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
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.
MALE REPRODUCTIVE SYSTEM
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
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.
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.
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.
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.
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.