42.1. How
Animals Reproduce
A. Two Patterns of Reproduction
1. Asexual—one parent involved.
2. Sexual—two parents
involved.
B. Asexual
1. Some animals usually reproduce asexually but most use a combination of
sexual and asexual
2. Hydra undergo
budding; a new individual arises as an outgrowth (bud) of a
parent. (Fig. 50.1)
3. Obelia polyps produce medusa by
budding; the medusa produce eggs and sperm.
4. Fragmentation
followed by regeneration is seen among sponges and echinoderms.
5. Parthenogenesis is
common among some insects, worms, fish, lizards and some other animals; an
egg develops
without fertilization.
6. In honey bees, the queen can
decide to fertilize or not fertilize eggs, producing workers or drones.
C. Sexual Reproduction
1. In sexual reproduction; the egg of one parent is fertilized by
sperm of the other.
2. Hermaphroditic organisms possess
both male and female organs.
a. Pair of
earthworms cross-fertilize each other.
b. Sex
reversal involves changing sex; a male wrass (reef fish) has a harem-if it
dies, the largest
female becomes male.
3. Gonads are organs
specialized to produce gametes.
a. Sponges
are an exception since their collar cells give rise to sperm and eggs.
b. Hydras
produce only temporary gonads in fall, when sexual reproduction occurs.
c. Animals
in other phyla have permanent gonads.
4. There are two types of gonads:
testes produce sperm and ovaries produce eggs.
5. Eggs and sperm cells derive from
germ cells that specialize early for this development.
6. Other cells in gonads support and
nourish developing gametes or produce hormones for reproduction.
7. Accessory organs
form ducts and storage areas that aid in bringing gametes together.
8. Sexually-reproducing animals must
ensure gametes find each other.
a. Aquatic
animals that practice external fertilization must synchronize egg release.
b. Lunar
cycles are one trigger that cues animals by tides.
c. Hundreds
of thousands of palolo worms rise to surface to release eggs during a 2-4 hour
period.
9. Copulation is sexual union
to facilitate reception of sperm by a female.
a. Penis
is a male copulatory organ typical of terrestrial males; deposits sperm into
female's vagina.
b. Aquatic
animals have other types of copulatory organs or employ other strategies for
delivering sperm:
1) Lobsters and crayfish modified swimmerets.
2) Cuttlefish and octopuses use an arm.
3) Sharks have a modified pelvic fin to pass packets of sperm to female.
c. Birds
lack a penis and vagina; they transfer sperm from cloaca to cloaca.
D. Life History Strategies
1. Many aquatic animals use external fertilization; eggs and
sperm join outside in the water.
2. Terrestrial animals tend to
practice internal fertilization; eggs and sperm join inside female's body.
3. Both types of animals are usually
oviparous; they deposit eggs in external environment.
4. Eggs are produced in ovaries;
they mature and increase in size as a result of accumulation of yolk.
a. Yolk
is stored food to be used by developing embryo.
b. To
prevent insect eggs from drying out, their eggshell has several layers of
protein or wax.
c. In insects,
small holes are left at one end to allow entry of sperm.
5. Some insects have a special organ
to store sperm so eggs can be fertilized later.
6. Larval stage is
often quite different in appearance and way of life from the adult form.
a. Larva is
able to seek its own food to sustain itself until it becomes an adult.
b. Metamorphosis
is a major change in form that some animals undergo during development.
c.
Incomplete metamorphosis lacks a pupal stage and nymphs look more like adults.
d. Larval
aquatic forms can utilize a different food source than the adults.
e.
Bilaterally symmetrical sea star larvae attach to substrate to become radially
symmetrical adults.
f.
Free-swimming barnacle larvae metamorphose into sessile adults with calcareous
plates.
g. Crayfish
lack a larval stage; eggs hatch into tiny juveniles with same form as adults.
7. Reptiles and birds provide their
eggs with plentiful yolk; there is no larval stage.
a. Complete
development takes place within a shell containing extraembryonic
membranes.
b. Chorion
is the outermost membrane that lies next to the shell and functions in gas
exchange.
c. Amnion
forms a water-filled sac around the embryo ensuring it will not dry out.
d. Yolk
sac hold yolk which nourishes the embryo.
e. Allantois
holds nitrogen waste products.
f. Shelled
egg frees animal from need to reproduce in water and helps it live
completely on land.
