Cleavage

● series of mitotic division in which the zygote divides into a number of smaller cells -> blastomeres

● preparations for cleavage● properties of cleaving embryo

● patterns/types of cleavage● cleavage in model organisms

What are in store for us?

From Fertilizationto Cleavage:Preparations for

cleavage

● shift in the cortical cytoplasm toward site of sperm entry

● initiated by entry of sperm

● influences cleavage pattern

1. rearrangement of cytoplasmic materials

Sea urchin egg

● encoded by mRNAs present egg cytoplasm

● cyclical activity – rises during M– decreases during S

2. activation of mitosis promoting factors (MPF)

a) cyclin B● larger subunit● accumulates at S and degraded after M– breakdown initiated by Ca++

Subunits of MPF

b) cyclin-dependent kinase● regulated by cyclin B● activates mitosis by phosphorylating proteins– histones– lamin of nuclear membranes– regulatory subunit of myosin

Properties of a cleaving embryo

● requires sperm centriole to organize mitotic spindles– mechanical agent of karyokinesis

● cells skip G1 and G2

– cells increase in number– cell size decreases = no net growth of embryo

– divisions are rapid(mammals excluded)

● transcriptionally inactive zygotic genome (except mammals)

– dependent on maternal gene-products in cytoplasm

= rate of cleavage, spatial arrangement and size of blastomeres

– divisions are synchronous

– cell division slows on the onset of zygotic gene expression; cells become motile ● Midblastula transition (MBT)

● requires sperm centriole to organize mitotic spindles– mechanical agent of karyokinesis

KaryokinesisCytokinesis

* Mech'l mitoticcontractile agent spindlering* Maj tubulin actin protein* Loc'n central cortical

cytoplasmcytoplasm* Maj colchicine cyto- disruptor nocodazole chalasin B

● formation of an internal, fluid filled cavity ->blastocoel– originates as small gaps between cleaving cells

– prevents premature contact of cells; provides space for later movement of cells; maintains integrity of embryo

Patterns / Types of Cleavage

● amount and distribution of yolk protein– rate of division– size of cells

Factors affecting cleavage pattern

● factors in egg cytoplasm that influence mitotic spindle formation– transforming acidic coiled

coil (TACC) proteins– maternal gene products

● tubulin

Animal

Vegetal

Vertical (Meridional)

Vertical

Horizontal(Equatoria

l)

cc

cc

cc

cc

● 1st and 2nd = vertical and perpendicular to each other

● 3Rd = horizontal/equatorial

1. Radial cleavage

● 4Th = animal tier divides vertically of same volume

= vegetal divides horizontally, unequal

5th 6th4th128

● blastocoel formation after the 7th cleavage division (at 128 cell stage)– polarized cells● apical surface covered with microvilli

● base secretes basal lamina lining the blastocoel

● first 2 divisions are almost vertical/meridional

● third horizontal division occurs at an oblique angle– tilting of mitotic spindle– macro and micromeres

2. Spiral cleavage

● 4th division– macromere = 1 macro and 1 micro

– micromere = 2 micro● same pattern with further cleavage

● = direction of coiling of shells

● cleavage planes are not parallel or perpendicular to a-v axis of egg in spiral

● fewer divisions before gastrulation in spiral

Differences between spiral and radial cleavage

● cells touch one another at more places in spiral– most thermodynamically stable arrangement

● blastula produced by spiral cleavage has no blastocoel– = stereoblastula

● first division is vertical – slower rate at vegetal pole

● second division is vertical– starts while first furrow bisects the vegetal pole

Radial unequal cleavage in amphibians

● 3rd division is horizontal but not equatorial

● micromeres divide faster than macromeres– morula of unequally sized blastomeres

– blastocoel formed at 128 cell stage

● first division is vertical -> L and R side of embryo– establishes axis of symmetry

● all further divisions on each side are mirror images of the other

3. Bilateral cleavage

32-cell

Cytoplasmic rearrangement in the fertilized egg of Styela partita. (A) Before fertilization, yellow cortical cytoplasm surrounds the gray yolky inner cytoplasm. (B) After sperm entry (in the vegetal hemisphere of the oocyte), the yellow cortical cytoplasm and the clear cytoplasm derived from the breakdown of the oocyte nucleus stream vegetally toward the sperm. (C) As the sperm pronucleus migrates animally toward the newly formed egg pronucleus, the yellow and clear cytoplasms move with it. (D) The final positions of the clear and yellow cytoplasms mark the locations where cells give rise to mesenchyme and muscles, respectively.

● first division is vertical but asymmetrical– founder cell and stem cell

● second division is vertical in posterior stem cell and horizontal in founder cell

4. Rotational cleavage

● succeeding division– founder cell = founder cells

– stem cell = vertical division -> 1 founder and 1 stem cell

● cell division is asynchronous– embryo usually contains odd number of cells

Rotational cleavage in mammals

● cell division is slow (12-24 hrs per cycle)

● zygotic genome is activated during early cleavage

● compaction after 3rd cleavage

● compacted embryo divides to form 16 cell morula– small group of nonpolar cells (embryoblast) -> ICM

– large group of polar cells (trophoblast/ trophectoderm)

● delamination at 64-cell stage

● trophoblast secrete fluid into the morula to create the blastocoel– cavitation

● escape from ZP prior to implantation

● karyokineses without cytokinesis

● nuclei migrate to periphery

● plasma membrane grows inward, partitioning the nuclei into individual cells

5. Superficial

Cellular blastoderm

Formation of the cellular blastoderm in Drosophila. (A) Developmental series showing the progressive cellularization. (B) Confocal fluorescence photomicrographs of nuclei dividing during the cellularization of the blastoderm. While there are no cell boundaries, actin (green) can be seen forming regions within which each nucleus divides. The microtubules of the mitotic apparatus are stained red with antibodies against tubulin. (C) Cross section during cellularization. As the cells form, the domain of the actin expands into the egg.

● divisions are restricted at the animal pole

● first 2 divisions are vertical and perpendicular to each other

● vertical cleavage until 32 cell stage

6. Discoidal

● midblastula transition at 10th division– yolk syncytial layer (YSL)

– enveloping layer (EVL)● periderm

– deep cells

● blastoderm is 5-6 layers thick

● subgerminal cavity between yolk and blastoderm

● extensive cell death to thin out the blastoderm

Discoidal cleavage in birds