Egg Activation and Beginning of Cleavage Flashcards
what is constant between species that triggers the slow reaction (and resumes cell cycle)?
wave of elevated Ca2+ crosses egg originating from point of sperm entry
note in mammals: Ca2+ levels oscillate for hours
how is Ca2+ elevation levels related to the slow block? how are increases in Ca2+ elicited?
cortical granule exocytosis is caused by increase in Ca2+ concentration
increased levels of Ca2+ may come from intracellular stores (e.g. ER)
OR
may be a result of the IP3 pathway in some species
are pH changes important for egg activation? in which species, if yes?
describe how pH changes affect egg activation.
yes, e.g. sea urchins but NOT mammals pH changes are important
DAG (diacylglycerol) activates PKC (protein kinase C) which phosphorylates Na+/H+ antiporter (Na+ pumped in, H+ pumped out); increases cytostolic pH from 6.8 -> 7.2.
leads to increased DNA + protein synthesis
what are the three major models of the initiation of egg activation events?
1) receptor model - activation w/ cell-surface G-protein
2) fusion model - fusion of gametes causes initiation
3) hybrid model - fusion + signaling events responsible
describe the receptor model of egg activation in detail
noted species it may occur in?
activation initiated at cell surface (binding of sperm to surface receptor, perhaps G-protein-associated receptor, receptor tyrosine kinase and/or integrins?) prior to sperm-egg fusion
heterotrimeric G-protein involvement in activation observed in some species. purified putative surface ligands from sperm cannot bring about complete activation pathway in some species; likely in sea urchin tho.
describe fusion model in detail
evidence? species it may occur in?
fusion of gametes leads to egg activation - perhaps thru release of small tyrosine kinase or PLC from sperm cytosol (or upstream molecules that activate these 2). may occur in mammals
evidence - Ca2+ oscillations induced by microinjection of sol. sperm proteins.
describe the “hybrid” model in detail
fusion occurs and signaling after thru a receptor (after fusion) - may be a theoretical possibility in some species
what is the nitric oxide (NO) model? what does it entail w/ respect to NO release (when it happens, what it results in?)
was found that in sea urchins, NO released by sperm induces Ca2+ release and NOS (nitric oxide synthase) activity in egg, leads to MORE Ca2+ release.
initial NO release from sperm appears to arise when NOS is activated in sperm after contact w/ vitelline env.
was NO release detected in mammalian fertilization?
no
what are new studies saying about the NO model, with respect to increases, capacitation?
some say that NO increases (when present) are slow to occur, Ca2+ actually does come first.
recent study in boars showed it could help w/ capacitation - if NOS is inhibited, some protein kinase substrates had lower levels of phos. + fewer acrosome-reacted sperm (thus fewer successful sperm).
what role does Ca2+ play in releasing the egg from meiotic block?
activates Ca2+-depdendent enzymes: calmodulin dependent kinase II (inactivates cdk2 and thus MPF) and calpain II (degrades c-MOS); meiosis can then be completed
how does a diploid nucleus form in sea urchins?
occurs via syngamy - pronuclear fusion occurs (facilitated by motor proteins on MTs)
how does a diploid nucleus form in mammals?
membranes of pronuclei do not fuse - break down before first mitosis, chromosomes condense + become oriented on new mitotic spindle.
first 2n nucleus is not formed until after 1st mitosis
what four major things does cleavage accomplish?
1) generation of a large number of cells
2) generation of many copies of the genome (many nuclei)
3) segregation of cytoplasmic components into diff. blastomeres
4) increasing nucleocytoplasmic ratio (nuc. vol./cyto. vol. increases over that which existed in egg)
2 + 3 important for cellular gene expression
why is increasing nucleocytoplasmic ratio important?
with a large amount of cytoplasm, you must have a great deal of molecules like cytoskeleton, RNAs, proteins, etc. to allow the cell to function - with a small amount of cytoplasm, there’s a more minimal amount of molecules required to properly upkeep the cell.
how are cleavage divisions different from later cell divisions?
1) blastomeres do not grow between cleavages
2) divisions occur at a rapid pace (albeit slower for mammals than for other species)
why do blastomeric cleavages occur so fast in non-mammals?
predation - non-mammal development occurs outside the mother and thus developing embryos are wholly susceptible to predation. thus, they must develop a body very fast to survive
predation is not a factor in almost all mammalian embryonic dev - embryo develops inside mother and gets nurtured in uterus
describe the four major patterns of yolk distribution + example animals in which they occur
1) isolecithal - small-to-moderate amount of yolk evenly dist. (e.g. sea urchins, humans, other mammals)
2) mesolecithal - mod. amount of yolk, mostly in vegetal hemisphere (e.g. amphibians)
3) telolecithal - large amount of dense yolk filling entire egg except for small area near amal pole (fish, birds, reptiles)
4) centrolecithal - yolk concentrated in center of egg, small bound of cytoplasm around edge (insects)
describe the cleavage types of each of the four types of yolk distributions
1) isolecithal - holoblastic cleavage (complete and even) - cleavage goes fully through each blastomere, equal in shape
2) mesolecithal - holoblastic - cleavage is still complete, but blastomeres are larger near vegetal pole due to yolk (harder for actin-myosin complex to cleave thru yolk)
3) telolecithal - discoidal cleavage - incomplete, occurs at very top of egg near animal pole due to dense yolk
4) centrolecithal - superficial cleavage - occurs around outside of yolk, strange furrowing mechanisms
what are the two types of holoblastic cleavage?
radial - blastomeres arranged w/ radial symmetry around A-V axis
spiral - first two cleavages run parallel to A-V axis, first 4 blastomeres form tetrahedron. pattern follows
in which species does radial cleavage occur? describe the first 3 cleavages
occurs in sea urchins
first two cleavages are meridional (furrows run thru animal/vegetal poles like a meridian on globe)
3rd cleavage is equatorial (think of equator on the globe)
describe the fourth cleavage in radial cleavage
in animal hemisphere, occurs w/ slightly oblique orientation - results in 2 tiers of blastomeres that are slightly offset such that spindles of V blast. are tilted to V pole.
blastomeres in V hem. div unequally to give rise to micromeres (small cells) at vegetal-most pole + large blastomeres (macromeres)
medium sized cells in animal hemisphere = mesomeres.
describe the fifth cleavage in radial cleavage
gives rise to 8 blastomeres in each of two layers (an1, an2) in animal hem due to meridional cleavage. meridional cleavage in V hem. also leads to 8 macromeres. micromeres divide unequally to give rise to 4 large + 4 small micromeres
describet the sixth cleavage in radial cleavage
macromeres, mesomeres, large micromeres divide equatorially. result is a top tier (an1-derived) + 2nd-tier (an2-derived), 2 layers of macromeres (veg1 and veg2), and set of large + small micromeres. 60 cells total, and now exists a blastocoel
