Physiology of Reproduction The following is an actual

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)? n 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. n 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. n 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. n 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. “

Gonadal development: n Both the testes and the ovaries are derived from the same gonadal primordium. n n 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,

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

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

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

n Without DHT, the external genitalia are feminized. n n n The genital tubercle becomes the clitorus. The genital folds become the labia minora. The genital swelling becomes the labia majora. n Currently, the structure of MIS is not known, but it appears to be a glycoprotein. n 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

n Development of secondary sexual characteristics: n n n 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,

This drastic drop in melatonin secretion (>75%) may trigger the secretion of sex steroids by the adrenal glands and/or the testes. n In females, the situation may be different. n There is good evidence that the hormone leptin is also involved. n Leptin is a hormone released by adipose tissue. n Circulating leptin levels may reflect total body fat storage by the body. n In females, a certain minimum total-body fat content is required for puberty to progress and for maintenance of the menstrual cycle. n

Male reproductive system:

Spermatogenesis I: n n 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: n 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. n 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. n As they approach the lumen, they shed much of their cytoplasm. They are attached to the Sustentacular cells, via specialized junctions, which provide nutrients. n When the spermatids reach the lumen, they remain embedded within the sustentacular cells, where they undergo tail development, acrosome formation and nuclear condensation. n 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: n The spermatozoa traverse the epididymus in 2 to 4 weeks. n During this time, they lose most of the remaining cytoplasm, as well as increase in mobility. n 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. n The sperm migrate into the ductus (or vas) deferens, where they can be stored for several months. n The vas deferens runs up through the spermatic cord, conducting the sperm to the prostate gland.

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

Male sexual response: n n 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.

n n During sexual excitement, a parasympathetic reflex is triggered that causes these arterioles to dilate (NO 2). 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 n n 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: The seminal vesicles are paired glands that produce about 60% of the semen. n Their secretions contain fructose sugar, ascorbic acid, and prostaglandins. n These are sac shaped glands, approximately 5 centimeters long, which lie along side the ampullae of the ductus deferens. n They each empty into a short duct, the ejaculatory duct, which merges with the terminal end of the ductus deferens. n

n These, in turn, fuse with the prostatic urethra which runs from the bladder through the prostate gland. n The alkalinity of the fluid serves to neutralize the normally acidic environment in the distal urethra and in the vagina. n 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. n This is thought to facilitate the uptake of sperm into the uterus.

n 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. n 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.

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. n After ejaculation, Sg. II and fibronectin aggregate to form a gelatinous mass, which is believed to trap the spermatozoa within the vagina. n Liquefaction occurs 5 -20 minutes later, through cleavage of the semenogelins by PSA prostate-specific antigen). n

Semen Production n Remember, Sperm + seminal fluid = semen. n 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. n 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: Freshly ejaculated sperm are incapable of fertilizing an egg. n As the sperm travel up the female reproductive tract, they lose cholesterol from their membranes n When the sperm reach the fallopian tubes, the membranes around the acrosome are fragile enough to allow the release of the acrosomal enzymes. n

Brain-testicular axis:

Female reproductive system:

OOGENESIS I: n n n 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 (2 n). n n 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: n n n 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: n As with Males, arousal is controlled by parasympathetic stimulation. Involves engorgement of the erectile tissues. n Increased bloodflow to the external genitalia and vaginal walls. n Stimulation of secretion of cervical mucous glands and greater vestibular glands. n

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. n As with male climax, female climax results in rhythmic peristaltic contractions of the uterus and vaginal walls and associated skeletal muscles. n n This is thought to enhance the migration of sperm up the female reproductive tract.

Fertilization and pregnancy:

Fertilization:

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

n 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. n 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: n HCG is similar to LH and maintains the corpus luteum in a functional state for 3 -4 months. n n n 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: n n 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.

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

Labour and delivery: n As the head of the fetus is pressed down against the cervix, it thins and then starts to dilate. n n 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. n This is known as the Expulsion Stage.

Labour and delivery cont. : n There is also a chance that the fetus will get stuck in the birth canal (usually caused by insufficient molding of the head. n n 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. n This is known as the Placental Stage.

Nursing: n Two hormones are involved, PRL and oxytocin. n PRL stimulates milk production, while oxytocin is required for the expression of milk from the breast.

Fertility issues: