Ch.46
Animal Reproduction
• I.
Animal reproduction
• A. Asexual reproduction – mitotic division, no fertilization
• 1. Fission- separation of parent into two or more individuals of the
same size.
• 2. Budding new individual splits off original (Cnidaria).
• 3. Fragmentation – breaking of the body into several pieces which
will develop into new individuals.(sponges,cnidaria,annelids and
tunicates).
• 4. Allows for production of offspring without mates.
• 5. Allows for reproduction in a short time.
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B.
Sexual reproduction – Fusion of gametes forming a diploid zygote.
C.
Reproductive cycles
1. Usually animals have seasonal cycles
a. Conservation of resources and energy
b. Allows for reproduction under favorable conditions
c. Controlled by hormones and environment
2. Animals have various reproductive patterns – may be sexual or asexual or
alternating.
a. Parthenogenesis – egg develops without fertilization
b. Daphnia switches between asexual and sexual depending on conditions.
c. Male honey bees are produced by parthenogenesis, females develop from
fertilized eggs.
d. Hermaphrodites – each individual has both functional male and female
reproductive parts. Most mate, each producing cross-fertilized eggs.
e. Sequential hermaphroditism – reverses sex during lifetime.
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II.
Mechanisms of sexual reproduction
A.
Fertilization
1. Internal – sperm deposited in or near the female reproductive tract and
fertilization occurs inside the female body.
2. External – Eggs are shed and fertilized outside the body.
3. Pheromones – Chemical signals between organisms of the same species.
B.
Internal fertilization usually produces fewer offspring but survival
through development is greater due to protection and care of offspring.
C.
Complex systems
1. Some systems do not have gonads
2. Parasitic flatworms are have very complex reproductive systems.
3. Believe it or not insects have testis(male) and vagina(female).
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III.
Mammalian reproduction
A.
Human reproduction
1. Male anatomy
a. Testes develop in abdomen and descend into scrotum before birth – this
keeps the sperm 2 degrees below body temp.
b. Seminiferous tubules in the testes is where sperm is produced.
c. The sperm pass to the epididymus
d. At ejaculation sperm enter the vas deferens then the urethra and out.
e. Glands called the seminal vesicles secrete mucus, amino acids, fructose
(energy), and prostaglandins. Prostate secretes milky alkaline fluid which
balances the acidity of the vagina. Bulbourethral gland – secrete clear mucus
right before ejaculation.
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2. Female anatomy
a. Ovaries – contains many follicles all formed at birth. One follicle a month
will mature after puberty
b. During ovulation the egg is expelled leaving the corpus luteum which
secrete progesterone and additional estrogen
c. If the egg is not fertilized the corpus luteum degenerates.
d. The egg cell travels through the oviduct to the uterus
e. Cervix – neck of the uterus, open to the vagina
f.
Vagina – repository for semen and birth canal.
g. Hymen – covers vaginal opening until ruptured.
h. Clitoris – small hood of skin
i.
Bartholin’s glands – secrete mucus to lubricate vagina.
j.
Mammary glands – small sacs of epithelial tissue that secrete milk.
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B.
Spermatogenesis/oogenesis
1. Spermatogenesis is continuous in adult males (100-600 million per
ejaculation)
2. Occurs in seminiferous tubules of testes
3. Oogenesis is the development of the ova.
a. begins in embryo
b. all potential ova present at birth
c. After puberty FSH stimulates follicle to enlarge and completion of
meiosis
d. LH triggers ovulation
4. Differences
a. All four cells produced during meiosis during spermatogenesis become
sperm, in oogenesis only one of the four will become a mature ova
b. Spermatogenesis is continuous throughout adult life, oogenesis happens
only before birth
c. Spermatogenesis is uninterrupted oogenesis has “resting periods”
• C.
Hormone regulation
• 1. Male pattern
• a. Testosterone is the most important androgen
which is responsible for male development.
• b. GnRH from hypothalamus stimulates
pituitary to release LH (for androgen production)
and FSH (increases sperm production)
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2. Female patterns
a. Estrous cycle – non-primate mammals
i.
Ovulation after the endometrium thickens
ii.
No pregnancy,endometrium reabsorbed
iii.
Affected by behavior and seasonal changes
iv.
Estrus is the only time copulation occurs
b. Menstral cycle- primate mammals
i.
Ovulation as in estrous
ii.
No pregnancy, endometrium is shed- menstral flow
iii.
In humans ranges from 20-40 days(menstral flow = day 1)
iv.
Proliferative phase – regeneration of endometrium, 1- 2 weeks
v.
Secretory phase – 2 weeks, further thickening of endometrium
vi.
Ovarian cycle parallels menstral cycle
vii.
Follicular phase- follicles in ovaries begin to grow, egg is expelled
viii.
Luteal phase – after ovulation, left over follicular tissue becomes
corpus luteum which secretes hormones
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c. Hormones
i.
GnRH from hypothalamus stimulates pituitary
ii.
FSH from pituitary stimulate the follicles to grow and produce
estrogen
iii.
Estrogen stimulates hypothalamus to increase production of GnRH
(positive feedback) which increases FSH and LH
iv.
LH stimulates final maturation of follicle and ovulation
v.
Corpus luteum secretes progesterone which along with estrogen
inhibits GnRH production
vi.
LH declines and corpus luteum atrophies
vii.
Estrogen and progesterone levels drop and GnRH is produced –
process begins again
viii.
Menopause – ovaries lose responsiveness to LH and FSH
Ch.51- reproduction