Physiology of Physiology of ReproductionReproduction

Gonadal development:Gonadal development: Both the testes and the ovaries are derived from Both the testes and the ovaries are derived from

the same gonadal primordium.the same gonadal primordium. There are two sets of ducts, the Wolfian duct and the There are two sets of ducts, the Wolfian duct and the

Mullarian duct. Mullarian duct. Development of the primary sexual Development of the primary sexual

characteristics depends directly on the endocrine characteristics depends directly on the endocrine environment during development.environment during development.

An individual can be forced into either a female An individual can be forced into either a female development or a male development by development or a male development by application of the appropriate hormones, application of the appropriate hormones, regardless of genetic makeup.regardless of genetic makeup.

In the absence of hormonal stimulation, the In the absence of hormonal stimulation, the gonadal primordium will develop into ovaries and gonadal primordium will develop into ovaries and the Mullarian ducts will develop into the uterine the Mullarian ducts will develop into the uterine ducts, uterus and vagina.ducts, uterus and vagina.

Development:Development:

The sex organs themselves, along with The sex organs themselves, along with all their associated ducts and glands all their associated ducts and glands are referred to as the Primary sexual are referred to as the Primary sexual characterscharacters

Secondary sexual characteristics are Secondary sexual characteristics are structures which will enhance structures which will enhance reproduction, but are not necessarily reproduction, but are not necessarily required. For example, beard growth required. For example, beard growth in men.in men.

Without hormonal stimulation, the Without hormonal stimulation, the Wolffian duct regresses.Wolffian duct regresses.

In males, the gonadal primordium In males, the gonadal primordium begins to secrete testosterone and begins to secrete testosterone and Mullarian Inhibiting Substance (MIS).Mullarian Inhibiting Substance (MIS).

Testosterone stimulates the Testosterone stimulates the development of the Wolffian ducts, development of the Wolffian ducts, which subsequently differentiate into which subsequently differentiate into the vas deferens, epididymis and the vas deferens, epididymis and seminal vesicles.seminal vesicles.

MIS causes the Mullarian ducts to MIS causes the Mullarian ducts to degeneratedegenerate Estradiol can prevent MIS from stimulating Estradiol can prevent MIS from stimulating

Mullarian duct regression.Mullarian duct regression. Testosterone is converted into Testosterone is converted into

dihydrotestosterone (DHT) by the dihydrotestosterone (DHT) by the enzyme 5α-reductase.enzyme 5α-reductase. DHT influences the development of the DHT influences the development of the

external genitalia.external genitalia. The genital tubercle becomes the penis.The genital tubercle becomes the penis. The genital folds become the shaft of the The genital folds become the shaft of the

penis.penis. The genital swellings become the scrotum.The genital swellings become the scrotum.

Without DHT, the external genitalia are Without DHT, the external genitalia are feminized.feminized. The genital tubercle becomes the clitorus.The genital tubercle becomes the clitorus. The genital folds become the labia minora.The genital folds become the labia minora. The genital swelling becomes the labia majora.The genital swelling becomes the labia majora.

Currently, the structure of MIS is not known, Currently, the structure of MIS is not known, but it appears to be a glycoprotein.but it appears to be a glycoprotein.

Circulating levels of androgens (and possibly Circulating levels of androgens (and possibly estrogens) also trigger differential development estrogens) also trigger differential development in the brain. Animals exposed to androgens in the brain. Animals exposed to androgens during a specific critical window will develop during a specific critical window will develop male reproductive behavior, regardless of the male reproductive behavior, regardless of the genotype or the physical phenotype.genotype or the physical phenotype.

Development of secondary sexual characteristics:Development of secondary sexual characteristics: This usually coincides with the final maturation of the This usually coincides with the final maturation of the

gonads. In humans, this is referred to as puberty.gonads. In humans, this is referred to as puberty. Mechanism controlling onset is unclear, but appears to Mechanism controlling onset is unclear, but appears to

involve the loss of inhibition of gonadal development.involve the loss of inhibition of gonadal development. One potential candidate (at least in males) is One potential candidate (at least in males) is

melatonin.melatonin. During childhood, melatonin is produced in the pars During childhood, melatonin is produced in the pars

intermedia of the pituitary gland.intermedia of the pituitary gland.

However, after childhood the pars intermedia However, after childhood the pars intermedia stops producing melatonin. Melatonin synthesis stops producing melatonin. Melatonin synthesis and secretion are taken over by the pineal gland, and secretion are taken over by the pineal gland, but at a much reduced rate.but at a much reduced rate.

This drastic drop in melatonin secretion This drastic drop in melatonin secretion (>75%) may trigger the secretion of sex (>75%) may trigger the secretion of sex steroids by the adrenal glands and/or steroids by the adrenal glands and/or the testes.the testes.

In females, the situation may be In females, the situation may be different.different.

There is good evidence that the There is good evidence that the hormone leptin is also involved.hormone leptin is also involved. Leptin is a hormone released by adipose Leptin is a hormone released by adipose

tissue.tissue. Circulating leptin levels may reflect total Circulating leptin levels may reflect total

body fat storage by the body.body fat storage by the body. In females, a certain minimum total-body fat In females, a certain minimum total-body fat

content is required for puberty to progress content is required for puberty to progress and for maintenance of the menstrual cycle.and for maintenance of the menstrual cycle.

Male reproductive Male reproductive system:system:

Spermatogenesis I: The immature germ cell in the male is referred to as The immature germ cell in the male is referred to as

the spermatogonium.the spermatogonium. These cells are located just under the basement These cells are located just under the basement

membrane of the seminiferous tubules, between membrane of the seminiferous tubules, between adjoining sustentacular (Sertoli) cells. adjoining sustentacular (Sertoli) cells.

Since sperm production continues throughout adult Since sperm production continues throughout adult life and at the peak, 100-200 million sperm can be life and at the peak, 100-200 million sperm can be produced daily, the spermatogonia are constantly produced daily, the spermatogonia are constantly renewed. renewed.

The first step in spermatogenesis is a mitotic The first step in spermatogenesis is a mitotic division of the spermatogonium. One of the division of the spermatogonium. One of the daughter cells remains, to replace the original daughter cells remains, to replace the original spermatogonium, while the other cell (now called a spermatogonium, while the other cell (now called a primary spermatocyte) undergoes meiosis. primary spermatocyte) undergoes meiosis.

Spermatogenesis II:Spermatogenesis II:

The first meiotic division yields two secondary spermatocytes. The first meiotic division yields two secondary spermatocytes. Usually, these secondary spermatocytes do not fully separate during Usually, these secondary spermatocytes do not fully separate during cell division, leaving a direct cytoplasmic connection between the cell division, leaving a direct cytoplasmic connection between the cells. cells.

Following the second meiotic division (again, an incomplete division), Following the second meiotic division (again, an incomplete division), the cells are known as spermatids. As the germ cells are undergoing the cells are known as spermatids. As the germ cells are undergoing meiosis, they also migrate towards the lumen of the seminiferous meiosis, they also migrate towards the lumen of the seminiferous tubule. tubule.

As they approach the lumen, they shed much of their cytoplasm. As they approach the lumen, they shed much of their cytoplasm. They are attached to the Sustentacular cells, They are attached to the Sustentacular cells, viavia specialized specialized junctions, which provide nutrients. junctions, which provide nutrients.

When the spermatids reach the lumen, they remain embedded within When the spermatids reach the lumen, they remain embedded within the sustentacular cells, where they undergo tail development, the sustentacular cells, where they undergo tail development, acrosome formation and nuclear condensation. acrosome formation and nuclear condensation.

Finally, the fully-formed spermatozoa are shed into the lumen of the Finally, the fully-formed spermatozoa are shed into the lumen of the seminiferous tubule, where they are carried to the epididymus. This seminiferous tubule, where they are carried to the epididymus. This whole process takes between 60 and 70 days. whole process takes between 60 and 70 days.

Spermatid migration:Spermatid migration:

Mitosis Mitosis vsvs. Meiosis:. Meiosis:

Male reproductive ducts:Male reproductive ducts: The spermatozoa traverse the epididymus in 2 to 4 The spermatozoa traverse the epididymus in 2 to 4

weeks. weeks. During this time, they lose most of the remaining During this time, they lose most of the remaining

cytoplasm, as well as increase in mobility. cytoplasm, as well as increase in mobility.

