semen released during ejaculation is
relatively small, about 2-6 ml but it contains 50-100
million sperm per ml.
Sperm capacitance:
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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.
Brain-testicular axis:
Female reproductive system:
OOGENESIS I:
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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).
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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.
OOGENESIS II:
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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.
Female Sexual Response:
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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.):
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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.
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This is thought to enhance the migration of sperm
up the female reproductive tract.
Female climax is NOT required for fertilization.
Fertilization and
pregnancy:
Fertilization:
Blocks to polyspermy
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If more than one sperm were to fertilize the egg, then
the genetic complement would be 3n.
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In order to prevent multiple sperm penetrations, two
responses have evolved in the egg.
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First, as soon as the first sperm head penetrates the egg,
it triggers a massive influx of Na+.
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This influx depolarizes the egg, making it positive inside. This
repels the positively charged sperm, inhibiting penetration of
more sperm.
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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.
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The contents of these vesicles surrounds the egg,
swells with water and gels, pushing other sperm
away from the egg and blocking their entry.
Implantation:
Chorionic villi
Placental hormones:
During early
pregnancy, HCG is
secreted by the
syncitial trophoblasts.
Later, the placenta
secretes estradiol,
progesterone, relaxin
and somatomammotropin.
Function of placental hormones:
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HCG is similar to LH and maintains the corpus luteum
in a functional state for 3-4 months.
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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.
Labour:
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Fetal growth results in
distortion of the myometrium.
Placental secretion of relaxin
prevents uterine contraction.
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.
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This pressure on the
cervix stimulates the
release of oxytocin
from the maternal
pituitary gland.
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The maternal oxytocin
causes more
contractions of the
uterus, forcing the
head of the fetus
against the cervix even
harder.
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This is a positive
feedback system.
Labour and delivery:
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As the head of the fetus is pressed down against
the cervix, it thins and then starts to dilate.
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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. :
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There is also a chance that the fetus will get
stuck in the birth canal (usually caused by
insufficient molding of the head.
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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.
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This is known as the Placental Stage.
Nursing:
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Two hormones are
involved, PRL and
oxytocin.
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PRL stimulates milk
production, while
oxytocin is required for
the expression of milk
from the breast.
Fertility issues: