Lecture Notes: Reproduction

O. Big Questions...

How do you go from one cell to a complex organism of many tissues, organs, etc.?

Where does the one cell come from?

How do you get a complex organism that interacts (i.e. behaves) with its enviornment?

I. Reproductive Stratagies - Sexual vs. Asexual

A. Asexual Reproduction - no fusion

1. Budding - (hydra, Cnidaria)
2. Fragmentation / Regeneration - (starfish arms)
3. Fission - Planeria (flat worm)
4. Parthanogenesis - (insects: walking sticks)


B. Sexual Reproduction

0. Thoughts...

a. Why?

- Creates Genetic Diversity
- BUT risks losing stable gene complexes

b. Natural sex reversals (certain fish): regulating sex ratios?
c. Maleness vs. Femaleness?


1. Requires Fusion of Sperm and Egg

a. dioecious - male individual, female individual
b. monoecious - individual with male and female repro organs

- polychaeates (earth worms)
- hermaphrodites

c. Requirement: get sperm and egg together


2. External Fertilization (vs. Internal Fertalization)

a. Requires production of many gametes

- Spawning - release of gametes
- "random" encounter

b. Primarily Aquatic
c. Stratagies to ensure fertilization.

- Population level control coordinating spawning
- coordination of Spawning
- Seasonality
- sperm attractants
- Copulatory behaviors

-- external but males and females join to ensure fertilization


3. Internal Fertilization (really "outside" the body)

a. Terrestrial adaptation? Primary in terrestrial environment.
b. permits increased nutrient investiment into single egg
c. complex courtship and mating behaviors / social behaviors

-animals usually join to eat each other;
--complex behaviors evolve to get past this awkward stage...
- pheromones - sex attractants
- sexual selected physical traits

d. seasonality

II. Mammalian Reproduction - Internal Fertilization (actually pretty general)

A. Male Gonad: Testis / Sperm Formation

1. Anatomy

a. Testis -- Seminiferous Tubule
b. Epididymis
c. Vas Deferens
d. Urethra
e. Penis
f. Seminal Fluids : Exocrine Glands into Urethra

- Seminal Vesicle
- Prostrate gland
- Bulbourethral glands


2. Sperm formation

a. Spermatogenesis occurs in Seminiferous Tubules
b. Two Cell Types:

- Sperm forming cells
- Sertoli Cells (supply nutrients; substrate, stimulating factors)

c. Development

- Primordial Germ Cell - meiosis
- Spermatogonium (2N) - divides to form

-- Spermatogonium (2N - diploid)
-- Primary Spermatocyte (2N)

- Primary Spermatocyte - Meiosis 1

-- Two Secondary Spermatocytes - Meiosis II
-- Four Spermatids (N) - Differentiate
-- Four Sperm Cells (N - haploid)

d. Sperm cells migrate to epididymis, where they acquire motility
e. semen: sperm + fluids + etc. (sugar, enzymes, mucus, amino acids)
f. spermatogonia to motile sperm, humans:

several hundred million sperm per day per day
time to motility: 65-75 days
human ejaculate: several ml, 10⁷ sperm/ml


3. Hormonal Regulation

a. Hypothalamus > GnRH (Gonadotropin-releasing Hormone)
b. GnRH > Anterior Pituitary > LH and FSH release
c. LH > interstititial cells in testes > testosterone
d. testosterone + FSH > Sertoli Cells and Spermatogonium >

>> Sperm formation

e. Testosterond - negative feedback on GnRH, LH and FSH cells
f. Testerone also modulates...

- Secondary Sexual Traits (facial hair, breasts, etc)
- behavior (testerone poisoning)

-- social behavior
-- intercourse


B. Female Gonad: Ovaries / Egg Formation

1. Anatomy

a. Ovary
b. Fimbrae -
c. Oviduct - likely site of fertilization
d. Uterus - embryogenesis
e. Vagina - sperm transfer


2. Egg Formation

a. Human embryo : primary germ cell > mitosis > oogonia (2N)

- finite number - ca. 400,000

b. Each oogonium develops into primary oocyte - latent until puberty

- arrest in mid meiosis I (prophase)
- Follicle:

each oocyte is surrounded by follicle cells
entirety is termed Follicle
human females: ca. 400,000 Follicles (eggs)

- one follicle will mature per month, puberty > menopause

c. Primary oocyte completes meiosis I (one per month)

- TO: Secondary oocyte (large) + first polar body (small)

