lec 12- Early development in C elegans Flashcards
what is a C elegans?
A nematode worm that, in the wild, lives in the soil
why is the C elegans an ideal model system?
-has transparent embryos which makes easy to study
-small number of cells
-suitable for genetic analysis
are C elegans hermaphrodite?
yes, they make sperm and oocytes (eggs) and are capable of self fertilization
how many hours does it take for hatching after fertilization for a C elegans ?
15 hours, very fast
what are the two cells that form from the first cleavage called?
large anterior cell called AB and small posterior cell called P1
what do the AB and P1 cells then divide into?
AB divides into dorsal ABp and ABa and P1 divides into posterior P2 and EMS
when does gastrulation occur in C elegans?
in the 28 cell stage
descendants of the EMS cell (Ea and Ep) move inside the embryo during gastrulation, what will these cells produce?
the gut
is the C elegans cleavage pattern for its cells invariant and known for every cell?
yes
what do the P1 cells give rise to?
germ line cells (reproductive cells)
what is apoptosis?
cell death during development
what sets up the anterior posterior axis in C elegans?
-sperm entry when fertilization occurs
what segregates to act as a sign for the AP axis in the zygotes?
P-granules
what do P-granules contain?
Maternal mRNA and proteins required for development of germ cells
are P-granules determinants?
no, but they are markers for it
after the segregation of P-granules, what occurs?
-the first cell division is then positioned to the AP axis
-two cells with different cytoplasmic components are produced
do P-granules remain in P-cell daughters after each cell division?
yes, they eventually end up in the P4 cells that give rise to the germline
what are P granules determinants of?
the development of the germ cells
how does sperm entry set up the AP axis?
-sperm enters the egg, contractions in the acting network around the outer edge of the zygote are triggered
-this causes cytoplasm on the outer edge to flow towards the anterior end of the zygote and the interior cytoplasm towards posterior
-the contraction causes P granules to become localized to the posterior end
-also causes PAR proteins to become asymmetrically localized
-PAR protein asymmetrical localization is needed to ensure proper segregation of developmental determinants that are needed to specify cell fates
what are the 6 steps that lead to cleavage starting from sperm entry?
- sperm enters, contradictions in actin network
- cortical cytoplasm flows away from the sperm pronucleus and central cytoplasm towards it
- pronuclear migration begins
- the pronuclei meet and fuse
- the mitotic spindle is aligned with the distribution of PAR proteins
- cleavage occurs
what does PIE-1 do?
it determines cell fates in the germ cells lineage, it segregates with P-granules into the cells that give rise to the germ cells lineage
what do PAR genes do?
encode proteins that are critical for positioning the asymmetric cell division and proper partitioning of cytoplasmic determinants
what happens to zygotes that lack PAR genes?
cleave symmetrically and produce similar daughter cells that can both contain P granules
what are PAR genes?
-maternal effect genes
-the mRNA and proteins are produced in the mothers genome
what is autonomous cell specification?
developmental process that occurs without any extracellular signals
what is regulative cell specification?
external signals that direct development
what happens when the first two blastomeres are cultured in isolation?
-the AB cells make only some cells that it would normally develop into, meaning conditional specification
-the P1 cells form all the structures that it would normally develop into, meaning autonomous specification
how does formation of the DV axis occur?
-cell rotates after first cleavage
-DV axis is determined after second cleavage
-the two AB cells lay on the dorsal side and EMS becomes located on the ventral side, ABp contacts the P2 cell
how does formation of DV axis occur when AB is mechanically rotated in the opposite direction?
-EMS is located on the original dorsal side and ABa now contacts P2 instead of ABp
-ABp develops into anterior cell instead of posterior one and ABa develops into the posterior cell
-the EMS cell still gives rise to ventral structures
-the worm that develops is normal, but it develops upside down in the egg case
this indicates that ABa and ABp are not determined
how are ABa and ABp specified?
-involves the Notch-Delta Pathway
-APX-1 (Delta) in P2 cells activates GLP-1 receptors (Notch) present in the ABa and ABp cells
explain the Notch Delta pathway:
-in Notch Delta pathway, both the receptor and the ligand are membrane-bound, meaning cell contact is required
-activation of Notch receptor results in cleavage of its cytoplasmic domain, it is now able to enter the nucleus where it activates gene expression
how is EMS specified?
-P2 cells produce Wnt signals (Wnt = MOM-2)
-EMS cell has frizzled receptor (MOM5 = frizzled receptor)
-only one side of the EMS cell receives MOM-2 signal from P2, this causes asymmetrically localized cytoplasmic factors in EMS
-when EMS divides, the E and MS cells each receive different cytoplasmic components that cause adoption of different fates