8. Birds tend their eggs.
a. Newly
hatched birds have to be fed before they develop to where they can seek food on
their own.
b. Parent
bird reproductive behavior involves complex hormone and neural regulation
9. Oysters and sea horses are ovoviviparous;
eggs remain in body until they hatch fully-developed.
10. Garter snakes and pit vipers
retain eggs until they hatch and give birth to live young.
11. Mammals are viviparous;
embryo remains in female's body during development.
a. Nutrients
needed for development are constantly supplied by the mother.
b.
Viviparity represents the ultimate in caring for the zygote and the embryo.
c. Evolution
of viviparity can be seen in primitive mammals.
1) Exceptions are the duckbill platypus and the spiny anteater, which are
egg-laying mammals.
2) Marsupials give birth to immature offspring that finish developing within a
pouch.
3) In all other mammals, development occurs in a placenta.
12. Placenta is a
complex organ comprised of maternal and embryonic tissues.
a. Placenta
exchanges O2, CO2, nutrients, wastes, etc. between fetal and maternal
circulations.
b. Evolution
allowed embryos to exchange materials with mother; this made shell unnecessary.
42.2. Male
Reproductive System
A. Male Gonads
1. Paired testes are suspended in scrotal sacs of scrotum.
(Fig. 50.3)
2. Testes began development in
abdominal cavity but descend into scrotal sac during development.
3. If testes do not descend, without
surgery or hormonal therapy, sterility results.
4. Lower temperature of scrotum is
vital to normal sperm production.
B. Sperm
1. Sperm produced in testes mature within epididymides.
a. These are
tightly coiled tubules outside testes in which sperm undergo maturation.
b.
Maturation time in epididymis is required for sperm to swim to egg.
2. Once sperm have matured, they are
propelled into vasa deferentia by muscular contractions.
3. Sperm are stored in both the
epididymides and the vasa deferentia.
4. When a male is sexually aroused,
sperm enter urethra, part of which extends through penis.
5. Penis is a
cylindrical copulatory organ used to introduce spermatozoa into female vagina.
a. Three
columns of spongy, erectile tissue extend down shaft.
b. During
sexual arousal, nervous reflexes cause an increase in arterial blood flow to
penis.
c. Increased
blood flow fills and distends erectile tissue, and the penis stiffens and
increases in size.
d. These
changes cause erection; failure to achieve erection is called impotency.
6. Semen (seminal
fluid) is thick whitish fluid that contains sperm and glandular
secretions.
a. Seminal
fluid is formed by seminal vesicles, prostate gland, and bulbourethral
glands.
b. Seminal
vesicles lie at the base of urinary bladder.
1) Each joins a vas deferens to form an ejaculatory duct that enters urethra.
2) They secrete into ejaculatory duct a thick fluid containing nutrients for
use by sperm.
c. Prostate
gland is located just below urinary bladder and surrounds upper portion
of urethra.
1) It secretes a milky, slightly alkaline solution that promotes sperm motility
and viability.
2) In older men, prostate gland may become enlarged constricting the urethra.
3) Prostrate cancer is also common in older men.
d. Bulbourethral
glands are located below prostate gland on either side of urethra;
release mucus
secretions that provide lubrication.
7. Urethra also
conducts urine from bladder during urination.
C. Ejaculation
1. Ejaculation results in expulsion of semen; it is achieved at
peak of sexual arousal.
2. First phase of ejaculation
is emission.
a. Nerve
impulses from spine trigger epididymides and vasa deferentia to contract.
b.
Subsequent motility causes sperm to enter ejaculatory duct; seminal vesicles,
prostate gland, and
bulbourethral glands release their secretions.
c. Small
amount of secretion from bulbourethral glands may leak from end of penis;
function to clean
urethra of acid and may contain sperm.
3. Second phase of
ejaculation is expulsion.
a.
Rhythmical contractions at base of penis and within urethral wall expel semen
in spurts.
b.
Rhythmical contractions are a release from myotonia, or muscle tenseness, a sexual
response.
4. An erection lasts for a limited
time and the penis returns to a flaccid state following ejaculation.
5. Refractory period
follows during which stimulation does not bring about erection.
