Reproductive System II (copy)

What separates in meiosis I?

What separates in meiosis II?

Compare the differences in
gametogenesis in males and
females.

Homologous Chromosomes

- Sister Chromatid

• Spermatogenesis: Continuous
• Oogenesis: Cyclical and have arrest point:

1) Prophase I

2) Metaphase II

The Hypothalamic-Pituitary-Gonadal Axis (HPG axis)

• Hypothalamus → GnRH( Gonadotropin Releasing Hormone pulses) → Anterior pituitary → secretes FSH (Follide Stimulating Hormone) and LH ( Luteinzing Hormone)

• FSH/LH act on gonads (testes or ovaries) → sex hormones → feedback loops

• Key idea: “GnRH drives FSH and LH, which drive follicle development and ovulation.”

Overview of Ovarian Follicles

• Definition: follicle = oocyte + surrounding support cells
• Role: protects, nourishes, and secretes hormones (help it develop)

Primordial Follicle (Resting/Waiting)

Present at birth

Single layer of squamous (sometimes call flat cells) cells around a primary oocyte (arrested in Prophase I)

Resting pool for future cycles (ready to go into oogenesis and cause ovalution)
- Most never develop
- Atresia (if it not dominant follicle)

Primary Follicle

FSH activates the follicle

Cuboidal granulosa cells appear - Granulosa cells begin producing estrogen precursors
(FSH become a primary hormone at this step)

Zona pellucida (layer of glocoprotein) forms around oocyte

Key Point: oocytes from being dormant to active

Secondary Follicle

• Granulosa layers multiply

• Theca cells form around follicle → respond to LH → make androgens → converted to estrogen by granulosa cells

(LH → Theca Cell → Androgen → Granolosa → Estrogen)
• Follicular fluid begins to accumulate

Mature (Graafian) Follicle

• Large antrum filled with fluid
• Oocyte on a stalk of granulosa cells
• High estrogen output (positive feedback) (day 12 to 14)
• Triggers LH surge

(estrogen does negative feedback on hypothalamus and anterior pituitary gland → secretes more LH)

(LH also signal to oocyte to finish meiosis I)

Ovulation

Triggered by LH surge (positive feedback from rising estrogen)

(more estrogen => more LH)

LH Surge causes 2 things:

1) Ovulation

2)Finish Meiosis I

• Follicle ruptures → oocyte enters uterine tube
• Secondary oocyte released, arrested in Metaphase II

Corpus Luteum Formation

• Remaining follicle cells transform into corpus luteum
• Secretes progesterone (and some estrogen)(help prepare the uterus)
• Prepares endometrium for possible implantation

(progesterone is primary hormone for luteal phase)

Progesterone does:

1) Prepare the uterus for pregnancy

2) prevent another (second) ovulation

a) Negative feedback on hypothalamus → decrease GnRH

b) Negative feedback anterior pituitary gland → decreases FSH and LH

(Lower FSH → follicle cannot start developing)(LH also prevent follicle develop)

If no fertilization occurs...

• Corpus luteum degenerates → corpus albicans
• Progesterone & estrogen drop (progesterone inhibit secretes FSH) → menstruation begins
• FSH rises to start new cycle

(corpus luteum stays 10-12 days without fertilization, and secretes estrogen and progesterone)

Hormonal Regulation in Females

• FSH: stimulates follicle growth & granulosa (turn androgen → Estrogen) activity
• LH: stimulates theca (androgen → turn antrogen → estrogen) cells, triggers ovulation
• Estrogen: promotes growth of endometrium; positive feedback for LH surge
• Progesterone: stabilizes endometrium, inhibits GnRH/FSH/LH post-ovulation

Key Ideas

Hormone Level Graph

• Estrogen levels rise as follicle grows
• High estrogen → LH Surge
• Day 14: LH surge → ovulation
• Luteal phase: progesterone peak
• Drop in hormones → menstruation

(follicular phase from 1-13 days, luteal phase from 14-28 days)

Male Hormonal Regulation

• Same HPG framework:
• GnRH → FSH & LH
• LH → Leydig cells → testosterone (does negative feedback for hypothalamus and pituitary gland)
• FSH → Sertoli cells → ABP & sperm support (also do negative feedback)
• Negative feedback: testosterone & inhibin

(negative feedback keep everything in stable range and continous rather than cyclical)

Comparing Male vs. Female Feedback Loops

Hormone Disruption

• Low body fat / stress → ↓ GnRH → amenorrhea → ability to menstructe

(if we have healthy body fat → secretes leptin hormones → hypothalamus → secretes GnRH → enough energy to support pregnancy)

(stress → adrenal glands secretes cortisol → inhibit to hypothalamus → not secretes GnRH)
• Polycystic ovary syndrome (PCOS): excess androgens, anovulation (string of pearls, “testosterone - like”)
• Testosterone therapy in males → ↓ FSH/LH → low sperm count

(take exogenous testosterone is negative, It inhibit hypothalamus → turn of spermatogenesis and testosterone)

Summary

• Follicles develop from primordial → primary → secondary → Graafian → ovulation → corpus luteum → regression (if no fertilization)
• HPG axis controls the process
• Hormone feedback ensures only one ovulation per cycle (progesterone → inhibit anterior pituitary and hypothalamus → less FSh & LH)
• Males use same axis but maintain constant gamete production