Cleavage: cutting up the embryo
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Cleavage: cutting up the embryo
cleavage patterns: why cleavage is important for development.
cytokinesis mechanisms: the role of the cytoskeleton, and variation in how membrane is added.
regulation mechanisms: the role of the cyclins and mpf: experiments
cleavage types: the problem with yolk
Mitosis lite.
• MPF = cyclin B + cyclin-dependent kinase. Cdc2 phosphorylates proteins involved directly in karyo-and cytokinesis.
MPF is necessary and sufficient for progression through the cell cycle
Experimental test of MPF function in frogs.
early embryos incubated in cycloheximide stop cleaving.
inject MPF and cleavage resumes.
purified MPF
Embryo
1. cycloheximide inhibits protein synthesis
How are cleavages accomplished?
•Different molecular mechanisms of cleavage (contractile ring vs. addition of vesicles)
actin: bluetubulin: orangephoto: G. von Dassow, Center for Cell Dynamics
Cleavage type: the problem of yolk
•Different animals cleave in different ways:
How many types of cleavage?
1. (i) Holoblastic (complete) with Isolecithal eggs
Observed in eggs with low amounts of yolk. Yolk homogeneously distributed in egg and
blastomeres are often the same size.
Cleavage can be divided into two categories:
How many ways to cleave?1. (ii) Holoblastic with mesolecithal eggs. Yolk
is distributed heterogeneously (concentrated in the vegetal pole).
Mesolecithal (intermediate amount of yolk).Cleavage furrow can still penetrate the yolk, but vegetal cells are bigger than animal cells b/c at any given moment in time have undergone fewer cell divisions.
How many ways to cleave?2. (i) Meroblastic: incomplete cleavage
with telolecithal yolk distribution.
Squid
SquidSquid
Large yolk contribution precludes complete cleavage
Fish
Also called discoidal cleavage as there is a disc of cytoplasm on a
massive yolk sac.
•2. Meroblastic (incomplete) cleavage
2.(ii) Meroblastic: incomplete cleavage with centrolecithal yolk distribution.
Many arthropods have this type of
cleavage.
narrow band of cytoplasm on the outer surface of the egg
yolky centercytoplasm
In fruit flies karyo- and cytokinesis are uncoupled for the first 12 mitoses.
Fruit fly zygoteegg plasma membrane
Fly karyokinesis (nuclear cleavage) occurs in a syncitium
This pattern is thought to have evolved to maximize speed of embryonic development.
only seen among holometabolous insects
Still other ways of dealing with yolk.
•gastropods and other animals (e.g. annelids) form polar lobes, generating a quasi-3 cell stage called the trefoil stage.
Gastropod Scallop Trefoil
Cleavage patterns
•cleavage planes are:
(i) the principal mechanisms by which maternal determinants are sequestered into particular lineages (cytoplasmic determinants) and
• cleavage is not just about dividing up the embryo: its about dividing it up in a very specific way.
(ii) the means by which intercellular interactions are determined
In most animals cleavage patterns are not random.
•The plane of cleavage is always perpendicular to the spindle.
•Therefore, the orientation of the spindle determines the pattern of cleavage.
What determines the orientation of the
spindle?
The anatomy of an egg
Vegetal hemisphere
Animal hemisphere
Equator
Cortex
Vegetal pole
Animal pole
Germinal vesicle (nucleus)
Plasma membrane
Cytoplasm
Maternal axis
The animal-vegetal axis (also called the maternal axis) is the reference point for
distinguishing among cleavage patterns.
Cleavage patterns
Radial (biradial): the mitotic spindle is oriented at right angles to the maternal
axis
Can broadly be divided into two categories, based upon the orientation of the mitotic spindles to the maternal axis.
Spiral (duet): the mitotic spindle is oriented at oblique angles to the
maternal axis
The difference between radial cleavage and spiral cleavage is the orientation of the mitotic spindle to
the maternal (A/V) axis.
A
V
Radial and Spiral Cleavage
Radial: Deuterostomes
Spiral: Protostomes
Biradial: radial + left/right symmetry established by first cleavage.
L/R axis
Some modified cleavages: Biradial and Duet
Duet Cleavage
Acoel flatworms (Phylum Platyhelmintha)
The difference between spiral and duet is that the micromeres arise one cleavage earlier (at the four
instead of eight cell stage)
Why does cleavage plane matter? The plane of cleavage has importance consequences for
development.
movement of the ABa and ABp relative to P2
reverses the polarity of the worm
intercellular communication requires
precise orientation of cleavage planes.