Oogenesis & the Sexual Cycle

Oogenesis

production of female gametes (ova/ eggs)

How is oogenesis different from spermatogenesis

it happens before birth in the fetal period

(spermatogenesis happens at puberty)

Oogenesis as a fetus (before birth)

Oogonia (2n) → mitosis + store nutrients → primary oocytes → begins meiosis but stops/ stalls in prophase 1

where do primary oocytes develop

primordial follicles

Oogenesis after puberty

each month after puberty, few primary oocytes are activated → selects one to resume meioisis I → 2 haploids (one secondary oocyte and one first polar body which dies)

Secondary oocyte vs first polar body

secondary oocyte gets all the cytoplasm while first polar body dies

What happesn to the secondary oocyte

begins meioiss 2 → stops at metaphase 2 → ovulated / released into uterine tube

What happens if sperm does not arrive
what happesn if it does

not penetrated by sperm → dies

penetrated by sperm → finishes meioisis 2 → ovum/ova/egg (functional gamete) + second polar body (which dies)

Full oogenesis pathway

Pre birth

oogonium (2n) → mitosis → primary oocytes(n) arrested at prophase 1 → meiosis I → secondary oocyte (n) + first polar body → secondary oocyte arrested at metaphase 2 → ovulation → penetrated by sperm → complete meiosis 2 → ovum + second polar body

What happens right after meiosis 2 is complete

fertilization with sperm → zygote

Reproductive cycle

from fertilization to giving birth to fertility returning

Sexual cycle and what are the 2

events that happen every months when not pregnant

• menstrual and ovarian

Ovarian cycle - what are the 2 phases
and what happens in between

happens in ovaries

1. follicular phase (days 1-14) → follicle growth (immature egg with surrounding tissues)

2. Luteal phase (days 14-28) → corpus luteum activity (developed from ruptured follicles)

in between → ovulation midcycle (ejection of oocytes)

Follicular phase (first transformation)

1. primordial follicles activate (squamous like cells → cuboidal)

2. primordial follicles enlarge → primary follicles

Follicular phase (second transformation)

Primary follicles → secondary follicles

• stratified epithelium (granolsa cells) form around oocyte

• granulosa cells and oocyte guide each others development

Follicular phase (third transformation)

Secondary follicle becomes late secondary follicle

• connective tissue (theca folliculi) + granulosa cells cooperate to produce estrogen

• zona pellucida forms around oocyte

• fluid accumulates

Follicular phase (fourth transformation)

late secondary follicle → tertiary follicle/ vesicular follicle

• antrum (filled with fluid) forms and expands → isolates oocyte

• vesicular follicle/ graafian follicle bulging from ovary surface

• primary oocyte completed meioisis 1 → secondary oocyte + first polar body

Ovulation

ovary walls rupture → expels secondary oocyte and corona radiata

mittzschmerz

pain felt at ovulation

1-2% of ovulation releases

more than one secondary oocyte → fraternal twins

Luteal phase

ruptured follicle collapses

• granulosa cells and internal thecal → corpus luteum → secretes progesterone and estrogen

What happesn to corpus lutuem

if pregnant → produces hormones until placenta takes over at 3 months

if not pregnat → corpus luteum degenerates → corpus albicans in 10 days

Establishing ovarian cycle - childhood → puberty

childhood → ovaries release estrogen → inhibit hypothalamus from release gonadotropin rleasing hormones

near puberty → Leptin (from adipose tissue) decRease estrogen inhibition → GnRH is released → anteriorior pituitary gland releases FSH and LT → act on ovaries → until mature cycle appears and menarche

menarche

first menstrual cycle

Ovarian cycle day 1

hypothalamus release GnRH → release of FSH and LH

• FSH and LH act on ovaries → activating and developing follicles → follicles secrete estrogen (remeber late secondary follicle → granulosa cells and theca folliculi secrete estrogen)

• as estrogen rises (negative feedback)

  • inhibit reelase of FSH and LH but promote production and storage
    so keep making them but dont secrete them

Near mid cycle

as one follicle becomes the dominant vesicular graafian follicle (before FSH and LH was inhibited) → increases production of estrogen

→ high estrogen level by midcycle → positive feedback effect on pituitary gland at midcycle → surge of LH at day 14

Effect of LH surge midcycle (day 14)

(3)

• meiosis I completed → secondary oocyte → continues until metaphase 2

• triggers ovulation

• transforms ruptured follicle → corpus luteum

Functions of corpus luteum (3)

produce inhibin, estrogen, progesterone →inhibit fsh and Lh secretion

decline lh and fsh levels

end luteal activity and inhibit follicle development

Day 26-28 of ovarian cycle

corpus luteum degenerates into corpus albican + ovarian hormones drop (inhibin estrogen progesterone) → inhibition on FSH and LH drop → starting cycle again

Uterine/ menstrual cycle

cyclic changes in endometrium of uterus in response to ovarian hormones

3 phases of menstrual uterine cycle

• day 1-5 -menstrual phase

• day 6-14 - proliferative (preovulatory) phase

• Day 15-28 - secretory (postovulatory) phase

Menstrual phase of menstural uterine cycle

Days 1-5

ovarian hormones are at the lowest → no inhibition → gonadotropins begining to rise → stratum functionalis is shed and menstrual flow occurs

Proliferative / preovulatory phase of menstrual uterine cycle

Days 6-14

estrogen levels rise → regeneration of stratum functionalis + increase progesterone receptors in endometrium

glands enlarge + spiral arteries increase

Secretory / posovulatory phase of menstrual uterine cycle

days 15-28

progesterone → further endometrium development , glands secrete glycoge, formation of cervical mucus plug (hard for sperm to pass through)

If fertilization doesn’t happen

corpus luteum degenerates → progesterone levels fall → spiral arteries kink / spasm (constrict then relax and open wide) → blood fragments rush in→ stratum functionalis breaks down/sheds → menstruation

Effects of estrogen

• promotes what 2

• what type of grwoth on female reproductive tract

• supports what at puberty

• induces what

• promote oogenesis and follicle growth

• anabolic growth

• rapid but shortlived growth spurt

• secondary sexual characteristics (breast, subcutanous fat at hips and breasts, widening of pelvis)

Metabolic effects of estrogen (2)

maintain low blood cholesterol and high hdl (good cholesterol)

calcium uptake

Progesterone effects

works with estrogen to establish and regulate uterine menstrual cycle

Placenta progesterone during pregnacny does what

inhibit uterine motility (contracting too much) + prepare breast for lactation

Menopause

permenant end of menstruation (no menstrual period for a year)

• decreased estrogen → atrophy of reproductive organs and breast + hot flashes (vasodilation) + skin and bone loss + increased blood cholesterol and decreased HDL