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 gonad2. Seminiferous tubules – make up most of the

testes / lots of small coils / sperm made here3. Leydig Cells – produce androgens, in testes4. Scrotum – sac that holds the testes5. 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 Semena. 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, citratec. 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 skin11. 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 babya. Endometrium – lining of uterus / rich in blood vesselsb. Cervix – neck of uterus

4. Vagina – forms birth canala. 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 cells1. 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. Hormonesa. 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. Fertilization occurs in fallopian tubes2. Cleavage (24 hours)3. Ball of cells is now in the uterus (3-4 days)4. Blastocyst (embryo) 1 week / 5 more days to

implant5. 2-4 weeks it get nutrients from endometrium

and then the placenta forms6. Organogenesis occurs and most organs are

formed by 8 weeks7. 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. Rhythm Method

2. Barrier Methods / Condom, diaphragm

3. IUD

4. Withdrawal

5. Birth Control Pills / Synthetic Estrogen and Progesterone / Both stop GnRH, estrogen stops FSH, Progesterone stops LH

6. Sterilization / tubal ligation, vasectomy

7. Abstinence