Scanning electron micrographs of early cleavage in a fertilized frog egg. Thefurrowing of the cell membrane is caused by the activity of the contractile ringunderneath 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 micrographof the midbody of a dividing animal cell. Cleavage is virtually complete, but the daughtercells remain attached by this thin strand of cytoplasm.The midbody reveals a dense matrix of tightly packed polar microtubules remaining fromthe mitotic spindle.
Activation of RhoA triggers assembly and contraction of the Contractile Ring
(a) Microtubule disassembly stimulates RhoA activity (left). Rho stabilizes microtubules through theformin 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 1shows a protein that possesses both actin- and microtubule-binding sites and could provide a staticcrosslink 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 amotor (yellow) and a binding protein (pink). Both types of interaction could move actin andmicrotubules relative to one another.
An experiment that shows the influence of the position of microtubule asters on the subsequent plane of cleavage. If amitotic spindle is mechanically pushed to one side of the cell, the membrane furrowing is incomplete, failing to occur on theopposite side of the cell. Subsequent cleavages occur not only in the conventional relation to each of the two subsequentmitotic spindles (yellow arrowheads) but also between the two adjacent asters that are not linked by a mitotic spindle (butin this abnormal cell share the same cytoplasm)(red arrowhead). Apparently, the contractile bundle of actin filaments thatproduces the cleavage furrow always forms in the region midway between two asters, which implies that the asterssomehow alter the adjacent region of cell cortex.