07 Cleavage

Cleavage

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

What are in store for us? preparations for cleavage ● properties of cleaving embryo ● patterns/types of cleavage ●

From Fertilization to Cleavage: Preparations for cleavage

1. rearrangement of

cytoplasmic materials shift in the cortical cytoplasm toward site of sperm entry ● initiated by entry of sperm ●

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Sea urchin

2. activation of mitosis

promoting factors (MPF)

encoded by mRNAs present egg cytoplasm ● cyclical activity – rises during M – decreases during S ●

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

b) cyclin-dependent

kinase regulated by cyclin B ● activates mitosis by phosphorylating proteins – histones – lamin of nuclear membranes ●

Properties of a cleaving embryo

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

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cells skip G1 and G2 cells increase in number – cell size decreases = no net growth of embryo – divisions are –

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transcriptionally inactive zygotic genome (except mammals)

dependent on maternal gene-products in cytoplasm = rate of cleavage, spatial arrangement and

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

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

Karyokinesis * Mech'l mitotic Cytokinesis contractile agent spindle ring * Maj tubulin actin protein * Loc'n central cortical cytoplasm cytoplasm * Maj colchicine cytodisruptor nocodazole

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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 –

Patterns / Types of Cleavage

Factors affecting cleavage pattern ●

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

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factors in egg cytoplasm that influence mitotic spindle formation transforming acidic coiled coil (TACC) proteins

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maternal gene products

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Vertical (Meridional)

Animal

Vertical

Vegetal cc

cc

cc

cc

Horizontal (Equatoria

1. Radial cleavage 1 and 2 = vertical and perpendicular to each other Rd ● 3 = horizontal/equatorial ●

st

nd

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4 = animal tier divides vertically of same volume = vegetal divides horizontally, unequal Th

4th

5th

6th 128

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

2. Spiral cleavage first 2 divisions are almost vertical/meridional ● third horizontal division occurs at an oblique angle ●

4 division – macromere = 1 macro and 1 micro – micromere = 2 micro ● same pattern with further cleavage ● = direction of coiling of ●

th

Differences between spiral and radial cleavage ● cleavage planes are not parallel or perpendicular to a-v axis of egg in spiral ● fewer divisions before

cells touch one another at more places in spiral – most thermodynamically stable arrangement ● blastula produced by spiral cleavage has no blastocoel ●

Radial unequal cleavage in amphibians ●

first division is vertical – slower rate at vegetal pole

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second division is vertical

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3 division is horizontal but not equatorial rd

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micromeres divide faster than macromeres – morula of unequally sized blastomeres – blastocoel formed at 128 cell stage

3. Bilateral cleavage ●

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

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all further divisions on

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

4. Rotational cleavage first division is vertical but asymmetrical – founder cell and stem cell ● second division is vertical in posterior stem ●

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

Rotational cleavage in mammals ●

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

cell division is slow (1224 hrs per cycle) ● zygotic genome is activated during early cleavage rd ● compaction after 3 cleavage ●

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

delamination at 64-cell stage ● trophoblast secrete fluid into the morula to create the blastocoel – cavitation ● escape from ZP prior to implantation ●

5. Superficial karyokineses without cytokinesis ● nuclei migrate to periphery ● plasma membrane grows inward, ●

Cellular

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.

6. Discoidal divisions are restricted at the animal pole ● first 2 divisions are vertical and perpendicular to each other ● vertical cleavage until 32 ●

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midblastula transition at th 10 division – yolk syncytial layer (YSL) – enveloping layer (EVL) ● periderm – deep cells

Discoidal cleavage in birds

blastoderm is 5-6 layers thick ● subgerminal cavity between yolk and blastoderm ● extensive cell death to ●