Lecture 4 - Oogenesis, beginning of fertilization Flashcards
how does oocyte production differ from sperm production (in terms of final product)?
end result of oogenesis - 1 large gamete (egg) w/ formation of polar bodies - meant to get rid of extra DNA
end result of spermatogenesis (per spermatocyte) - 4 haploid sperm gametes
what marks the site of the animal pole in eggs? where is it in relation to vegetal pole (and what is near each)?
polar bodies form at site of animal pole (end nearest nucleus)
vegetal pole is 180 degrees from animal pole, end w/ most yolk. these landmarks are important for cleavage.
what do eggs become endowed with during arrest, and how (general processes)
eggs become endowed w/ abundants RNAs - contain processes for making their own + acquiring from neighbor cells (nurse cells)
1) nucleus becomes very enlarged (called germinal vesicle in some species)
2) chromosomes (actively transcribed) assume “lampbrush” conformation w/ many loops to allow machinery to transcribe DNA. eggs develop numerous nucleoli to help w/ rRNA synthesis for increased trnsl.
or
1) in many insects/invertebrates, eggs pumped w/ RNA by nurse cells (which are essentially sister cells to oocyte, stay connected via cytoplasmic bridges in nurse-oocyte complex). nurse cells present next to adjacent future anterior pole.
drosophila ovary - general contents, describe nurse cell and oocyte conformations
contain 16 ovarioles - strings of egg chambers in developmental series. most developed egg chambers found at base of ovariole.
follice cells (somatic) surround nurse cell-oocyte complex and mediate uptake of yolk proteins, synthesize eggshell proteins. also play important roles in establishing polarity.
nurse cells - general properties, how do they help w/ egg-pumping during oogenesis?
nurse cells do not undergo meiosis; they are polyploid. they transport RNA/protein into oocyte by MTs + cytoplasmic bridges during early/mid-oogenesis.
can make large quantities of genetic products due to polyploidy.
what happens with nurse cells during mid-late oogenesis?
nurse cells contract (microfilament-dependent mechanism) - pump cytoplasm into oocyte thru pores called ring canals. MT-depending mixing (or “churning”) of cytoplasm occurs (homogenizes contents).
BUT
certain mRNAs have high affinity for cytoskeleton, will be asym. dist.’d
give an example of asymmetrical dist. after nurse cell cytoplasmic mixing w/ oocyte.
bicoid mRNA - transported by MTs out of nurse cells, remains anchored at anterior end of egg prior to fertilization; necessary for head dev.
some bcd mRNA transported further down MTs to give rise to RNA (therefore protein) gradient after fertilization
what does the drosophila nurse cell/oocyte complex represent? how does it relate with mammals?
maternal RNA “donation” system (e.g. maternal effect genes in relation to development).
mammalian maternal effect gene exps are still being developed, but few have been found so far; indicates possible fundamental difference in dev. strategies.
what are some possible hypotheses w/ maternal effect gene communication in fertilization w/ mammals?
granulosa cells connected to egg by gap junctions could provide route for communication, BUT
only small molecules (e.g. ions) can fit (not proteins/macromolecules).
a few maternal effect genes have been identified buy its not sure how they work.
evidence for maternal miRNAs + proteins regulate gene expression, assist w/ spindle assembly, etc. exists
what is yolk? when does it accumulate, precursors?
mixture of proteins, lipids, glycogen used to nourish developing embryo. minimal-to-nonexistent in mammals, abundant in birds, reptiles, sharks.
yolk components accumulate during meiotic arrest.
precursors of yolk proteins (vitellins) are vitellogenins - made in liver in vert., or in fat body + follicle cells in insect.
more characteristics of yolk proteins - where are released/taken up, modification
1) released into blood (vert.) or hemolymph (invert.)
2) taken up by oocyte by receptor-mediated endocytosis, modified into vitellins
3) microvilli and follice cell-created passage ways (temp. form channels between themselves) allow uptake
4) once inside egg, vitellogenins modified into less soluble form (phosvitin + lipovitin remember 2 vitellins formed)
5) vitellins sequestered into crystalline-like arrays in membrane-bound yolk bodies
what is oocyte maturation?
adjustment of membrane permeability, receptivity, chromatin condensation, etc.
describe germinal vesicle breakdown (+ when does it occur)?
occurs early on (prior to fert.)
is controlled hormonally (progesterone)
1) prog. binds to surface receptor, leads to decrease in cAMP levels
2) leads to trsnl. of certain maternal mRNAs including those that encode
3) c-mos protein, which initiates
4) MAPK cascade which results in
5) promotion of MPF activity (maturation promoting factor, consists of cyclin + a CDK)
6) leads to phosphorylation of histones + nuclear lamins (germinal vesicle breakdown)
when does oocyte get arrested? what is it caused by?
caused by cytostatic factor (MOS + multiple kinases and Emi2), inhibits anaphase promoting complex (so chromosomes stay lined up on metaphase plate)
how is meiotic arrest broken?
fertilization:
1) leads to rapid increase in intracellular Ca2+
2) Ca2+ increase leads to destruction of MPF + c-mos (Ca2+ dependent enzymes - calmodulin dependent kinase II, inactivates cdk2, and calpain II, degrades c-MOS).
3) egg is released from second arrest, “free” to begin cleavage
