Reproductive Physiology 1

Question 1

What is oogenesis?

Answer 1

• process of oogenesis begins in the fetal ovaries where stem cells (oogonia) produce primary oocytes
• at birth there are at least 2 million primary oocytes in the ovary
• the primary oocytes are diploid (full set of 46 chromosomes)
• at the onset of puberty (>11 years) there are approx 300 000 primary oocytes and they slowly decrease over the reproductive life
• menopause starts when there are no more functional primary oocytes
• a max of about 500 eggs that could ever be released if you never became pregnant and always ovulated with every menstrual cycle for your entire reproductive life

Question 2

Describe the process of oogenesis

Answer 2

• primary oocytes are held in prophase of meiosis I unchanged from the way they were produced in the fetus
• a number of primary oocytes restart meiosis I early in the menstrual cycle and a few become secondary oocytes
• this cohort of eggs is produced at the same time but only one of the secondary oocytes will be released from the ovary at ovulation
• completion of meiosis II begins exactly at the time when the sperm contacts the egg (fertilization)
• development of eggs in every cycle is called ovarian cycle

Question 3

Describe the follicular phase of the ovarian cycle

Answer 3

-egg and granulosa cells develop up to the point of ovulation

Question 4

Describe the luteal phase of the ovarian cycle

Answer 4

-where the leftover bits of the granulosa hang around and secrete hormones (particularly progesterone) to prepare the lining of the uterus (endometrium) for a fertilized egg (zygote)

Question 5

Describe the process of the follicular and luteal phases

Answer 5

• granulosa cells develop around the follicle
• secondary follicle start to make antrum with a primary oocyte inside
• mature follicle with secondary oocyte and full antrum
• granulosa cells become the corpus luteum
• corpus luteum lasts about 14 days

Question 6

Answer 6

Question 7

Answer 7

• antrum is not large enough in this secondary follicle
• will get much larger

Question 8

Describe the change from a primordial to primary follicle

Answer 8

• primordial has thin layer of flat follicular cells around primary oocytes
• cells become cuboidal granulosa cells in primary follicle
• zona pellucida begins to form
• primary follicle continues to develop with more granulosa on the outside
• zona pellucida stays around the primary oocyte

Question 9

Describe the change from a primary to secondary follicle

Answer 9

• begin to make antrum
• cells continue to proliferate but gaps begin to occur between granulosa cells
• secrete follicular fluid which forms intracellular spaces to make it a secondary follicle

Question 10

Describe the change from a secondary to mature follicle

Answer 10

• antrum areas will begin to come together to form a large antrum filled with fluid
• primary oocyte has completed meiosis I to become a secondary oocyte
• granulosa cells around the oocyte are referred to as the corona radiata

Question 11

What stage of follicle is this?

Answer 11

• ovulated secondary oocyte
• end up with a polar body in meiosis with oocytes
• all of the nuclear material ends up split between the polar body and the oocyte (half and half)

Question 12

Describe the homeostatic process leading to ovulation

Answer 12

1. Stimulus: low estrogen levels in blood
2. Hypothalamus (detector and control centre) has receptors that detect low estrogen
3. Hypothalamus releases GnRH to the anterior pituitary
4. Anterior pituitary releases LH and FSH which leads to follicle growth
5. LH and FSH cause egg to develop into primary follicle, secondary, then mature follicle
6. LH stimulates estrogen production from eggs which inhibit the GnRH signal then LH and FSH release
7. Developing follicle continues to produce more and more estrogen which when estrogen levels get high enough, stimulates GnRH, LH, and FSH
8. LH surge causes follicle to rupture and egg to be released

Question 14

Describe the levels of FSH, LH, estrogen, and progesterone across a 28 day cycle

Answer 14

• estrogen increases causing LH and FSH surge
• drops a bit because corpus luteum takes a bit of time to start making estrogen
• if you don’t get pregnant, estrogen levels drop off and start the next cycle
• progesterone produced mainly by corpus luteum which doesn’t start producing hormones until after ovulation

Question 15

What part of the endometrium grows during the menstrual cycle?

Answer 15

• stratum functionalis
• grows to approx 10mm
• menstruation is the loss of the stratum functionalis
• stratum basalis is always there

Question 16

Answer 16

Question 17

Describe the purpose/length of menstrual cycle

Answer 17

• to prepare the lining of the uterus for the fertilized egg to implant
• it takes from about 21 to 40 days to complete the endometrium preparation
• development of endometrium is highly variable in length but it all begins with the growth of the stratum functionalis which is lost in the previous menstrual cycle above the stratum basalis which remains
• development of the functionalis is chiefly due to the proliferation of the endometrial stroma and the elongation and growth of endometrial glands
• the glands will produce material to provide nutrition for the zygote

