Egg Activation and Beginning of Cleavage

Question 1

what is constant between species that triggers the slow reaction (and resumes cell cycle)?

Answer 1

wave of elevated Ca2+ crosses egg originating from point of sperm entry

note in mammals: Ca2+ levels oscillate for hours

Question 2

how is Ca2+ elevation levels related to the slow block? how are increases in Ca2+ elicited?

Answer 2

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

Question 3

are pH changes important for egg activation? in which species, if yes?

describe how pH changes affect egg activation.

Answer 3

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

Question 4

what are the three major models of the initiation of egg activation events?

Answer 4

1) receptor model - activation w/ cell-surface G-protein
2) fusion model - fusion of gametes causes initiation
3) hybrid model - fusion + signaling events responsible

Question 5

describe the receptor model of egg activation in detail

noted species it may occur in?

Answer 5

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.

Question 6

describe fusion model in detail

evidence? species it may occur in?

Answer 6

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.

Question 7

describe the “hybrid” model in detail

Answer 7

fusion occurs and signaling after thru a receptor (after fusion) - may be a theoretical possibility in some species

Question 8

what is the nitric oxide (NO) model? what does it entail w/ respect to NO release (when it happens, what it results in?)

Answer 8

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.

Question 9

was NO release detected in mammalian fertilization?

Answer 9

no

Question 10

what are new studies saying about the NO model, with respect to increases, capacitation?

Answer 10

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).

Question 11

what role does Ca2+ play in releasing the egg from meiotic block?

Answer 11

activates Ca2+-depdendent enzymes: calmodulin dependent kinase II (inactivates cdk2 and thus MPF) and calpain II (degrades c-MOS); meiosis can then be completed

Question 12

how does a diploid nucleus form in sea urchins?

Answer 12

occurs via syngamy - pronuclear fusion occurs (facilitated by motor proteins on MTs)

Question 13

how does a diploid nucleus form in mammals?

Answer 13

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

Question 14

what four major things does cleavage accomplish?

Answer 14

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

Question 15

why is increasing nucleocytoplasmic ratio important?

Answer 15

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.

Question 16

how are cleavage divisions different from later cell divisions?

Answer 16

1) blastomeres do not grow between cleavages
2) divisions occur at a rapid pace (albeit slower for mammals than for other species)

Question 17

why do blastomeric cleavages occur so fast in non-mammals?

Answer 17

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

Question 18

describe the four major patterns of yolk distribution + example animals in which they occur

Answer 18

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)

Question 19

describe the cleavage types of each of the four types of yolk distributions

Answer 19

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

Question 20

what are the two types of holoblastic cleavage?

Answer 20

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

Question 21

in which species does radial cleavage occur? describe the first 3 cleavages

Answer 21

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)

Question 22

describe the fourth cleavage in radial cleavage

Answer 22

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.

Question 23

describe the fifth cleavage in radial cleavage

Answer 23

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

Question 24

describet the sixth cleavage in radial cleavage

Answer 24

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