6. Orgasm is
physiological and psychological sensations that occur at climax of sexual
stimulation.
D. The Testes
1. Longitudinal section shows compartments called lobules; each contains one to
three seminiferous tubules.
a.
Altogether, seminiferous tubules have a combined length of about 250 meters.
b. In
microscopic cross section, tubules show cells undergoing spermatogenesis.
c. Sustentacular
(Sertoli) cells support, nourish, and regulate spermatogenic cells.
2. Mature sperm (spermatozoa)
have three parts.
a. Head
contains nucleus covered by an acrosome.
1) Acrosome is caplike covering over anterior end of nucleus;
stores enzymes to penetrate egg.
2) Human egg is surrounded by several layers of cells and thick membrane sperm
must penetrate.
b. Middle
piece contains mitochondria wrapped around microtubules of flagellum;
provides energy for movement.
c. Tail
also contains microtubules as components of a flagellum; its movement propels
sperm.
3. Ejaculate of a normal human
male contains several hundred million sperm.
4. Fewer than 100 ever reach
vicinity of egg; and only one sperm normally enters an egg.
E. Hormonal Regulation in Males
1. Hypothalamus has ultimate control of testes' sexual function
through gonadotropic-releasing hormone
(GnRH).
2. There are two
gonadotropic-releasing hormones, follicle-stimulating hormone (FSH)
and luteinizing
hormone (LH)
found in both males and females.
3. FSH stimulates
spermatogenesis in seminiferous tubules which also release the hormone inhibin.
4. In males, luteinizing
hormone is also called interstitial cell-stimulating hormone (ICSH); it
stimulates
testosterone
secretion by interstitial cells of testes.
5. Hormones of
hypothalamus-pituitary-testis system are involved in a negative feedback
relationship that
maintains
fairly constant production of sperm and testosterone.
F. Functions of Testosterone
1. Testosterone is main sex hormone in males.
2. It is essential for development
of male secondary sex characteristics and for maturation of
sperm.
a. It causes
growth of a beard, axillary hair and pubic hair.
b.
Testosterone causes larynx and vocal cords to enlarge, causing a deeper voice.
c. It is
responsible for greater muscle strength of males; some athletes take
supplemental anabolic steroids
that are testosterone or mimics (but which have serious side effects).
d.
Testosterone is largely responsible for sex drive and may contribute to
aggressiveness.
e. It causes
oil and sweat glands in skin to secrete; it is largely responsible for acne and
body odor.
f.
Testosterone is also involved in triggering baldness if genes are present.
42.3. Female
Reproductive System
A. Internal Reproductive System
1. Female reproductive system includes: ovaries, oviducts,
uterus, and vagina.
2. Ovaries produce an
egg each month; are located in the abdominal cavity. (Fig. 50.7)
3. Oviducts (Fallopian
tubes) extend from ovaries to uterus.
a. Oviducts
are not attached to ovaries.
b.
Fingerlike projections called fimbriae sweep over ovaries and
waft in egg when it erupts.
c. This is
normal site for fertilization; embryo is slowly moved by ciliary movement to
uterus.
4. Uterus is a hollow,
thick-walled muscular organ the size and shape of an inverted pear.
a. Embryo
completes development by embedding itself in uterine lining, the endometrium.
b. A small
opening at cervix of uterus leads to vaginal canal.
5. Vagina is a tube at
a 45 degree angle with the small of the back.
a. Mucosal
lining lies in folds and can extend.
b. It
receives the penis during copulation and also serves as birth canal.
B. External genitalia of women are known collectively as the vulva.
1. Mons pubis and labia minora and labia
majora are to side of vaginal and urethral openings.
2. At
juncture of labia minora is the clitoris.
a. This is homologous
to the penis in males.
b. Clitoris
has a short shaft of erectile tissue, capped by a pea-shaped glans.
c. It
contains many sensory receptors that allow it to function as a sexually
sensitive organ.
3. Orgasm involves
release of neuromuscular tension in muscles of genital area, vagina, and
uterus.
C. The Ovaries
1. Ovaries alternate in producing one oocyte each month.
2. Ovaries produce an egg (ovum) and
female sex hormones, estrogens and progesterone,
during ovarian cycle.
D. The Ovarian Cycle
1. In longitudinal section, an ovary shows many cellular follicles,
each containing an oocyte (egg).
2. A female is born with as many as
two million follicles; the number is reduced to 300,000-400,000 by the
time of
puberty; and only a small number of follicles (about 400) ever mature.
3. As a follicle matures, it
develops from a primary follicle to secondary follicle to a
Graafian follicle.
4. Oogenesis is
occurring; a secondary follicle contains a secondary oocyte
pushed to one side of fluid-filled cavity.
5. A Graafian follicle
fills with fluid until follicle wall balloons out on surface and bursts,
releasing a
secondary
oocyte surrounded by zona pellucida and follicular cells.
6. Ovulation is
rupture of Graafian follicle with discharge of a secondary oocyte into pelvic
cavity.
7. Secondary oocyte completes a
second meiotic cell division when fertilization occurs.
8. Meanwhile, the follicle develops
into corpus luteum; if pregnancy does not occur, corpus luteum
begins to
degenerate in 10 days.
9. Ovarian cycle is under control of
gonadotropic hormones, follicle-stimulating hormone (FSH) and
luteinizing
hormone (LH).
10. Gonadotropic hormones are not
present constantly but are secreted at different rates during cycle.
11. During follicular phase,
FSH promotes development of a follicle that secretes estrogen.
12. As estrogen level in blood
rises, it exerts feedback control over anterior pituitary secretion of FSH; the
follicular
phase comes to an end.
13. Estrogen levels in blood rise,
causing hypothalamus to secret more GnRH; this causes surge in LH secretion.
14. LH spike triggers ovulation.
15. Luteal phase is
second half of ovarian cycle, following ovulation.
a. LH
promotes development of corpus luteum, which secretes large amounts of
progesterone.
b. Progesterone
causes endometrium to build up.
c. As blood
level of progesterone rises, negative feedback to anterior pituitary's
secretion of LH causes
corpus luteum to degenerate.
d. Luteal
phase ends and menstruation occurs.
E. The Uterine Cycle
1. Estrogens and progesterone affect endometrium of uterus to cause a cycle of
events known as the
uterine
cycle.
2. An average 28-day uterine cycle
is divided into four sections.
a. During days
1-5, low levels of estrogen and progesterone in the body cause
menstruation.
1) Menstruation is periodic shedding of tissue and blood from
endometrium; lining disintegrates
and blood vessels rupture.
2) Flow of blood and tissues, known as menses, passes out vagina.
b. During days
6-13, increased production of estrogens by an ovarian follicle causes
endometrium to
thicken and become vascular and glandular (proliferative phase).
c. Ovulation
usually occurs on day 14 of 28-day cycle.
d. Days
15-28 see increased production of progesterone by corpus luteum causes
endometrium to
double in thickness; uterine glands mature, producing thick mucoid secretion (secretory
phase).
1) Endometrium is now prepared to receive a developing embryo.
2) If no pregnancy occurs, progesterone and estrogen levels decline and corpus
luteum degenerates.
3) With low levels of progesterone, uterine lining also begins to degenerate.
4) During menstruation, anterior pituitary increases FSH production; a new
follicle begins to mature.
3. Ovarian cycle controls uterine
cycle.
F. Events Following Fertilization
1. If fertilization occurs, embryo begins development as it
travels down oviduct to uterus.
2. Embryo becomes embedded in
endometrium several days following fertilization.
3. Placenta develops
from both maternal and embryonic tissues.
a. Placenta
functions to exchange gases and nutrients between the fetal and maternal
circulation.
b. There is
normally no mixing of blood itself.
4. Initially, placenta produces human
chorionic gonadotropin (HCG) which maintains corpus luteum.
5. Corpus luteum is maintained by
HCG until placenta produces its own progesterone and estrogens.
6. Progesterone and estrogens have
two effects at this stage.
a. They shut
down the anterior pituitary so that no new follicles mature.
b. They
maintain lining of uterus so corpus luteum is not needed.
7. There is no menstruation during
pregnancy.
G. Functions of Estrogen and Progesterone
1. Estrogens maintains secondary
sex characteristics of females.
2. There is less body and facial
hair; more fat beneath skin provides a more rounded appearance.
