Endocrine System & Reproduction

20-18

Sexual Differentiation

• Early embryonic gonads can become testes or ovaries– Y chromosome induces formation of testes

• Testosterone (T) from testes induces formation of male sex organs

• In absence of T, female sex organs develop

20-4

Sexual Reproduction

• Gametes (sperm or eggs) are formed in gonads by meiosis– Each has haploid

number of chromosomes (23)

– Fusion of sperm & egg during fertilization results in diploid zygote with 43 chromosomes

20-5

Fig 20.1

Hypothalamus & Pituitary

• Hypothalamus controls release of LH & FSH from anterior pituitary (Ant Pit) with GnRH

• LH & FSH stimulate production of sperm & eggs, & gonadal sex steroids; & maintain size of gonads

• Sex steroids provide negative feedback on hypothalamus & Ant Pit

• Gonads also secrete hormone inhibin which negatively feeds back on FSH secretion

20-19

Fig 20.9

Male Reproductive System

20-25

Male Reproductive System• Testes contain

seminiferous tubules (STs) where spermatogenesis occurs; & interstitial tissue housing T-secreting Leydig cells

• Sertoli cells of STs contain receptors for FSH– FSH stimulates

spermatogenesis

• Leydig cells contain LH receptors– LH stimulates

secretion of TFig 20.12

20-26

Control of LH & FSH Secretion

• Is by negative feedback– T inhibits LH & GnRH

secretion but not FSH

– Inhibin from Sertolis provides negative feedback on FSH

Fig 20.1320-27

Spermatogenesis

• Germ cells that migrate from yolk sac during development become spermatogonia (stem cells)– Spermatogonia

replicate selves throughout life by mitosis

– Give rise to haploid sperm by meiosis

Fig 20.16

20-31

• Have oval-shaped head that contains DNA & the acrosome (a cap of digestive enzymes)

• Have a midpiece & flagellar tail– Tail will become motile in epididymus

Spermatozoa

20-36

Fig 20.19

Female Reproductive System

20-46

Female Reproductive System

• Ovaries contain large number of follicles that produce female gametes (eggs or ova) in ovarian cycle

• Extensions of fallopian tubes called fimbriae partially cover each ovary– The cilia of its lining

draw in ovulated eggs

20-47

Fig 20.24

Female Reproductive System continued

• Uterus has 3 layers:• Perimetrium is outer

layer of connective tissue

• Myometrium is middle layer of smooth muscle

• Endometrium is hormonally-responsive inner epithelial layer that is shed during menstruation

• Between uterus & vagina is cervix

Fig 20.24

20-48

Ovarian Cycle

• 1o oocytes are contained in 1o follicles– Consisting of oocyte + follicle cells

• In response to FSH some follicles enter ovarian cycle & grow, producing layers of granulosa cells

• Some 1o follicles continue, developing vesicles & becoming 2o follicles

20-51

Fig 20.29

Ovarian Cycle continued

• One follicle continues growth– Vesicles fuse, forming fluid-filled cavity called an antrum

• Is now called a Graafian follicle

20-52

Fig 20.29

Ovarian Cycle continued

• As Graafian follicle develops, 1o oocyte completes meiosis I

• This is an assymetric division because 1 daughter cell (the 2o oocyte) gets all cytoplasm– Other daughter becomes a

small polar body which will degenerate

– 2o oocyte arrests at metaphase II

• Only fertilized eggs complete meiosis II

20-53

Fig 20.31

Ovarian Cycle continued

• 2o oocyte is part of graafian follicle• Granulosa cells form layer around outside of follicle• Oocyte sits on mound in this layer called cumulus oophorus• 2o oocyte is enclosed by ring of granulosa cells called corona

radiata• Between oocyte & radiata is gelatinous layer called zona pellucida

– Which forms barrier to sperm penetration

20-54

Ovulation

• By 10-14 days after menstruation begins, only 1 follicle survives– Others become

atretic follicles (degenerate)

• Surviving graafian follicle forms bulge on surface of ovary– Secretes increasing

levels of E

20-55

Fig 20.29

Ovulation continued

• Graafian follicle releases egg into Fallopian tube at ovulation

20-56

Ovulation continued

• If fertilized, egg completes meiosis II with formation of another polar body

• Egg degenerates in 2 days if not fertilized

20-57

Fig 20.31

Ovulation

• LH causes empty follicle to become corpus luteum (CL)– Which secretes

progesterone (P) & estrogen

– If no fertilization, becomes corpus albicans (non- functional remnant)

20-58

Fig 20.32

Menstrual Cycle

20-59

Menstrual Cycle

• Is ≈ month-long cycle of ovarian activity seen in higher primates

• Characterized by shedding of endometrial lining & bleeding (menstruation)– & sexual receptivity throughout cycle

• Other animals have estrous cycles in which there is no shedding of endometrium & receptivity is limited– In estrous animals that bleed (dogs & cats), it is due to high

estrogen (E) that accompanies receptive period

20-60

Menstrual Cycle continued

• In humans is ≈28 days• Day 1 is taken to be first day of menstruation• Days 1 thru ovulation constitute the follicular

phase• Time from ovulation to menstruation is luteal

phase• Endometrial changes are called: menstrual,

proliferative, & secretory phases

20-61

Follicular Phase• Lasts from day 1 to

about 13• Is dominated by

growth & death of a cohort of 1o follicles into 2o follicles– With 1 survivor

becoming a graafian follicle

• Which will undergo ovulation

– As follicles grow, secrete larger amounts of E

• Reaching peak about day 12 Fig 20.33

20-62

Follicular Phase continued

• Follicular growth & E secretion are dependent on FSH from Ant Pit– FSH & E induce formation

of FSH receptors in granulosa cells

• Causing follicles to become increasingly sensitive to same level of FSH

• At same time, FSH & E recruit LH receptors in graafian follicle

Fig 20.3320-63

Follicular Phase continued

• Rapidly rising E secretion causes:– Hypothalamus to increase

pulses of GnRH – & Ant Pit sensitivity to

GnRH to increase, resulting in greater LH secretion

• A positive feedback between E & Ant Pit secretion develops– Resulting in LH surge that

peaks 16 hrs before ovulation

• & causes ovulation

Fig 20.3320-64

Luteal Phase

• After ovulation, LH causes empty follicle to become the corpus luteum (CL) which secretes E & P

20-65

Fig 20.34

Luteal Phase continued

• P levels rise & peak about a week after ovulation• Development of new follicles & another ovulation are inhibited

by:– High P & E exert strong negative feedback on LH & FSH – Inhibin from CL further suppresses FSH

Fig 20.34

20-66

• With no fertilization, CL regresses – Consequently E & P levels decline

• Causing menstruation & allowing new cycle of follicle development

Luteal Phase continued

20-67

Fig 20.34

Cyclic Changes in the Endometrium• Are driven by cyclic changes in E & P levels • Proliferative phase occurs during follicular phase; E levels

increase– E stimulates growth of endometrial lining

• Spiral arteries develop

– E causes cervical mucus to become thin & watery to allow sperm penetration

20-68

Fig 20.34

• Secretory phase occurs during luteal phase; endometrium becomes ready for implantation– P stimulates development of uterine glands – P & E cause endometrium to become thick, vascular, & spongy– P causes cervical mucus to thicken & become sticky

Cyclic Changes in the Endometrium continued

20-69

Fig 20.34

• Menstrual phase results from drop in P & E following CL degeneration– Low P causes constriction of spiral arteries– Blood flow stops, causing necrosis & sloughing of endometrium

Cyclic Changes in the Endometrium continued

20-70

Fig 20.34