Cytokinesis and Organelle Inheritance

Push the spindle of a furrowing sea urchin blastomere off-centre with a blunt needle, and within three to five minutes a band of active signalling protein on the cell surface moves to sit over the spindle’s new middle. The furrow follows.

Mitosis has sorted the chromosomes into two sets. The cell still has to cut itself in two, in the right place, and give each daughter a working set of organelles. This lesson asks three questions. How does the cortex, the layer of actin just under the plasma membrane, learn where to cut? How does a plant cell, boxed in by a wall, divide at all? And how are organelles shared, by chance or by control?

The spindle tells the cortex where: RhoA zones and moved spindles

In animal cells, the cut is made by a cleavage furrow, an inward fold of the surface driven by actin filaments and myosin II. The furrow forms midway between the two sets of chromosomes, and that position is not an accident. The anaphase spindle, the array of microtubules that pulled the chromosomes apart (see mitosis), sends the signal.

An old experiment showed the spindle’s reach. Raymond Rappaport reshaped a fertilized sand dollar egg into a ring. At the second division, a furrow formed between two neighbouring asters, the star-shaped arrays of microtubules at the spindle poles, even though no spindle joined them. In that egg, the microtubule array alone could mark a furrow site.

What does the marking? The answer is a small GTPase, RhoA. Like other GTPases it is a switch: on when it holds GTP, off when it holds GDP. The cycle belongs to GTPase switches and integrins and Rho GTPases. At the equator, RhoA is switched on by Ect2, a guanine-nucleotide exchange factor (GEF), an enzyme that swaps GDP for GTP.

In 2005 William Bement, Hélène Benink and George von Dassow made active RhoA visible. They fused a green fluorescent protein to a fragment of rhotekin that binds only GTP-bound RhoA, so the glow marks where RhoA is on, not where the protein simply sits. In embryos of two sea urchin species and of the frog Xenopus laevis, active RhoA gathered in a bounded band at the equator one to four minutes before the furrow. The band still formed in sea urchin embryos treated with cytochalasin D or latrunculin B, drugs that block actin assembly. So RhoA activation comes before the actin ring and does not need it.

Then came the test of cause. In green sea urchin blastomeres that had already formed a RhoA zone and begun to furrow, they pushed the spindle aside with a blunt needle. The RhoA band relocated over three to five minutes, and the furrow followed it. Correlation became instruction: move the spindle, and the signal moves first. The delay also argues against the spindle simply dragging the cortex along. These timings come from large embryonic cells; do not assume the same minutes in a human cell.

The contractile ring and the final cut

Think of a drawstring at the waist of a bag. Active RhoA does two things through its effectors. It turns on formins, proteins that build long, unbranched actin filaments, and it promotes the activity of nonmuscle myosin II, a motor that pulls actin filaments past each other (see molecular motors and cytoskeleton dynamics). Together they make a contractile ring under the membrane at the equator, and as it tightens, the furrow ingresses.

The drawstring is the standard cartoon, and it is a cartoon. How the filaments are arranged in the ring differs between systems, and no one geometry is settled for all cells.

Ingression does not finish the job. It leaves the two daughters joined by a thin intercellular bridge packed with spindle microtubules, with a dense midbody at its centre. Abscission, the final cut, comes later. ESCRT-III, a set of proteins that polymerize into spirals on membranes, assembles in the bridge, constricts it and cuts it, in the several animal-cell systems covered by a 2022 review. If the cut fails, the bridge can regress and leave one cell with two nuclei and doubled DNA, a route to the tetraploid cells discussed in chromosome changes.

Plants build a wall from the inside out

A plant cell sits inside a rigid wall, so it cannot pinch. In the prevalent mode of somatic plant cytokinesis there is no furrow at all. The new membrane starts in the middle of the cell and grows outward.

The builder is the phragmoplast, an array of microtubules and actin filaments that forms between the two daughter nuclei. It guides vesicles from the Golgi to the centre of the division plane. There they fuse with one another to form the cell plate, a flat compartment of new membrane filled with wall material. The plate expands sideways toward the parental cell surface, and cytokinesis is complete when it joins the old wall.

Fusion needs machinery. KNOLLE, an Arabidopsis protein related to syntaxins, the target-membrane SNAREs you met in vesicle traffic, sits at the division plane. In knolle mutant embryos, serial-section electron microscopy showed bands of vesicles in the plane and walls left incomplete. The vesicles had arrived; their fusion appeared to fail.

Where the plate meets the old wall is decided earlier. Before mitosis, many plant cells build a preprophase band, a cortical ring of microtubules that marks the future division plane and then disappears, leaving a cortical division zone behind. The band is not indispensable everywhere. In Arabidopsis roots, disrupting it reduced the precision of division orientation without abolishing the division zone. In Arabidopsis cambium stem cells, trm678 mutants that make no band kept regular division planes, while the kinesins POK1 and POK2 were needed for ordered planes. Keep each result in its tissue.

Live imaging now gives the plate a clock. In Arabidopsis root cells tracked by lattice light sheet microscopy, untreated plates developed over about 20 to 30 minutes, and their diameter grew at about 4.09 μm per minute early on. Blocking callose deposition with Endosidin 7 disrupted the switch from collecting vesicles to maturing the plate.

Dividing the organelles: chance and control

A cell does not count out its organelles. Many are shared by chance, and the arithmetic shows when chance is good enough.

Worked problem: partition by chance

The numbers are illustrative. Suppose each unit of an organelle goes to either daughter independently with probability 0.5, like a coin toss. The count in one daughter then follows a binomial distribution.

mean = n × 0.5; standard deviation = square root of (n × 0.5 × 0.5); P(one daughter gets none) = 2 × 0.5^n

Case 1: n = 100 Golgi stacks. Mean = 50. Standard deviation = square root of 25 = 5. About 95% of divisions fall within two standard deviations, so most daughters get 40 to 60 stacks. P(one daughter gets none) = 2 × 0.5^100, about 1.6 × 10^-30. That never happens.

Case 2: n = 10. P = 2 × 0.5^10 = 2/1024, about 0.002, or 0.2%. One division in five hundred leaves a daughter with none.

Case 3: n = 2. P = 2 × 0.25 = 0.5. Half of all divisions fail.

Check: case 2 by counting. All ten to daughter A is one outcome of 1,024, all ten to daughter B is another, so 2 of 1,024. The factor of 2 is there because either daughter can be the empty one.

Conclusion: chance is safe only for many independent units. A single connected structure, such as the Golgi ribbon of a mammalian cell, a tubular mitochondrial network or a centrosome pair, is one unit, not many. It must be broken up first or segregated actively.

Cells do both. In PtK1 cells, a rat-kangaroo kidney line, the Golgi ribbon (see the secretory pathway) disassembles during mitosis, its membranes cluster around the spindle poles, and the ribbon reforms after division. In HeLa cells, mitochondria fragmented in early mitosis, and cells depleted of Drp1, a dynamin-related GTPase that drives mitochondrial fission, kept tubular mitochondria through mitosis, as Taguchi and colleagues reported in 2007. That is evidence from one cell line, so treat it as a strong lead rather than a general rule.

Position is a separate question from shares. In HeLa cells mitochondria gather toward the furrow, and a dominant-negative Miro-1 reduced that gathering, yet no obvious inheritance defect followed. Organelles also differ in how they divide (see organelles as environments), and in some cells division is unequal on purpose, as in asymmetric division.

Design problem

The Golgi’s mitotic membranes cluster around the spindle poles. Is the Golgi inherited by riding on the spindle? Design an experiment in cultured cells that could tell, and say what each model predicts before you look at any data.