3. Pelvic girdle enlarges and pelvic
cavity is larger; therefore, women have wider hips.
4. Both estrogen and progesterone
are required for breast development.
H. The Female Breast
1. Female breast contains 15-24 lobules, each with
a mammary duct.
2. Mammary duct begins at nipple
and divides into numerous ducts which end in alveoli (blind
sacs).
3. Prolactin hormone
is needed for lactation (milk production) to begin.
4. Production of prolactin is
suppressed by the feedback inhibition that estrogens and progesterone have
on the
anterior pituitary during pregnancy.
5. It takes a couple of days after
delivery of a baby for milk production to begin.
6. Breasts produce a watery,
yellowish white fluid (colostrum) similar to milk but containing
more protein,
less fat and
rich in Ig A antibodies providing some immunity to newborn.
42.4.
Control of Reproduction
A. Infertility
1. Two major causes of female infertility are blocked oviducts
and failure to ovulate due to low body weight.
a. Endometriosis, spread of uterine tissue beyond
uterus, is another cause.
b. If no
obstruction is apparent and body weight is normal, HCG from urine of pregnant
women, along
with HMG (human menopausal gonadotropin) from postmenopausal women, causes
multiple ovulations.
2. Most frequent causes of male
sterility and infertility are low sperm count and abnormal sperm.
a. Disease,
radiation, chemical mutagens and too much heat near testes can cause sterility.
b. Use of
psychoactive drugs can also contribute to this condition.
B. Birth-Control Methods
1. Most reliable birth control is abstinence; it has advantage of
preventing transmission of a STD.
2. Numerous birth-control methods
and devices are available to prevent pregnancy.
3. Effectiveness is based on number
of sexually active women per year who do not get pregnant.
4. Male and female condoms also
offer protection against STDs.
5. Researchers are searching for a "male
pill."
a. Analogues
of gonadotropic-releasing hormone prevent hypothalamus
from stimulating anterior
pituitary but causes feminization.
b. Inhibin
inhibits spermatogenesis but must be administered by injection.
6. Morning-After Pills
a. These
regimens either prevent fertilization or stop fertilized egg from ever
implanting.
b. Previn is
a kit of four synthetic progesterone pills.
1) Two are taken up to 72 hours after intercourse.
2) Two more are taken 12 hours later.
3) Previn upsets the normal uterine cycles making implantation difficult; 85%
effective.
c. Mifepristone,
also known as RU-486, causes the loss of an implanted embryo.
1) It blocks the progesterone receptors of the cells of the uterine lining.
2) Without functioning receptors for progesterone, uterine lining sloughs off
carrying embryo with it.
3) Taken in conjunction with a prostaglandin to induce uterine contractions, it
is 95% effective.
4) It can be taken by women with delayed menstruation without knowing if they
are pregnant.
C. Sexually Transmitted Diseases (STDs)
1. STDs are caused by organisms ranging from viruses to
arthropods.
a. Humans
cannot develop lasting immunity to any STDs; therefore, prompt medical
treatment should be
received when exposed to an STD.
b. To
prevent STDs, a condom can be used; spermicide with nonoxynol 9 gives added
protection.
c. It is
difficult to cure STDs caused by viruses; treatment is available for AIDS and
genital herpes.
d. STDs
caused by bacteria (e.g., gonorrhea, chlamydia, and syphilis) are treatable
with antibiotics.
D. AIDS
1. Acquired immunodeficiency syndrome (AIDS) is caused by human
immunodeficiency virus (HIV).
2. HIV attacks helper T cells that
stimulate activities of B lymphocytes to produce antibodies.
3. After HIV infection begins,
helper T cells decline in number; a person becomes susceptible to infections.
4. AIDS has three stages of
infection called category A, B, and C
a. Category A stage may last about a year.
1) Individual is asymptomatic but can pass on infection.
2) Immediately after infection but before testing positive, a large number of
infectious viruses are in blood.
3) After testing positive, a person may remain well as long as he or she can
maintain sufficient helper T
cells (above 500 mm3).
b. Category B stage may last six to eight years.
1) Lymph nodes swell.
2) There is weight loss, night sweats, fatigue, fever, and diarrhea.
3) Infections such as thrush and herpes recur.
c. Category C stage is full-blown AIDS.
