(10) Meiosis Flashcards
(Meiosis)
- Meosis practiced by germ cells - what happens?
- Progeny (gametes) end up with what relative to parent?
- During what division is genetic material reshuffled as DNA is exchanged between the original maternal and paternal chromosomes?
- DNA replicates –> 2 rounds of cell division (meiotic divisions)
- half chromosome complement of parent
- during first division
(Thus the gametes produced are genetically mixed and will combine with other genetically mixed gametes to form the next generation. As such, meiosis is fundamental to the advantage of sexual reproduction)
(Cell Division During Female Meiosis is Unequal)
- The end result of meosis is what?
- Does this happen in both sexes?
- Why is this?
what are smaller remnants called?
- How is this beneficial?
- 4 daughter cells (1N1C) (half chomosomal content of parent)
- only in male (female ends up with one 1N oocyte)
- in females cytokinesis is uneven
polar bodies
(1st polar body = 1N2C)
(2nd polar body = 1N1C)
- preserves oocyte products needed for development of early embryo (microtubules, mitochondria)
(Arrest of Meiosis in Females)
- in males once meiosis commences after puberty it is continuous
- In females, germ cells become oocytes by entering meioisis when?
- In females the first meiotic division continues to what point?
- Reinitiaion of 1st meiosis occurs when?
as a result of what?
limited to what?
why?
- during fetal life
- diplotene stage of prophase (usually at time of birth) (aka arrested in diplotene of the first meiotic prophase)
- after puberty
preovulatory LH surge
limited to Graafian follicles which have survived selection
only in these do granulosa cells have sufficient LH receptors
(Arrest of Meiosis in Females Cont)
- The oocyte completes this meiosis and the first polar body is expelled
- It immediately enters the 2nd meiotic divison - then what happens?
- In most species this second block occurs when?
- When does completion of the second meiotic division occur?
- is again arrested
- metaphase stage of second meiosis (it is this state that oocytes are ovulated in most species)
- when the oocyte is reactivated by sperm
(Because of these two blocks, and especially the first one, meiosis in females can take years.)
(In women for example, all oocytes have entered the first stage of arrest by birth. If we assume puberty doesn’t occur till 12 and menopause is not complete till 50 then meiosis for the oocyte could take anywhere from 12 to 50 years to be completed.)
(Corpus Luteum)
(Formation of the Corpus Luteum)
- The corpus luteum forms from what?
- What is CL formation called?
- When does it begin?
when follicles secrete more what and less what?
- the inital stages of luteinization in all species is due to what?
- remnants of the ovulated follicle
(species with multiple ovulations form multiple corpora lutea)
- luteinization
- following the LH surge but prior to ovulation
less estrogen and more progesterone
- LH surge
(Corpus Luteum)
(Formation of the Corpus Luteum)
- after ovulation the collapsed follicle reorganizes.
- What form the large (>20 um) (granulosa lutein) and small (<20 um) (theca lutein cells) luteal cells
- Which cells are responsible for most of the baseline progesterone secretion?
which cells do respond to LH with increased progesterone secretion?
- granulosa and theca cells
- large luteal cells
small luteal cells
(Corpus Luteum)
(Formation of the Corpus Luteum)
- What is the function of the CL?
- prevents what?
- further ovulations are prevented by what?
- can follicular growth sill occur?
- secretion of progesterone to prepare the uterus of the ensuing pregnancy
- sexual receptivity
- action of progesterone to suppress gonadotropin surge release from HPA axis
- yeah
(Luteotropins)
- The Cl requires hormonal support to function. Hormones that do this are called what?
(There are many luteotropins and they vary in importance between species. They probably do not act in isolation but form a “luteotropic complex”. The hormone that is luteotropic in a species may also change over the estrous cycle or pregnancy.)
- Which is luteotropic in all species examined and is responsible for at least the early changes of luteinization?
- What is an important luteotropin in carnivores, mice, rats and hamsters. It may also be important in other species
3. What is a vital luteotropin in the rabbit, but in some other species may have the opposite action (be luteolytic)?
- What is the luteolytic hormone in all mammals examined?
- luteotropins
- LH (aka universal luteotropin)
- Prolactin (also from anterior pituitary)
- Estradiol
- PGF2α
(other prostaglandins may have luteotropic functions. Prostacyclin (PGI2) in cattle and PGE2 in women are 2 examples.)
(The Lifespan of the Corpus Luteum Determines Cycle Length)
In species with short cycles there has to be a mechanism whereby if the animal is not pregnant she is returned to estrus so another attempt at fertility can be made.
Since the CL inhibits the hypothalamo-pituitary axis by its secretion of progesterone, in order for a new cycle to commence the influence of the CL must be removed.
In these species the natural lifespan of the CL is not so short as the ~2 weeks of activity we see from it. Rather the CL of these species is actively killed off in the absence of pregnancy to allow the next cycle to occur. This process is termed luteolysis.
(General Model of Luteolysis)
- The sustained high levels of progesterone throughout the luteal phase finally cause what?
What does this mean?
- What are the two results of the increase in Estogen/Progesterone?
- What does increased oxytocin release and increase of its receptors cause?
which acts to do what?
- down-regulation of the progesterone receptor in both the hypothalamus and uterine endometrium
progesterone has less influence at tissue level and small amounts of estrogen from developing follicles can have more
- receptors for oxytocin up regulated in uterine endometrium
activity of central oxytocin pulse generator (involving hypothalamus and posterior pituitary) is increased
- PGF2α release
PGF2α acts on the corpus luteum to cause luteolysis
(Counter-Current Exchange (local transfer) vs Systemic Route of PGF2α Action)
- in many species uterine vein and ovarian artery are next to each other
- how does prostaglandin in these get to CL?
- what happens to PGF2a entering systemic cirulation?
- this system found in what?
- in what species is there no local transfer?
- what use a combo of both?
- prostaglandin from uterine endometrium goes from uterine vein into ovarian artery (can directly access CL)
- rapidly and completely (99%) metabolized by lungs - can’t cause luteolyssis - so local transfer is vital
- sheep and guinea pig
- horse and rabbit (therefore less metabolism)
- pigs and cows
(Luteolysis in Ruminants)
- In ruminants the above system (including counter-current) applies.
Have an additional soure of oxytocin… what is it?
- In these species release of oxytocin from the pituitary is backed up by release from the ovary, setting up a positive feedback loop where release of PGF2α from the uterine endometrium causes oxytocin release from the CL, causes more uterine PGF2α release, and so on.
- the CL itself
(Primates are Different)
- in primates, unlike all other species examined… what is different?
- What is the theory?
- no role for uterine endometrium in prostaglandin production that causes luteolysis
- central oxytocin release acts on luteal oxytocin receptors which gives local formation of PGF2α (within the CL).
A positive feedback loop is then established within the ovary between oxytocin and prostaglandin,
that results in luteolysis
(figure out what to do about pages 99-101)
(Pseudomenstruation)
- Blood coming from the vagina is a natural part of the estrous cycle in some species. But it is not menses as we know it in women (and old-world primates) with destruction and sloughing of the endometrium at the end of the luteal phase.
(Bitch)
- Vaginal bleeing is a normal part of what in the bitch?
- High estrogen levels in proestrus cause rapid development of the uterine endometrium –> blood cells are lost into the uterine lumen at this time by what?
- how is this blood discharged from vagina?
- proestrus (Generally the discharge clears to a straw color as they enter estrus, but this is far from a hard and fast rule, with some continuing to bleed.)
- diapedesis and following rupture of small capillaries
- this blood can seep through the relaxing cervix –> discharged from vagina
