Cleavage: cutting up the embryo

1. Cleavage: cutting up the embryo regulation mechanisms: the role of the cyclins and mpf: experiments cytokinesis mechanisms: the role of the cytoskeleton, and variation in how membrane is added. cleavage types: the problem with yolk • cleavage patterns: why cleavage is important for development.

2. 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

3. Embryo 1. cycloheximide inhibits protein synthesis purified MPF Experimental test of MPF function in frogs. early embryos incubated in cycloheximide stop cleaving. inject MPF and cleavage resumes.

4. How are cleavages accomplished? • Different molecular mechanisms of cleavage (contractile ring vs. addition of vesicles) photo: G. von Dassow, Center for Cell Dynamics actin: blue tubulin: orange

5. Cleavage type: the problem of yolk • Different animals cleave in different ways:

6. How many types of cleavage? Cleavage can be divided into two categories: 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.

7. Mesolecithal (intermediate amount of yolk). How many ways to cleave? • (ii) Holoblastic with mesolecithal eggs. Yolk is distributed heterogeneously (concentrated in the vegetal pole). 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.

8. Squid Squid Fish How many ways to cleave? • (i) Meroblastic: incomplete cleavage with telolecithal yolk distribution. Large yolk contribution precludes complete cleavage Squid Also called discoidal cleavage as there is a disc of cytoplasm on a massive yolk sac.

9. yolky center cytoplasm • (ii) Meroblastic: incomplete cleavage with centrolecithal yolk distribution. • 2. Meroblastic (incomplete) cleavage Many arthropods have this type of cleavage. narrow band of cytoplasm on the outer surface of the egg

10. In fruit flies karyo- and cytokinesis are uncoupled for the first 12 mitoses. egg plasma membrane Fruit fly zygote 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

11. Gastropod Scallop Trefoil 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.

12. Cleavage patterns • cleavage is not just about dividing up the embryo: its about dividing it up in a very specific way. • cleavage planes are: (i) the principal mechanisms by which maternal determinants are sequestered into particular lineages (cytoplasmic determinants) and (ii) the means by which intercellular interactions are determined

13. 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?

14. The anatomy of an egg Animal pole Plasma membrane Germinal vesicle (nucleus) Animal hemisphere Equator Vegetal hemisphere Cortex Cytoplasm Vegetal pole Maternal axis The animal-vegetal axis (also called the maternal axis) is the reference point for distinguishing among cleavage patterns.

15. Cleavage patterns Can broadly be divided into two categories, based upon the orientation of the mitotic spindles to the maternal axis. Radial (biradial): the mitotic spindle is oriented at right angles to the maternal axis Spiral (duet): the mitotic spindle is oriented at oblique angles to the maternal axis

16. The difference between radial cleavage and spiral cleavage is the orientation of the mitotic spindle to the maternal (A/V) axis. A V

17. Radial and Spiral Cleavage Radial: Deuterostomes Spiral: Protostomes

18. L/R axis Some modified cleavages: Biradial and Duet Biradial: radial + left/right symmetry established by first cleavage.

19. 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)

20. 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.