The epithelial cells which line the epididymus The epithelial cells which line the epididymus secrete proteins which bind to the sperm cell secrete proteins which bind to the sperm cell membranes, to enhance their forward mobility and membranes, to enhance their forward mobility and ability to fertilize an ovum. ability to fertilize an ovum.

The sperm migrate into the ductus (or vas) The sperm migrate into the ductus (or vas) deferens, where they can be stored for several deferens, where they can be stored for several months. months.

The vas deferens runs up through the spermatic The vas deferens runs up through the spermatic cord, conducting the sperm to the prostate gland. cord, conducting the sperm to the prostate gland.

The end of each ductus deferens (two) The end of each ductus deferens (two) enlarges to form ampullae, where enlarges to form ampullae, where sperm are stored until ejaculation. sperm are stored until ejaculation.

The prostate contains the first part of The prostate contains the first part of the urethra (prostatic urethra) which is the urethra (prostatic urethra) which is where the ejaculatory ducts merge with where the ejaculatory ducts merge with the urethra. the urethra.

The urethra exits the prostate, The urethra exits the prostate, penetrated the urogenital diaphragm penetrated the urogenital diaphragm and runs the length of the penis.and runs the length of the penis.

Male sexual response:Male sexual response: ErectionErection The first phase of the male The first phase of the male

sexual response is erection sexual response is erection of the penis, which allows of the penis, which allows it to penetrate the female it to penetrate the female vagina. vagina.

This occurs when the This occurs when the erectile tissue of the penis erectile tissue of the penis becomes engorged with becomes engorged with blood. blood.

When a male is not When a male is not sexually aroused, the sexually aroused, the arterioles supplying the arterioles supplying the erectile tissues are erectile tissues are constricted. constricted.

During sexual excitement, During sexual excitement, a parasympathetic reflex a parasympathetic reflex is triggered that causes is triggered that causes these arterioles to dilate. these arterioles to dilate.

As a result, the vascular As a result, the vascular spaces of the penis fill spaces of the penis fill with blood causing the with blood causing the penis to become enlarged penis to become enlarged and rigid. and rigid.

Expansion of the penis Expansion of the penis also compresses the veins also compresses the veins retarding the outflow of retarding the outflow of blood and further blood and further contributing to the contributing to the swelling of the penis. swelling of the penis.

This reflex is initiated by This reflex is initiated by a variety of stimuli a variety of stimuli ranging from thought to ranging from thought to touch. touch.

EjaculationEjaculation A spinal reflex is initiated, producing a A spinal reflex is initiated, producing a

sympathetic discharge to the genital organs. sympathetic discharge to the genital organs. As a result, the reproductive ducts and As a result, the reproductive ducts and

accessory glands contract peristaltically accessory glands contract peristaltically discharging their contents into the urethra. discharging their contents into the urethra.

The muscles of the penis undergo a rapid The muscles of the penis undergo a rapid series of contractions propelling semen from series of contractions propelling semen from the urethra. the urethra.

This is followed by muscular and This is followed by muscular and psychological relaxation and vasoconstriction psychological relaxation and vasoconstriction of the arterioles serving the penis, allowing of the arterioles serving the penis, allowing blood to drain out of the erectile tissue, blood to drain out of the erectile tissue, which subsequently causes the penis to which subsequently causes the penis to become flaccid again. become flaccid again.

Role of the Accessory Role of the Accessory Glands:Glands:

The seminal vesiclesThe seminal vesicles are paired are paired glands that produce about 60% of the glands that produce about 60% of the semen. semen.

Their secretions contain fructose sugar, Their secretions contain fructose sugar, ascorbic acid and prostaglandins. ascorbic acid and prostaglandins.

These are sac shaped glands, These are sac shaped glands, approximately 5 centimeters long, approximately 5 centimeters long, which lie along side the ampullae of the which lie along side the ampullae of the ductus deferens. ductus deferens.

They each empty into a short duct, the They each empty into a short duct, the ejaculatory duct, which merges with the ejaculatory duct, which merges with the terminal end of the ductus deferens. terminal end of the ductus deferens.

These, in turn, fuse with the prostatic These, in turn, fuse with the prostatic urethra which runs from the bladder urethra which runs from the bladder through the prostate gland. through the prostate gland.

The alkalinity of the fluid serves to The alkalinity of the fluid serves to neutralize the normally acidic environment neutralize the normally acidic environment in the distal urethra and in the vagina. in the distal urethra and in the vagina.

