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.