Download Cleavage and cytoplasmic determinants

Cleavage and cytoplasmic determinants
Today’s questions:
I. What patterns occur during embryonic cleavage? II What runs cleavage? II.
What runs cleavage?
III. What are cytoplasmic determinants, and how do they q
g
affect the consequences of cleavage?
fertilized egg 8 cell (day 3) cell adhesion 32‐cell
32
cell (morula) blastocyst
(morula) blastocyst (day 5) hatching embryo
(day 5) hatching embryo
Cleavage in humans: These were taken at the same magnification. What can you conclude about cell size? g
y
Nemerteans, urchins, ascidians
Annelids (far left), Annelids
(far left)
cnidarians, ctenophores Scaphopods, bivalves, p p ,
,
insects (far right)
Wh t d
What do you notice about cleavage patterns in different lineages? ti
b t l
tt
i diff
t li
?
II. What runs cleavage?
Experiments with ‐amanitin
Experiments
with ‐amanitin (Amanita = =
“death cap” and “destroying Angel” mushrooms): inhibits RNA polymerase In frogs: cleavage occurs normally if embryos are treated with ‐amanitin.
embryos are treated with 
amanitin.
What can you conclude from this observation? b
?
III. What are cytoplasmic
determinants and how do they
determinants, and how do they affect the consequences of cleavage?
Stylea partita has partita has ~transparent
transparent embryos embryos
… can follow fate of cells, based on pg
pigmented areas in egg.
gg
Observational data: From “fate mapping”:
What can you conclude from these data?
Experimental data: If you displace the yellow pigment experimentally, so it ends up in different blastomeres, they p
y,
p
,
y
develop into muscle.
p
Are the experimental data consistent with the observational data, or do they conflict with it? A key technique in studying cytoplasmic
determinants (and other events in (
development): With in situ hybridization, you can With
in situ hybridization you can
visualize where specific mRNAs are located.
The red molecule: The black *:
Fly egg + red* molecule:
Fly egg + red* molecule: Red +yellow: Red*+yellow:
Localizing Vg1 mRNA in frog eggs
Early in egg development = Vg1
Early
in egg development = Vg1 mRNA distribution is mRNA distribution is
uniform But, in a mature egg (+ after ( f
fertilization) What do these observations suggest? Note: Vg1 knock‐out mutants lack endoderm and dorsal mesoderm.
Hypothesis: Vg1 mRNAs are transported along microtubule tracks, then become physically attached to microfilaments just inside the plasma membrane. Experiment: Do these data support the hypothesis, or conflict with it? Is bicoid a cytoplasmic determinant?
Anterior
Posterior
How can there be information in a gradient?