The fructose is supplied as an energy The fructose is supplied as an energy source for the sperm, and the prostaglandins source for the sperm, and the prostaglandins serve to stimulate smooth muscle serve to stimulate smooth muscle contractions in the vagina and cervix. contractions in the vagina and cervix.

This is thought to facilitate the uptake of This is thought to facilitate the uptake of sperm into the uterus. sperm into the uterus.

The bulbourethral glandsThe bulbourethral glands are paired glands are paired glands that secrete a small amount of thick clear mucus. that secrete a small amount of thick clear mucus. This secretion is released prior to ejaculation This secretion is released prior to ejaculation and is believed to neutralize traces of acidic and is believed to neutralize traces of acidic urine in the urethra.urine in the urethra.

The prostate glandThe prostate gland is a single gland, which is a single gland, which secretes about one third of the semen volume. It secretes about one third of the semen volume. It secretes a milky, slightly acidic fluid containing secretes a milky, slightly acidic fluid containing citrate, acid phosphatase and several proteolytic citrate, acid phosphatase and several proteolytic enzymes. These enzymes are probably involved enzymes. These enzymes are probably involved in breaking down the mucus plug in the cervix. in breaking down the mucus plug in the cervix. They also appear to contribute to the motility They also appear to contribute to the motility and viability of the spermand viability of the sperm

Semen ProductionSemen Production

Remember, Sperm + seminal fluid = semen.Remember, Sperm + seminal fluid = semen.

Semen provides a transport medium for the Semen provides a transport medium for the sperm. It also provides nutrients for the sperm. It also provides nutrients for the sperm and chemicals that protect them, sperm and chemicals that protect them, activate them and facilitate their movement. activate them and facilitate their movement.

The amount of semen released during The amount of semen released during ejaculation is relatively small, about 2-6 ml ejaculation is relatively small, about 2-6 ml but it contains 50-100 million sperm per ml.but it contains 50-100 million sperm per ml.

Sperm capacitance:Sperm capacitance:

Freshly ejaculated sperm are Freshly ejaculated sperm are incapable of fertilizing an egg.incapable of fertilizing an egg.

As the sperm travel up the female As the sperm travel up the female reproductive tract, they lose reproductive tract, they lose cholesterol from their membranescholesterol from their membranes

When the sperm reach the fallopian When the sperm reach the fallopian tubes, the membranes around the tubes, the membranes around the acrosome are fragile enough to allow acrosome are fragile enough to allow the release of the acrosomal enzymes.the release of the acrosomal enzymes.

Brain-testicular axis:Brain-testicular axis:

Female reproductive Female reproductive system:system:

OOGENESIS I:OOGENESIS I: This process is the equivalent of This process is the equivalent of

spermatogenesis in the male. However, the spermatogenesis in the male. However, the two processes are vastly different. two processes are vastly different.

In females, much of the process occurs during In females, much of the process occurs during fetal development. fetal development.

The primitive germ cells undergo numerous The primitive germ cells undergo numerous rounds of mitosis, which produces millions of rounds of mitosis, which produces millions of oogonia (2n). oogonia (2n). Most of these oogonia are resorbed (through a Most of these oogonia are resorbed (through a

process called atresia). process called atresia). However, a few hundred thousand begin However, a few hundred thousand begin

meiosis and enter prophase I. These are now meiosis and enter prophase I. These are now referred to as primary oocytes. referred to as primary oocytes.

OOGENESIS II:OOGENESIS II: There are no oogonia present in the adult There are no oogonia present in the adult

female. female. The primary oocytes are arrested in prophase I The primary oocytes are arrested in prophase I

and become quiescent until puberty. and become quiescent until puberty. Cyclical changes in LH and FSH will trigger Cyclical changes in LH and FSH will trigger

three or four primary oocytes to finish meiosis three or four primary oocytes to finish meiosis each uterine cycle. each uterine cycle.

During the two meiotic divisions, all the During the two meiotic divisions, all the cytoplasm will stay with a single daughter cell, cytoplasm will stay with a single daughter cell, which is destined to become the ovum. which is destined to become the ovum.

The other three daughter cells simply develop The other three daughter cells simply develop as small polar bodies that are eventually as small polar bodies that are eventually degraded and resorbed. degraded and resorbed.