Chapter 46
Animal Reproduction
I. Asexual Reproduction
Creation of new individuals whose
genes all come from one parent
A. Types
1. Fission – separation of parent into 2
individuals
2. Budding – new individuals from an
existing one
3. Fragmentation – breaking the body into
several pieces / regeneration
4. Parthenogenesis – egg develops without
being fertilized
B. Benefits
1. No mate needed
2. Lots of offspring fast
3. Advantageous in stable, favorable
environments
II. Mechanisms of Sexual
Reproduction
Both types use pheromones to
attract mates
A. Internal Fertilization
1. Fertilization occurs in or near the female
reproductive system
2. Requires cooperative behavior and
complex reproductive systems
3. Produces fewer offspring but greater
protection and care of young is seen
B. External Fertilization
1. Eggs are released and fertilized in the
environment
2. Need an environment where eggs will
not dry out
3. Timing is crucial
4. Produce more zygotes but less survive
C. Variations
• Complex : flatworm, insects
(spermatatheca)
• Simple : polychaete where gametes
develop from cells in the coelom
• Many use a cloaca
Flatworm
Insects
III. Mammalian / Human
Reproductive System
A. Male
1. Testes – male gonad
2. Seminiferous tubules – make up most of the
testes / lots of small coils / sperm made here
3. Leydig Cells – produce androgens, in testes
4. Scrotum – sac that holds the testes
5. Epididymis – tube from the seminiferous
tubules to the vas deferens / 6m long / sperm
gain mobility here
6. Vas Deferens – carry sperm from the
epididymis behind the bladder up to the
ejaculatory duct
7. Ejaculatory Duct – opens to urethra and goes
out
A. Male (cont)
8. 3 Glands That Together Make Semen
a. Seminal vesicles - 60%, yellow, thick,
alkaline, mucus, fructose, coagulating enzyme,
ascorbic acid, prostaglandins (thins uterine
mucus and cause contractions)
b. Prostate gland – thin, milky, anticoagulant
enzyme, citrate
c. Bulbourethral gland – before ejaculation,
clear mucus, neutralizes urine from urethra
9. Penis – erectile tissue fills with blood and
injects semen
10. Glans Penis – head, thin skin
11. Prepuce - foreskin
B. Female
1. Ovary – female gonad / has lots of follicles
(egg and nourishing follicle cells / turns into
corpus luteum) / follicles produce estrogen
2. Oviduct / Fallopian Tubes – tube leading from
ovary to uterus / cilia
3. Uterus – thick muscular organ for holding the
baby
a. Endometrium – lining of uterus / rich in
blood vessels
b. Cervix – neck of uterus
4. Vagina – forms birth canal
a. Hymen – covers vaginal opening
B. Female (cont)
5. Labia minora/majora – flaps of skin that
border the vestibule
6. Clitoris – mostly erectile tissue with lots
of nerve endings
7. Bartholin’s Glands – secrete mucus /
near vaginal opening
IV. Formation of Sperm and
Egg
A. Spermatogenesis
1. Occurs in the seminiferous tubules /
continuous
2. Spermatogonia are the special diploid
cells
3. Starts on the edge of the tubule and
moves towards the lumen
4. Spermatogonia – Primary Spermatocyte
– Spermatids – Sperm (Spermatocyte)
5. 65-75 days
B. Oogenesis
• Occurs in the ovary and fallopian tubes
• Oogonia are the specialized diploid cells
1. Oogonia multiply and begin meiosis I but
stop at prophase I (primary oocyte).
Stay this way till puberty.
2. FSH enters and stimulates a follicle to
grow and a primary oocyte to complete
meiosis I and start meiosis II but stop at
metaphase II (secondary oocyte)
B. Oogenesis (cont)
3. The secondary oocyte is released during
ovulation
4. Meiosis II completes after fertilization
occurs
* Uneven cytoplasm distribution results in
polar bodies
V. Hormones and
Reproduction
A. Male Hormones
1. Androgens (Testosterone) – produced by
leydig cells in testes
- increase primary and secondary
characteristics
- increase aggressiveness, deep voice,
hair, muscles, etc.
B. Female Hormones
1. Menstrual vs. Estrous Cycle
- Both Cycles: ovulation occurs after
endometrium thickens
- Estrous: reabsorb endometrium, pronounced
behavior changes, stronger seasonal effects,
estrus, does not occur in primates
2. Coordination of Cycles and Hormones
3. Menopause – no longer respond to
gonadotropins
B. Female Hormones
5.
Hormones
a. GnRH – gonadotropin releasing hormones
- Made by hypothalamus / uses estrogen levels to
regulate
- stimulates LH and FSH production by pituitary
- estrogen and progesterone together stop it
b. FSH – follicle stimulating hormone
- induces follicular phase
c. LH – luteinizing hormone
- causes change to corpus luteum
1. - Low amounts of LH and
FSH
- Receptors for FSH not
LH
- FSH stimulates follicle
growth
2. - Follicle produces
estrogen which thickens
the endometrium and
inhibits LH and FSH
3. - Later lots of estrogen
increases GnRH
production of LH and
FSH in high amounts
4. Now the follicle has LH
receptors and ovulation
occurs (secondary
oocyte)
5. - Left over LH changes
follicle to corpus luteum
6. - Corpus luteum secretes
estrogen and
progesterone
- Together they stop
production of GnRH
which stops LH and FSH
- This keeps the
endometrium happy
7. - Corpus luteum
disintegrates and
estrogen and
progesterone drop
allowing LH and FSH to
be produced
8. - Causes spasms in
uterus and endometrium
sheds
B. Female Hormones
• So what if there is a fertilized embryo
implanted into the endometrial lining?
- HCG (human chorionic gonadotropin) is
produced by the embryo which keeps the
corpus luteum intact.
- Eventually the placenta produces the
needed estrogen and progesterone
VI. Pregnancy
38-40 weeks
A. First Trimester
1.
2.
3.
4.
Fertilization occurs in fallopian tubes
Cleavage (24 hours)
Ball of cells is now in the uterus (3-4 days)
Blastocyst (embryo) 1 week / 5 more days to
implant
5. 2-4 weeks it get nutrients from endometrium
and then the placenta forms
6. Organogenesis occurs and most organs are
formed by 8 weeks
7. Mucus plug forms
B. Second Trimester
1. Movement (20 weeks)
2. Continued growth
3. HCG stops and corpus luteum falls apart
/ placenta secretes the necessary
progesterone
C. Third Trimester
1. High estrogen levels trigger oxytocin
receptors on the uterus
2. Oxytocin produced by the fetus and mom
trigger contractions
3. Oxytocin also trigger the placenta to
make prostaglandins to help too
- positive feedback, more stress more
hormones
4. Birth : dilation, birth, afterbirth
VII. Contraceptives
1.
2.
3.
4.
5.
Rhythm Method
Barrier Methods / Condom, diaphragm
IUD
Withdrawal
Birth Control Pills / Synthetic Estrogen and
Progesterone / Both stop GnRH, estrogen
stops FSH, Progesterone stops LH
6. Sterilization / tubal ligation, vasectomy
7. Abstinence