day lilyrose
Sex is going on all around us. Thanks to sexual reproduction, the world is a more colorful place-sexual reproduction is responsible for many pleasant sights, sounds, and smells.
a) sights: colored flowers, brightly colored birds, antlers on deer relatives, lightningbugs at night

wod duck
b) sounds: bird, insect, & frog song
c) smells: flowers, musks (many of which are used in perfumes)

Sexual reproduction is so common that throughout much of the year, it is impossible to breathe air without inhaling sex cells (pollen grains are, after all, either sperm or sperm-making cells in nitrogenous coats). Although we take it for granted, a legitimate question can be asked-why does sexual reproduction even exist at all? Why do males exist? After, all sexual reproduction can be a negative, even life -threatening, experience for both males and females.

For females, sexual reproduction is difficult because, more often than not, they alone bear the responsibility of caring for young. Pregnancy is a time of decreased mobility when it is harder to find food and escape predators. Certain species even incorporate violence in mating-such as in waterfowl and orangutans. In invertebrates, sexual reproduction can result in other uncomfortable aspects as well such as mating plugs and penises that try to remove them.
For males, sexual reproduction can also be dangerous because they often bear the responsibility of attracting females (after all, it isn't always easy to find a member of your species in a forest-one individual has to announce their presence). Many male insects, birds and frogs produce songs to attract females but this also informs predators of their location. Males are often more brightly colored, which also makes it easier for predators to find them.


Females often mate only with those males which win an intermale combat (which may even involve teeth, horns, headbutting, & antlers) or which perform other shows such as flying near cliff sides at 265 mph in peregrine falcons.

For both males and females, sexual contact between individuals offers a potential for disease-causing organisms to spread. There you have it: sex can be dangerous. Not only can sexual reproduction be dangerous, asexual reproduction has a number of advantages:

--populations reproduce more quickly when every individual can reproduce
--if you cross a mountain range or find a new island, you don't become celibate for life
--if you have good genes, your offspring have good genes (not true if your mate is a genetic nightmare)
--you escape the trials & tribulations mentioned above (useless energy spent, higher mortality)

So why does sex exist? No, not for pleasure. The brain is responsible for when you feel pleasure-your brain could be "wired" to induce pleasure or even cause orgasm when you eat spinach, or climb a tree, or cut your toenails. Pleasure is simply the bribe that your brain employs to encourage you to do something that may not be in your best interests. Sexual reproduction accomplishes one thing and one thing only: variation among offspring. Variation among offspring increases the probability of surviving disease or environmental change (and gives good genes the chance to cluster together).
There are two reproductive systems: the male and female.

After a certain amount of time, the male brain decides that puberty should begin. The hypothalamus releases GnRH which stimulates the release of FSH and LH from the pituitary gland. FSH stimulates cells called spermatogonia to start the process of making sperm (spermatogenesis) and LH stimulate other testicular cells to make testosterone from cholesterol. At puberty, testosterone and other androgen hormones from both the testes and the adrenal glands not only cause the development and enlargement of male organs which are directly involved in reproduction, they also cause the development of male secondary sexual characteristics. Secondary sexual characteristics are features that differ between the sexes but that are not crucial for reproduction such as muscular & skeletal growth (resulting in wide shoulders & narrow hips), hair growth (in pubic, axillary, facial, and chest regions), increased sebaceous gland secretion, and enlargement of vocal cords (causing deepening of voice).
Another secondary sexual characteristic, in both genders, is the odor produced by apocrine sweat glands over the groin, armpit, & nipple. These sweat glands secrete compounds that bacteria can use as food sources and the bacteria break these compounds into some which have odor. These compounds in human sweat, especially those of apocrine sweat glands (most abundant around the genitals, armpits, and nipples) that act as pheromones-- compounds meant to influence other individuals. Female sweat contains cues relating to the stage of the menstrual cycle; women who cohabitate unconsciously use these cues to allow their cycles to come in sync. In some male mammals, these pheromones give information as to a male's position in the social hierarchy (the alpha male which has won all his fights smells differently than the zeta male which has lost all of his). Some evidence suggests that smell affects attraction in a way to decrease inbreeding, making it less likely that you are attracted to those more closely related to you.

a) Scrotum
The scrotum is an outpocket of the abdominal wall
A vertical septum splits it into 2 sacs and it has a wrinkled appearance due to the dartos muscle. Sperm survive 3 degrees cooler than normal body temperature and infertility would result from maintaining the testes in the body. A muscle called the cremaster muscle contracts to elevate the testes during exposure to cold temperatures and during arousal and relaxes upon exposure to warmth and testes descend.

b) Testes
cat testestestis
The testes are split into 200-300 lobules, each of which contains 1-3 seminiferous tubules. These tubules are lined with spermatogenic cells in various stages of development.

Sperm require about 74 days to mature. Spermatogenesis begins as spermatogonia (diploid cells with two copies of each chromosome) undergo cell division. Daughter cells that remain near outside of the tubule retain their identity as spermatogonia (and, since males are always producing new spermatogonia, they can produce gametes for decades, ending only when they die). The daughter cells that are pushed toward the center of the tubule become primary spermatocytes which begin another type of cell division called meiosis which will produce sperm. Because of two processes which occur during meiosis (independent assortment and crossing over), each individual (male and female) has an enormous number of different gametes that they can produce. Because of independent assortment, each human has the ability to make 8 million genetically different gametes (each couple 64 trillion different zygotes). If crossing over occurs once per tetrad (it probably occurs 10 x); each individual could make 7 trillion different gametes (and each couple 5,000,000,000,000,000,000,000,000,000 different zygotes).
At the end of meiosis, four haploid cells (having only one copy of each chromosome) called spermatids lie near center of the seminiferous tubule.


Cross Section of Rat Testis
Human Testis Cross Section


What are spermatids good for? Nothing actually because they can't swim yet. After spermatogenesis is complete and spermatids are produced, they must undergo a second process called spermiogenesis. In spermiogenesis, spermatids develop a head with an acrosome (which contains enzymes we'll discuss later) and a flagellar tail. Now the cells are called spermatozoa. They separate leave the seminiferous tubule and enter the ductus epididymis where they will mature in 10-14 days. The average male makes about 300 million sperm per day.
These spermatozoa must travel about 8 meters to leave male body: from the seminiferous tube (31"; 3 weeks) to straight tubules to rete testis to efferent ducts to the epididymis (7 meters; 10-14 days) to the vas deferens (16-18"; may be stored up to several months) to ejaculatory duct (1") and finally to the urethra which exits the penis.

c) Epididymis
The epididymis (pictured below) is about 1mm wide & 7meters long (it is highly coiled). Its cells have a high surface area for the reabsorption of unused, degenerating sperm. These cells also may secrete/reabsorb to adjust composition of tubular fluid. Spermatozoa spend 10-14 days here during which their motility and fertility increase. They can be stored until they degenerate & are reabsorbed or until smooth muscle contractions them expel into the vas deferens.
d) Vas Deferens
The vas deferens is about 18" long; it proceeds from the epididymis, penetrates the inguinal canal, and enters the pelvic cavity. The vas deferens & seminal vesicle join to form the ejaculatory duct; this duct ejects spermatozoa into the urethra just before ejaculation. The spermatic cord is composed of the vas deferens plus accompanying arteries, veins, lymphatic vessels, and autonomic nervous supply. Interestingly, testes begin to develop in males above the kidneys during featl development from which point they must descend into the scrotum. The inguinal canal is the hole in the male abdomen through which they traveled and through which the blood vessels and nerves which service the testes still pass. The abdominal wall is weak where the spermatic cord passes through the inguinal canal; pressure in the abdomen (such as from intense muscle contractions) can force a piece of an organ through this canal (such as a loop of the small intestine causing an inguinal hernia.
Sperm may be stored in the vas deferens (for up to several months with very low metabolism) or moved by peristaltic contractions of the muscular coat of the vas deferens. A vasectomy removes a portion of the vas deferens; sperm production (but not complete transport) continues without any effect on sexual desire but the sperm can no longer reach the urethra.

inguinal canal

e) Urethra
Sperm reach the urethra from the ejaculatory duct. The urethra first passes through prostate gland and then the penis.

f) Accessory Sex Glands
Semen does not only contain sperm: it also contains a number of factors secreted by accessory sex glands.

1) seminal vesicles
Seminal vesicles secrete most of the liquid in semen(60%). They secrete a basic, viscous fluid that contains fructose (energy source), fibrinogen (semen coagulates after ejaculation), and prostaglandins ( which contribute to sperm motility and may contribute to muscular contractions of the female reproductive tract). Without a basic fluid, the acid of the female reproductive tract would kill the sperm.

2) prostate gland
The prostate gland secretes 25% of semen. Itcontributes to sperm motility & viability, it secretes clotting enzymes & fibrinolysin that breaks down the clot. Semen coagulates rapidly within female reproductive tract and fibrinolysin causes it to liquefy within 5-20 minutes; without this, sperm may be immobilized. The prostate grows until the end of puberty and it may continue to enlarge after age 45.
3) bulbourethral glands
The bulbourethral glands are located beneath the prostate. They secrete lubricating fluid that protects sperm from vaginal acid, lubricates the urethra, and lubricates the tip of the penis during intercourse.

Semen is a mixture of sperm & secretions of accessory glands; average ejaculate is 2.5 to 5 ml with 50-150 million sperm/ml (infertility frequently results from counts below 20 million/ml). It protects sperm from vaginal acids, transports & nourishes sperm.


g) Penis
The penis introduces sperm into the vagina. There are two types of tissue: 2 masses of corpora cavernosa penis which are rich in blood vessels & the urethra-containing corpus spongiform penis. During stimulation, large quantities of blood enter causing the tissue to swell; this swelling blocks the veins and prevents blood from leaving. Impotence is most commonly caused by the ability to maintain an erection; may be caused by depression, anxiety (most cases are psychological), drugs & illnesses that affect ANS, or plaques blocking arteries. Temporary periods of impotence are common in normal individuals.
Ejaculation causes the internal urethral sphincter to contract (so that urine doesn't enter semen and semen doesn't enter the bladder) and results in powerful muscle contractions of the 2 cavernosus muscles.
The distal end composes glans penis which is covered at birth by the foreskin. The foreskin is removed in circumcision. Some feel this lowers susceptibility to infection and disease (one of the reasons is that preputial glands on skin & neck of penis & foreskin secrete a waxy material known as smegma which can be used as a food source for bacteria). Others contend that the removal of this sensitive area deprives men of much of the pleasure they would otherwise experience in intercourse.