1) Nervous disorders and opportunistic diseases (e.g., unusual type pneumonia
or skin cancer) occur.
2) Without intensive medical treatment, an AIDS patient usually dies about 7-9
years after infection.
3) Recent combination therapy of several drugs allows AIDS patients to live
longer.
5. Transmission
a. AIDS is
transmitted by sexual contact with an infected person (vaginal or rectal
intercourse and
oral/genital contact).
b. Needle
sharing among intravenous drug users is a high-risk behavior.
c.
Transfusions of blood or clotting factors is now a
rare mode of transmission and can be screened.
d. Largest
increases in AIDS cases now involve heterosexual contact and intravenous drug
use.
e. Women now
account for 19% of all newly diagnosed cases of AIDS.
f. Increase
of AIDS among women of reproductive age causes an increase in AIDS in children.
g. HIV
crosses the placenta or is transferred through breast feeding.
E. Genital Warts
1. Genital warts are caused by human papillomaviruses (HPVs).
2. Many carriers are asymptomatic or
they have minimal symptoms.
3. If visible warts are removed,
they may recur.
4. HPVs are now associated with
cancer of cervix as well as tumors of vulva, anus, and penis.
5. Some researchers believe viruses
are involved in 90-95% of all cases of cancer of cervix.
F. Genital Herpes
1. Genital herpes is caused by herpes simplex virus.
2. Type 1 causes cold sores and
fever blisters; type 2 more often causes genital herpes.
3. Individuals infected with this
type can be an asymptomatic carriers.
4. Symptoms include painful ulcers
on genitals, fever, painful urination, and swollen lymph nodes.
5. Exposure to herpes in birth canal
can cause neurological disorders and even death in a newborn.
G. Hepatitis
1. Hepatitis A is acquired from sewage-contaminated drinking water; also an STD
by oral/anal contact.
2. Hepatitis B is spread in same
manner as AIDS but is more infectious; vaccine is available.
3. Hepatitis C is called
post-transfusion hepatitis.
4. Hepatitis infections infect liver
and can lead to liver failure, liver cancer, and death.
H. Chlamydia
1. Chlamydia is named for bacterium that causes it: Chlamydia
trachomatis.
2. New chlamydial infections are
higher than any other STD.
3. It also causes pelvic
inflammatory disease (PID).
4. It also causes cervical
ulcerations which increase risk of acquiring AIDS.
5. If a baby is exposed at birth,
inflammation of the eyes or pneumonia can result.
I. Gonorrhea
1. Gonorrhea is caused by bacterium Neisseria gonorrhoeae.
2. Male diagnosis is easy; typical
symptoms include urination pain and a thick, greenish yellow discharge.
3. Latent infections lead to PID;
vasa deferentia or oviducts become infected and inflamed.
4. As tubes heal, they may become
partially blocked, resulting in sterility or infertility.
5. If a baby is exposed at birth, an
eye infection can lead to blindness; all newborns are given eye drops.
6. Previously easily cured by
antibiotics, nearly 40% of modern strains are antibiotic resistant.
J. Syphilis
1. Syphilis is caused by bacterium Treponema pallidum.
2. Disease has three stages
typically separated by latent periods.
a. Primary stage
involves the appearance of a hard chancre (ulcerated sore).
b. Second
stage involves the appearance of a rash all over the body, including palms and
feet.
c. Third
stage involves neurological and cardiac disorders.
1) Infected individual may become mentally retarded, blind, walk with a
shuffle, or insanity.
2) Large destructive ulcers (gummas) develop on skin or within
internal organs.
3. Syphilitic bacteria can cross
placenta, causing birth defects or stillbirth.
4. Unlike other STDs discussed,
there is a blood test to diagnose syphilis.
5. Tracing sexual partners is very
important to control syphilis.
K. Vaginitis
1. Vaginitis is caused by Trichomonas vaginalis
(flagellated protozoa) or Candida albicans (yeast).
2. Protozoan infection causes
frothy, foul-smelling discharge with itching.
3. Protozoan infection transmitted
through sexual intercourse.
4. Yeast infection causes white,
curdy discharge with itching.
5. Candida albicans is
a normally-occurring organism in the vagina; yeast infections can result from
taking
birth-control pill or antibiotics