CLEAVAGE
I. DEFINITION: Initial series of mitotic cell divisions of the zygote.

A. Unicellular zygote --> multicellular blastula (cells = blastomeres).

B. No growth occurs (slight exception in mammals).

C. General shape of embryo does not change, except for formation of blastocoel.

D. Few real qualitative changes in chemical composition of cells.

E. Cytoplasmic substances not displaced to any great extent; remain in same relative positions as in zygote. (Note: this does not include the events between egg activation and cleavage.)

F. Nuclear volume/cytoplasmic volume is low at beginning of cleavage, and higher (near level of ordinary somatic cells) at the end.

G. Cell cycle during cleavage is relatively short in most organisms.



II. PATTERNS OF CLEAVAGE - affected by yolk distribution.

A. Classification of eggs based on yolk distribution.

1. Isolecithal (oligolecithal) - sparse, evenly distributed yolk. eg. echinoderms, Amphioxus, molluscs, mammals

2. Mesolecithal - moderately telolecithal. eg. amphibians

3. Telolecithal - dense yolk concentrated at one end of egg. eg. reptiles, fishes, birds

4. Centrolecithal - yolk concentrated in center of egg. eg. most arthropods

B. Holoblastic cleavage:

1. Isolecithal and mesolecithal eggs.

2. Cleavage furrows pass all the way through the egg.

3. Initial divisions occur along specific planes.

4. Radial: blastomeres about the same size; top 4 blastomeres directly over the bottom 4. eg. echinoderms, Amphioxus, amphibians?

5. Bilateral: variation of radial; 2 of 1st 4 blastomeres larger; plane of bilateral symmetry. eg. tunicates

6. Spiral: top 4 blastomeres over junctions of bottom 4, due to oblique mitotic spindles; dextral vs. sinestral (snail shell coiling).

7. Rotational: mitotic spindle of 1 blastomere at 2-cell stage rotates 90o. eg. mammals

C. Meroblastic cleavage:

1. Telolecithal eggs.

2. Incomplete cleavage restricted to blastodisc (non-yolky cytoplasm at one end of egg.

3. Forms blastoderm surrounded by periblast (a syncytium).

4. Cleavage planes vertical at 1st, followed by horizontal cleavages to separate blastoderm from underlying yolk.

5. Thickness of blastoderm varies among fishes, reptiles, and birds.

6. Components of blastula (birds and reptiles):

a. epiblast = superficial (top) layer.

b. hypoblast = formed by blastomeres under epiblast, arranged into a thin epithelium.

c. blastocoel = space between epiblast and hypoblast.

d. subgerminal cavity = space between hypoblast and yolk; only under central part of blastoderm.

e. area pellucida = body of embryo.

f. area opaca = outer region of blastoderm, directly on top of yolk; involved in breakdown of yolk.

D. Superficial cleavage:

1. Centrolecithal eggs (arthropods).

2. Karyokinesis without cytokinesis, resulting in syncytium.

3. Nuclei migrate from central island of cytoplasm to peripheral region near membrane.

4. Cleavage furrows form to separate nuclei.