Lecture 8 Cleavage.pptx

Cleavage (not what you think!) Lecture 8 Cleavage: Cell Division without Growth Cleavage divisions are the first mitotic divisions of the embryo. They divide up the single celled zygote into increasingly more and increasingly smaller cells (blastomeres). Cleavage: Altered Mitosis – no...

Cleavage (not what you think!) Lecture 8 Cleavage: Cell Division without Growth Cleavage divisions are the first mitotic divisions of the embryo. They divide up the single celled zygote into increasingly more and increasingly smaller cells (blastomeres). Cleavage: Altered Mitosis – no G1 G2 After Nigg et al, 1995 Cdk1-cyclinB (MPF – maturation promotion factor - Masui) Kinase promotes mitosis Cdk1-cyclinB normally active at M-phase, cyclin degraded after M-phase and resynthesized (new transcription) before next M-phase. Gap phase = low cyclin levels Cleavage embryo is filled with Cdk1-cyclinB. After mitosis, only cyclinB that was associated with nucleus is destroyed. Levels decline almost unnoticeably. No gap phase because Cdk1-cyclinB Cleavage: Characteristics After Nigg et al, 1995 Cleavage divisions very rapid compared to other cell divisions lack defined G1 and G2 phases due to constitutive Cdk1-cyclinB activity no cell growth – cells get smaller each division cells are transcriptionally silent – Zygotic genome inactive still – embryo relies on oocyte stores Cell divisions are often synchronous pattern of cleavage divisions is stereotypical for an organism, and affected by yolk thickness/distribution leavage: Characteristics Different organisms exploit different environments at different stages of life. Cleavage can be described using a few different parameters: Amount and distribution of yolk Whether or not early cleavage is complete Whether or not cleavage is symmetrical (ie; daughter cells are identical) How planes of cell division contribute to distribution of blastomeres  Cleavage divisions in frogs Egg yolk is concentrated in lower half of egg Complete divisions = holoblastic First 2 divisions \vertical, 3rd is horizontal and closer to top subsequently top cells divide faster (less yolk) After first two cell divisions (4 cell stage)  Meroblastic cleavage divisions in fish, birds Yolk is very thick and cell divisions cannot pass through Only upper region of egg is free of yolk first 5 cell divisions are vertical and incomplete - Meroblastic cleavage furrows stop when they reach yolk blastomeres are still not separated from yolk 6th division is horizontal and separates yolk from upper blastomeres Cephalopod Meroblastic Cleavage Galis et al. (2002). Divergence and convergence in early embryonic stages of metazoans. Contributions to Zoology. 71. 101-113. 10.1163/18759866-0710103008. Cleavage divisions in mammals Holoblastic cleavage Egg has little yolk that is evenly distributed (isolecithal) Cleavage divisions go completely through the egg/blastomeres each blastomere is of equal size different than other animals in that cleavage divisions are slower and zygotic transcription starts at minimal level early in embryo 24-48  Cleavage divisions, Planes, and Rotation Holoblastic cleavage 2nd cleavage is at right angles to 1st – rotational cleavage Rotational or Meridional Pattern of cleavage division is determined by orientation of mitotic spindle Cleavage divisions in C. elegans Holoblastic and asymmetric IE; only P cell and its progeny inherit P granules Fates of cells are very lineage restricted Ablate a cell, future vulva, or specific Centrosomes, Spindles, cleavage orientation Plane of cell division depends on orientation of mitotic spindles This depends on centrosome positioning Centrosome positioning depends on proteins at cell cortex that interact with centrosomes (usually via astral microtubules) “Meroblastic” Fruitfly Cleavage 1. Initially, DNA replication and nuclear division without cell cleavage 2. Nuclei migrate to periphery 3. Pole plasm-containing cells collect posteriorly (presumptive germ cells) 4. Cells partially partition but remain open at bottom (syncytial) 5. Finally, around division 13, cellular partition  Cleavage divisions Cleavage divisions divide up the zygote into multiple cells that will each eventually have different fates in the embryo Are cell fates established during the cleavage divisions? in one model, cell fate determinants are present in the zygote and are asymmetrically distributed at each cell division each successive cleavage division further restricts the fate of a cell Answer: Depends on organism Some are highly mosaic with regard to distribution of cytoplasmic components