Download Outline28 Reproduction

Biology 219 – Human Physiology
Clemens
Reproduction
A. Reproductive Systems
Text: Ch. 26
Organs:
gonads
ducts
accessory glands
external genitalia
Male
testes
Female
ovaries
B. Gamete Formation
1. Meiosis
46 chromosomes (2n, diploid) → 23 chromosomes (n, haploid)
↓
22 homologous pairs of autosomes + 1 pair of sex chromosomes (XX = female, XY = male)
DNA replicates prior to meiosis; each chromosome is duplicated, consists of 2 sister
chromatids
Meiosis I: homologous pairs separate, 2n (duplicated) → n (duplicated)
Meiosis II: sister chromatids separate, n (duplicated) → n (single chromosomes)
male: 1 primary spermatocyte (2n) → → 4 sperm cells (n)
female: 1 primary oocyte (2n) → 1 egg cell (n) … → 1 fertilized egg + polar bodies
C. Male Reproductive System
1. Spermatogenesis
- occurs in seminiferous tubules of testes
spermatogonia → primary spermatocyte → secondary spermatocytes → spermatids → sperm
meiosis I
meiosis II
maturation
Sertoli cells - support developing spermatocytes + endocrine function
blood testis barrier - tight junctions between Sertoli cells form barrier to immune system
2. Hormonal Control
a) General pattern
hypothalamus: GnRH → anterior pituitary: FSH & LH → gonads: steroid sex hormones + others
FSH - stimulates gamete production in male and female
LH - stimulates secretion of sex hormones by gonads and stimulates ovulation in females
Steroid sex hormones - ♂: testosterone, DHT; ♀: estrogen [estradiol], progesterone
- stimulate gamete production, overall reproductive function, development
of reproductive organs and secondary sexual characteristics
Negative feedback control regulates hormone levels in males and partly in females
Positive feedback control triggers ovulation in females
b) Hormonal Control of Spermatogenesis
Sertoli cells - respond to FSH, support developing spermatocytes
- androgen binding protein (ABP) binds and concentrates testosterone
Leydig (interstitial) cells - produce testosterone in response to LH
- testosterone binds to ABP in Sertoli cells → promotes spermatogenesis
Negative feedback control:
inhibin - secreted by Sertoli cells, inhibits FSH secretion
testosterone inhibits GnRH and LH secretion
D. Female Reproductive System
1. Oogenesis
- occurs in ovarian follicles in ovaries
follicular cells (granulosa cells and theca cells) surround a developing egg cell (oocyte)
Follicle:
primary follicle → secondary follicle → mature follicle → corpus luteum
Egg:
ovulation
oogonia → primary oocyte → secondary oocyte (egg) → …
meiosis I
meiosis II requires fertilization
2. Menstrual Cycle
a. Ovarian Cycle
follicular phase
day 0
↓
ovulation
luteal phase
14
28
follicular phase - growth and maturation of follicle, oocyte goes through meiosis I
ovulation - egg bursts through wall of ovary, swept into uterine tube
luteal phase - corpus luteum forms from remaining follicle cells;
- corpus luteum continues to secrete estrogen and progesterone;
degenerates at the end of the cycle unless fertilization occurs
b. Uterine Cycle
menses
proliferative phase
day 0
secretory phase
14
28
menses - uterine lining (endometrium) is sloughed off and discharged
proliferative phase - endometrium regrows, becomes thicker and vascularized
secretory phase - endometrium continues to thicken and becomes secretory
3. Hormonal Control of the Female Reproductive System
a. Early follicular phase
- FSH and LH secretion begins to increase
- follicular cells stimulated to produce steroid sex hormones:
theca cells: produce androgens → granulosa cells: convert androgens to estrogens
- estrogen levels rise → stimulates proliferative phase
- at low to moderate levels, estrogen inhibits FSH and LH secretion (negative feedback)
b. Late follicular phase
- estrogen secretion peaks
- at high levels, estrogen exerts positive feedback → stimulates FSH and LH secretion
- LH surge triggers ovulation
- high estrogen level prepares reproductive tract for ovulation and sperm entry
c. Luteal phase
- after ovulation, corpus luteum secretes estrogen and progesterone
- progesterone levels rise → stimulates secretory phase, prepares uterus for pregnancy
- estrogen, progesterone and inhibin exert negative feedback, inhibit FSH and LH secretion
If fertilization does not occur, corpus luteum degenerates (→ corpus albicans)
→ estrogen and progesterone levels decrease → menstruation
If fertilization does occur, zygote undergoes cell divisions to form a blastocyst (~100 cells)
- blastocyst implants in the uterus about 7 days after fertilization
- implanted embryo secretes human chorionic gonadotropin (hCG)
hCG “rescues” the corpus luteum → continues secretion of estrogen and progesterone,
keeps endometrium intact for the developing embryo
- placenta takes over production of progesterone and estrogen by the 7th week