Meiosis

Question 1

what is meiosis

Answer 1

• single round of DNA replication

- 2 consecutive rounds of cell division (end up with 1/2 chromosome complement of parent)

Question 2

meiosis result in females

Answer 2

each original 2N oogonia entering meiosis gives rise to only one 1N oocyte

Question 3

meiosis result in males

Answer 3

2 meiotic divisions –> 4 daughter cells each being 1N 1C (half chromosomal content of the parent)

Question 4

why does meiosis in females only give rise to 1 oocyte

Answer 4

cytokinesis is uneven –> nearly all cytoplasm goes to one of the cells at each division –> smaller remnant cells are polar bodies –> done to preserve oocyte products needed for early embryo development

Question 5

arrest of meiosis in females

Answer 5

• first meiotic division continues up to diplotene stage of prophase
• arrested there until birth (arrested in diplotene of first meiotic prophase)

Question 6

female reinitiation of first meiosis

Answer 6

• occurs after puberty

- result of a preovulatory LH surge

Question 7

action of LH on reinitiation of first female meiosis

Answer 7

-limited to large graafian folicles because only in these do the granulosa cells have sufficient LH receptors

Question 8

what happens when oocyte completes this meiosis

Answer 8

• first polar body is expelled

- enters 2nd meiotic division and is again arrested

Question 9

when does the second female meiosis block occur

Answer 9

at the metaphase stage of the second meiosis –> state that oocytes are ovulated in most species

Question 10

when does completion of second female meiotic division occur

Answer 10

when the oocyte is reactivated by a penetrating sperm at the time of fertilization

Question 11

what is the process of CL formation called

Answer 11

luteinization

Question 12

when does lutenization begin

Answer 12

following LH surge but prior to ovulation, when follicles begin to secrete less estrogen and more progesterone

Question 13

what are the initial stages of lutenization in all species due to

Answer 13

LH surge

Question 14

what happens to follicles following oculation

Answer 14

• collapsed follicle reorganizes

- granulosa cells and theca cells respectively form large and small luteal cells

Question 15

what do large and small luteal cells do with LH

Answer 15

• large: responsible for most of the baseline progesterone secretion but do not respond to LH by increasing progesterone secretion
• small: respond to LH with increased progesterone secretion

Question 16

functions of CL

Answer 16

• secretion of progesterone to prepare uterus for ensuing pregnancy
• prevents sexual receptivity
• suppresses gonadotropin release

Question 17

LH in luteotropic complex

Answer 17

• luteotropic in all species
• responsible for early stages of lutenization
• “universal luteotropin”

Question 18

prolactin in luteotropic complex

Answer 18

• from anterior pituitary

- luteotropin in carnivores, mice, rats, hamsters (many species)

Question 19

estradiol in luteotropic complex

Answer 19

vital luteotropin in rabbit, may be luteolytic in other species

Question 20

PGF2a in luteotropic complex

Answer 20

• luteolytic hormone in all mammals

- other prostaglandins may have luteotropic functions (PGI2, PGE2)

Question 21

what must happen for a new cycle to commence in species with short cycles

Answer 21

influence of the CL must be removed –> CL is actively killed off (luteolysis)

Question 22

what do sustained high levels of progesterone throughout luteal phase cause

Answer 22

down-regulation of the progesterone receptor in both the hypothalamus and uterine endometrium –> progesterone has less influence at the tissue level, small amounts of estrogen from developing follicles have more influence

Question 23

results of the effective increase in the radio of estrogen/progesterone

Answer 23

• receptors for ocytocin are up regulated in the uterine endometrium
• activity of the central oxytocin pulse generator is increased

Question 24

what does increased oxytocin release and the increase in its receptors on the uterine endometrium cause

Answer 24

PGF2a release –> acts on CL to cause luteolysis

Question 25

uterine counter-current exchange

Answer 25

• prostaglandin from uterine endometrium passes up from uterine vein
• portion of it transferreda cross vessel walls into ovarian artery
• directly accesses CL (doesn’t go through systemic circulation)

Question 26

what happens to PGF2a entering systemic circulation in species with counter-current exchange

Answer 26

• rapidly and almost completely metabolized on passage through lungs
• unable to cause luteolysis
• local transfer via ovarian artery is vital for luteloysis

Question 27

what animals have and don’t have uterine counter-current exchange

Answer 27

• have: sheep, guinea pig

- don’t have: horses, rabbits

Question 28

what happens in species without uterine counter-current exchange

Answer 28

• PGF2a transferred via systemic circulation

- much less active metabolism in the lungs makes it possible

Question 29

cows and pigs with uterine counter-current exchange

Answer 29

combination of local transfer mechanism and systemic circulation route

Question 30

luteolysis in ruminants

Answer 30

they have counter-current exchange and the CL as a source of oxytocin –> oxytocin release from pituitary backed up by release from ovary –> positive feedback loop (PGF2a from uterus causes oxytocin release from CL causes more PGF2a)

Question 31

Primates and luteolysis

Answer 31

• no role for uterine endometrium in prostaglandin production that causes luteolysis
• central oxytocin release acts on luteal oxytocin receptors –> local formation of PGF2a in CL
• positive feedback loop in ovary between oxytocin and prostaglandin –> luteolysis

Question 32

Pseudomenstruation in the bitch

Answer 32

• vaginal bleeding normal
• high estrogen levels cause rapid development of uterine endometrium
• blood cells lost from rupture of small capillaries

Question 33

Pseudomenstruation in the cow

Answer 33

• bleeding in metestrus

- source is caruncular capillaries that hemorrhage following rapid decline in estrogen post ovulation

Question 34

Menstruation in bats, elephant shrew

Answer 34

• real menstruation in both

- bat: associated with a decline in luteal function –> sloughing of endometrium