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CLEAVAGE

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http://biodev.obs-vlfr.fr/recherche/biomarcell/ascidies/Anim-fromegg.htm

Ciona intestinalis

http://www.mun.ca/biology/scarr/Garstang_Hypothesis.htm

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=.07EmbIt9ELpT-YFA1rDieIO4AMQFlmlIwWj

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E. G. Conklin, 1905 - In Styela partida, cells that contain yellow crescent pigment form muscle (mesoderm)

http://sdb.bio.purdue.edu/AboutThisSite/conklin.article.html

Swalla & Jeffery, 1995 - showed that a maternally derived mRNA was segregated into cells containing yellow crescent pigment.

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Clear cytoplasm = ectoderm

Yellow cytoplasm = mesoderm

Dark gray cytoplasm = endoderm

Light gray cytoplasm = neural tube and notochord.

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=.0f0icUJoQpTbPzZ-do7pAF-n3I2E18sYrYZ

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Frog

7http://www.md.huji.ac.il/~yisraeli/embryology/embryo.html

Frog

5-10 min after fertilization

Second polar body 15-30 min after

fertilization

Pronuclei will fuse 1.5 - 2 hr after

fertilization

Point of sperm fusion with egg determines:

1. Position of grey crescent2. Major axes of embryo3. Position of dorsal lip4. Position of first cleavage

furrow

Wilhelm Roux, 1885

L R

Ant

Pos

Dor

Ven

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1. In some species, the cytoplasm of the zygote is very organized.

2. That maternally derived cytoplasmic information is sequestered into specific cytoplasmic compartments.

3. The end result - maternal mRNAs end up in specific groups of cells - determines those cells eventual fate

a. the cells containing the yellow pigment cytoplasm of Stylea’s embryos form muscle

b. the cells of the frog embryo that contain gray crescent cytoplasm become the chordamesoderm and eventually the notochord.

What do these experiments tell us?

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1. Decrease cell size

2. Increase cell number - allows for formation of blastocoel in many

species by a process called cavitation.

3. In some cases, distribution of maternal cytoplasmic information to specific groups of cells

4. Beginning of Differentiation - as cleavage proceeds, you reach a point where the blastomeres are no longer totipotent

5. Synthesis of embryo derived mRNA for direction of future development begins during cleavage. Mammals - starts after a few cleavage divisions; Frogs - starts during late blastula stage

Purposes of cleavage

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11

A

V

A

V

Sagittal view

View of animal pole

View of vegetal pole

STARFISH CLEAVAGE - EQUAL HOLOBLASTIC

IIIIIIIIVV

~32 cell stage

Meiosis complete at ovulation Isolecithal, oligolecithal, microlecithal egg

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1st - 3.5 hr

2nd - 4.5 hr

3rd - 5.5 hr

A

Sagittal view

View of animal pole

View of vegetal pole

UNEQUAL HOLOBLASTIC (may be meroblastic cleavage, but not always)

Amphibians - meroblastic, unequal starting with 3rd cleavage division

V

II

2 cell stage4 cell stage

IIIII

8 cell stage

IV

16 cell stage

V

32 cell stage

http://www.luc.edu/depts/biology/dev/stages.htm

http://www.its.caltech.edu/~bi12/2003/movies/xclvgtop.mov

Ovulated as primary oocyte. First meiotic division completed in oviduct, second after fertilization.

Mesolecithal vs telolecithalFrog blastula

18 hr

A

V

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Structure of the chicken (bird) shelled egg

Polylecithal or megalecithal egg (huge amount of yolk)

Blastodisc

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AVIAN CLEAVAGE - DISCOIDAL

IIIIIIIIVV

Yolk

Yolk

Looking down on the animal pole.

Discoidal cleavage is a type of meroblastic cleavage

Ovulated as primary oocyte.

Meiosis I completed almost immediately after ovulation.

Meiosis 2 completed after fertilization.

1st - 3 hr

2nd - 3.5 hr

3rd - 4 hr

4th - 4.5 hr

5th - 5 hr

A

15

A

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Polyinvagination

Delamination

Movie of delamination in digital lab manual

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SUPERFICIAL CLEAVAGE - INSECTS

pole plasm http://www.luc.edu/depts/biology/dev/insect.htm

Superficial cleavage is a type of meroblastic cleavage

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19http://www.erin.utoronto.ca/~w3bio380/lecture/Lect11/L11.htm

Human embryo cleavage and cavitation.