Question 18

Describe the stages of the menstrual cycle

Answer 18

• begin counting at the first day of bleeding (sluffing of the endometrial layer)
• menstrual phase lasts approx 4 days (variable)
• proliferative phase from day 4 to 14 where the endometrium proliferates, more glands developing, more veins/arteries invading the area
• secretory phase: glands begin to secrete nutrition for zygote and try to make an area where the fertilized egg can implant
• if no egg implants, the endometrium will begin to sluff off which is the beginning of the menstrual phase

Question 19

Describe how the ovarian cycle and menstrual cycle are connected

Answer 19

• menstrual phase is due to lack of estrogen and progesterone which means there are only primary follicles around (estrogen and progesterone are made by developing follicles)
• secondary and mature follicles will produce estrogen
• with more estrogen, the functionalis layer is stimulated and it begins proliferating
• more estrogen triggers LH and FSH surge and ovulation
• corpus hemorrhagicum (good blood supply to follicle so it bleeds a bit)
• granulosa cells produce corpus luteum which produces estrogen and progesterone
• progesterone causes glands to secrete and estrogen supports growth of the endometrium
• if there is no fertilized egg, corpus luteum becomes corpus albicans which is not functional and so estrogen and progesterone drop
• menstrual phase begins again

Question 20

How does pregnancy stop the endometrium from sluffing?

Answer 20

• pregnancy stops the degeneration of the corpus luteum because the implanting egg and then the placenta release human chorionic gonadotropin (hCG)
• hCG acts like LH and stimulates corpus luteum to produce lots of progesterone and a little estrogen
• the progesterone inhibits the uterine contractions and disintegration of the stratum functionalis that precedes the menstrual flow

Question 21

What is the optimal time for fertilization?

Answer 21

• the day before to two days after ovulation are about the only times that sexual intercourse will result in fertilization of an egg that can go on to implant in the endometrium
• if the embryo were to implant a couple days after ovulation, it is too late because by the time the embryo arrives at the endometrium and causes hCG secretions the endometrium will already be entering the menstrual phase

Question 22

Describe the parts of the sperm

Answer 22

Head:

-acrosome: important for going to digesting the zona pellucida, contains vesicles of digestive enzymes

Nucleus: contains haploid number of chromosomes to put into the egg

Midpiece: has mitochondria to make energy for the flagellum so sperm can move

Question 23

Describe spermatogenesis

Answer 23

• process of making spermatozoa
• stem cells (spermatagonia) constantly divide and then undergo meiosis to give mature sperm
• the whole process takes 64-72 days
• process begins at about age 14 when there are large amounts of testosterone circulating and continues to happen throughout life
• about 400 million sperm are made per day
• each mL of semen has about 50-130 million sperm
• if counts fall below 20 million sperm/mL then infertility becomes a concern

Question 24

Where is semen produced?

Answer 24

• seminal vesicles produce majority of semen and secrete in through the ejaculatory duct into the urethra to meet the prostatic secretions
• vas deferens carry sperm themselves and meets the vesicular secretions in the ejaculatory duct

Question 25

What is semen composed of?

Answer 25

• semen volume is about 2.5-5mL but only a small fraction is sperm
• the majority of the volume is made up of other fluids secreted by the prostate, seminal vesicles, and bulbourethral glands
• the seminal vesicle secretions contain fructose (can be used for energy) and clotting proteins (ejaculate has to clot inside of the vagina or it would fall out), and are somewhat alkaline
• the alkaline fluid neutralizes the vaginal pH to some degree
• prostaglandins are also found in the seminal vesicular fluid which stimulate the sperm to swim and the uterus to contract
• seminal secretions are about 60% of the total volume of semen
• the prostate secretes a thin acidic or neutral fluid which provides a medium for sperm to swim, citric acid (a nutrient for sperm), and protein digesting enzymes (25-30% of semen volume)
• during arousal, the bulbourethral glands release alkaline fluid into the urethra to neutralize acidic urine and some mucous to decrease sperm damage later during ejaculation (does not make up much of semen volume)

Question 26

How are the processes of gamete production similar?

Answer 26

• both rely of pituitary gonadotropins (FSH, LH)
• FSH stimulates the growth of gametes (oocytes and sperm)
• LH stimulates the production of hormones from the gonads (LH stimulates ovarian follicle to make estrogen and progesterone and the (interstitial cells of) Leydig cells in the testes to make testosterone
• hormones produced by the ovary or testes feedback on the pituitary to decrease the production of the gonadotropins
• once sex steroids get low enough this feeds back to hypothalamus to start again

Question 27

What is needed for fertilization?

Answer 27

• on the male side, >20 million well formed sperm per mL of semen are required for reliable fertilization
• sperm must make it past the cervix, up the uterus and into the uterine (fallopian) tube
• meets the egg in fallopian tube
• on the female side, egg has to be in uterine tube, cervical mucus has to be thin, endometrium has to be almost ready (ie. secretory phase) for a blastocyst