Cytokinesis

Somanna A. N.

Scanning electron micrographs of early cleavage in a fertilized frog egg. Thefurrowing of the cell membrane is caused by the activity of the contractile ring underneath it.

The contractile ring

A schematic drawing of a cleavage furrow in a dividing cell.

(A) Scanning electron micrograph of an animal cell in culture in theprocess of dividing; the midbody still joins the two daughter cells. (B) Electron micrograph of the midbody of a dividing animal cell. Cleavage is virtually complete, but the daughter cells remain attached by this thin strand of cytoplasm.The midbody reveals a dense matrix of tightly packed polar microtubules remaining from the mitotic spindle.

Activation of RhoA triggers assembly and contraction of the Contractile Ring

(a) Microtubule disassembly stimulates RhoA activity (left). Rho stabilizes microtubules through the formin mDia and also results in actin-myosin contraction through stimulation of Rho kinase (b) Actin (red) and microtubules (green) can exhibit static or dynamic interactions. Interaction 1 shows a protein that possesses both actin- and microtubule-binding sites and could provide a static crosslink between the two polymers. Interaction 2 shows a complex between an actin-based motor (blue) and a microtubule-based motor (orange), whereas interaction 3 shows a complex between a motor (yellow) and a binding protein (pink). Both types of interaction could move actin and microtubules relative to one another.

An experiment that shows the influence of the position of microtubule asters on the subsequent plane of cleavage. If a mitotic spindle is mechanically pushed to one side of the cell, the membrane furrowing is incomplete, failing to occur on the opposite side of the cell. Subsequent cleavages occur not only in the conventional relation to each of the two subsequent mitotic spindles (yellow arrowheads) but also between the two adjacent asters that are not linked by a mitotic spindle (but in this abnormal cell share the same cytoplasm)(red arrowhead). Apparently, the contractile bundle of actin filaments that produces the cleavage furrow always forms in the region midway between two asters, which implies that the asters somehow alter the adjacent region of cell cortex.

The phases of cytokinesis or cleavage