-- first polar body (may divide, eventually degenerates)

- STOP / ARRESTMENT (mammals)

-- meiosis II does not occur until AFTER fertilization
-- in mammals, on fertilization, second meiosis completes

yielding egg and 2nd polar body


d. OVARIAN CYCLE - Migration

- Follicle development begins at one end of ovary (away from oviduct)
- migrates during meiosis to other end (end facing oviduct)
- Follicle ruptures, releaseing ovum

(secondary oocyte surrounded by follicle cells)

- Ruptured Follicle migrates back

- Ruptured Follicle differentiates into Corpus Luteum
- Corpus Luteum degenerates

- Ovulatory Phase -- Luteal Phase


3. Hormonal Regulation of Sperm / Egg Development

a. Coordination - Why have cycles?

- Internal Fertilization coordinates with internal gestation
- Preparation of suitable substrate for development

- uterus lining
- placenta

- None of this is human evolved - i.e. more ancient

- coordination of behavior
- annual cycles - environmental concerns


b. Menstrual Cycle vs. Estrous Cycle

- Shedding (MC) or reabsorption (EC) of endometrium
- MC: human, some primates / EC: other mammals
- EC: more pronounced behavioral and seasonal changes
- Cycle times variable

-- Human MC: ca. 28 days

--- variable within and between individuals - 20-40 days
--- can be coordinated among individuals

-- Rat EC: 5 days
-- Dog EC: annual


c. OVAIAN CYCLE, Ovulatory Phase

- Start:

-- GnRH >> FSH, LH
-- Follicle Cells express FSH-R (no LH-R)
-- no Estrogen

- early in follicle development

-- FSH stimulates Follicle Cells to express Estrogen -- small follicle, low levels of estrogen
-- low estrogen inhibits GnRH release maintaining low LH, FSH.

- later in follicle development

-- follicle cells start expressing LH-R
-- as follicle grows, estrogen release increases
-- high estrogen stimulates GnRH release, increasing LH, FSH
-- surge in LH causes surge in estrogen
-- high estrogen induces final maturation FAST!
-- OVULATION


d. OVARIAN CYCLE, Luteal Phase: following Ovulation

- Start: with Ovulation
- LH > follicle cells >>>> Corpus Luteum

(max. dev. 8-10 days post ovulation)

- Corpus Luteum > estrogen AND progesterone
- estrogen/progesterone inhibit GnRH, LH, FSH release

-- drop in LH allows Corpus Luteum to degenerate
-- degeneration of Corpus Luteum > drop in estrogen/progesterone -- drop in androgens, release inhibition GnRH, LH, FSH

- increase in GnRH, LH, FSH > cycle starts over on new follicle


e. MENSTRUAL CYCLE

- Regulation of endometrial lining of uterus
- During Follicular Phase

-- estrogen > endometrial thickening
-- at ovulation, endometria is prepared for embryo

- During Luteal Phase

-- estrogen AND progesterone maintain endometrial dev.

--- enlargement of uteran arteries
--- dev. of endometrial glands - embryonic nutrients

-- androgen crash > arterial spasms > no blood to endometrium
-- endometrium degenerates : menstruation


f. If fertilization is successful...

- embryo embeds in endometrium
- release of chorionic gonadotropin (from placenta)
- Corpus Luteum rescued,

secrete estrogen/progesterone for 2-3 months
placenta starts producing estrogen/progesterone



CLICK FIGURE FOR LARGER IMAGE

D. Fertilization
TO BE CONTINUED

1. Events
2. Hormonal Regulation of Incubation Environment

Lecture Outline: Reproduction and Development Chapters 39,40 10/28/96

O. Overall, Week of October 28: Reproduction and Development

A. Monday: Types, Formation of Sperm, Formation of Egg
B. Tuesday: Fertilization and Early Development
C. Wednesday: Formation of the Body Plan - Signal Coordination

I. Reproductive Stratagies - Sexual vs. Asexual

A. Asexual Reproduction - no fusion
B. Sexual Reproduction

1. Requires Fusion of Sperm and Egg
2. External Fertilization
3. Internal Fertilization

II. Mammalian Reproduction - Internal Fertilization (actually pretty general)

A. Male Gonad: Testis / Sperm Formation

1. Anatomy
2. Sperm formation
3. Hormonal Regulation

B. Female Gonad: Ovaries / Egg Formation

1. Anatomy
2. Egg Formation
3. Hormonal Regulation of Egg Development

Vocabulary: