Cytokinesis in Animal and Plant Cells

Cytokinesis is the final stage of the Cell Cycle, the physical division of the cytoplasm (and the cell as a whole) into two daughter cells following Chromosome Segregation and the Metaphase-to-Anaphase Transition. It is mechanistically distinct from mitosis proper: mitosis segregates genetic material, cytokinesis partitions cytoplasm, organelles, and membrane. The two processes are coordinated in time — cytokinesis normally begins in anaphase and completes by telophase — but they are executed by separate molecular machines, and experimentally they can be uncoupled (cells can complete chromosome segregation while failing to divide, producing binucleate cells). The core mechanical challenge differs fundamentally between kingdoms because of one structural fact: animal cells are bounded only by a flexible plasma membrane and actomyosin cytoskeleton, while plant cells are encased in a rigid cellulose cell wall. A flexible boundary can be pinched inward. A rigid boundary cannot; it must instead be divided by inserting a new partition. This single constraint explains essentially every difference in mechanism described below: animal cytokinesis is a contractile process (the cleavage furrow, driven by a contractile ring of actin filaments and myosin II), while plant cytokinesis is a constructive process (the phragmoplast, an array of microtubules and actin that directs membrane vesicles to fuse and build a cell plate, the precursor of the new cross wall). Despite this divergence in downstream machinery, both processes are triggered and spatially positioned by the same upstream structure: the anaphase spindle, specifically the region between the segregating chromosome sets called the spindle midzone (in animals) or phragmoplast midzone (in plants), where antiparallel microtubules from the two half-spindles overlap and interdigitate. This convergence is not superficial — many of the molecular players (kinesin-6 family motors, the chromosomal passenger complex, Rho-family GTPases and their regulators) are conserved or functionally analogous across the plant and animal lineages, reflecting a shared ancestral logic even though the terminal effector machinery (actomyosin ring vs. vesicle-fusion plate) evolved independently to solve the wall-vs-no-wall problem.

Animal cell cytokinesis: the contractile ring

Step 1 — Specification of the division plane. As cells enter anaphase, the mitotic spindle itself specifies where the cell will divide. Two microtubule populations carry this positional information to the cell cortex (the actin-rich layer beneath the plasma membrane):

• Astral microtubules, radiating from the spindle poles to the cortex, are thought to inhibit contractility in the polar cortex (astral relaxation) and/or to help center the division site between the poles.
• Central spindle (midzone) microtubules, antiparallel bundles that form between the separating chromosome masses, deliver a positive signal specifying the equatorial cortex as the site where the ring must assemble.

Step 2 — Rho-GTPase activation at the equator. The key molecular switch is RhoA, a small GTPase of the Ras superfamily. During anaphase, RhoA becomes locally activated in a narrow equatorial band of cortex overlying the spindle midzone. This activation is driven by centralspindlin, a heterotetrameric complex of the kinesin-6 motor MKLP1 and the Rho-family GAP-domain protein RacGAP1 (MgcRacGAP), which walks along midzone microtubules to the cortex and there recruits and activates ECT2, a guanine-nucleotide exchange factor (GEF) that converts RhoA-GDP to active RhoA-GTP. Active RhoA-GTP then: 1. Activates formins (e.g., mDia2), which nucleate and elongate linear actin filaments. 2. Activates Rho-associated kinase (ROCK), which phosphorylates the regulatory light chain of myosin II, activating its motor activity, and simultaneously inhibits myosin light-chain phosphatase, reinforcing activation.

Step 3 — Ring assembly and constriction. Actin filaments and bipolar myosin II filaments assemble into a circumferential band under the equatorial plasma membrane: the contractile ring. Myosin II motors slide antiparallel actin filaments past one another, generating contractile tension analogous to (but molecularly and architecturally distinct from) a muscle sarcomere. As the ring constricts, it draws the overlying plasma membrane inward, producing the visible cleavage furrow. New membrane is added locally by targeted vesicle trafficking (exocytosis, partly Rab11/Rab35-dependent recycling endosome delivery) to accommodate the increasing membrane curvature and surface area demand.

Step 4 — Abscission. Constriction proceeds until the furrow narrows to a thin cytoplasmic bridge, the midbody, containing the compacted remains of the central spindle (densely packed antiparallel microtubules and the midbody matrix, rich in centralspindlin, PRC1, and other midzone proteins). Final membrane fission (abscission) is carried out by the ESCRT-III membrane-remodeling machinery (recruited via CEP55 and ALIX/TSG101), which forms spiraling filaments that constrict the membrane tube down to molecular dimensions and sever it — a process mechanistically related to ESCRT-mediated membrane scission in multivesicular body formation and viral budding.

Plant cell cytokinesis: the phragmoplast and cell plate

Because the plant cell wall cannot be pinched, cytokinesis instead builds a new partition, the cell plate, from the inside out.

Step 1 — Preprophase band (positional memory, set before mitosis). Uniquely in plants, the future division site is marked long before anaphase. In late G2, a dense cortical ring of microtubules and actin, the preprophase band (PPB), forms around the cell cortex and marks the exact plane where the cell will eventually divide. The PPB itself disassembles before metaphase, but it leaves behind a persistent cortical molecular mark, the cortical division zone, containing proteins such as members of the TANGLED1 and POK (POLAR/PHRAGMOPLAST ORIENTING KINESIN) families, which "remember" the site for the phragmoplast to find later.

Step 2 — Phragmoplast formation at anaphase/telophase. As anaphase chromosomes separate, the spindle midzone microtubules reorganize into the phragmoplast: a barrel-shaped array of two opposing sets of antiparallel microtubules (plus actin filaments), with plus-ends overlapping at the equatorial midplane and minus-ends oriented toward the two daughter nuclei. This antiparallel microtubule overlap is functionally the direct analog of the animal central spindle, and indeed uses homologous machinery — a kinesin-6 motor (in plants, part of the MAP65/kinesin system) and the MKLP1-family motor PAKRP1/kinesin-related proteins, together with MAP65, a microtubule-bundling protein that cross-links antiparallel microtubules at the midzone, functionally analogous to animal PRC1.

Step 3 — Vesicle delivery and cell plate formation. Golgi-derived vesicles, carrying membrane lipids and cell-wall precursor materials (pectins, hemicelluloses, and callose), are transported along the phragmoplast microtubules by kinesin motors to the equatorial midplane. There they aggregate and fuse with one another — a process dependent on the plant-specific cytokinesis-related SNARE protein KNOLLE (a syntaxin) paired with its partner SNAP33 and the Sec1/Munc18-family regulator KEULE — building a tubulovesicular network that matures into a flattened membrane compartment, the cell plate.

Step 4 — Centrifugal expansion and fusion with the parent wall. The cell plate does not simply appear fully formed; it grows centrifugally, expanding outward from the center of the cell toward the cortex, as the phragmoplast microtubule array itself expands outward (old microtubules near the plate center depolymerize while new microtubules are nucleated at the leading edge). This continues until the growing plate reaches the parental plasma membrane and cell wall at the position marked earlier by the cortical division zone, where it fuses to complete a continuous new partition. Callose (a β-1,3-glucan) is deposited transiently to stabilize the young cell plate, and is later replaced by cellulose and pectin as the mature cross wall (new primary cell wall) matures, with plasmodesmata (cytoplasmic channels) forming at sites where ER strands were trapped in the plate, preserving symplastic connections between the two new daughter cells.

Rappaport's off-center furrow induction (1961–1985) proved the spindle midzone, not chromosomes, positions the furrow. Ray Rappaport worked with large, experimentally tractable echinoderm (sand dollar) eggs, which he could physically manipulate under simple micromanipulation. Using glass rods, he compressed dividing eggs into unusual shapes — dumbbells, rings, and doughnuts — so that a single mitotic spindle could be placed adjacent to cell cortex that was far from any chromosomes but still between two asters. The critical finding: a cleavage furrow formed wherever cortex lay between two asters, even in cortical regions that had never been near chromosomes, and even in binucleate cells where two independent spindles were positioned so that only their facing (inner) astral/midzone regions overlapped — producing a single furrow at that region of overlap rather than two furrows at each spindle. This showed that the stimulus for furrow formation is generated by the interaction zone between two sets of microtubules (later shown to be the central spindle/astral overlap) and is transmitted to the cortex, not by the chromosomes themselves and not by the asters acting alone. Rappaport's "furrow-stimulus" experiments, decades before RhoA or centralspindlin were known, established the conceptual framework — a signal traveling from the spindle midzone to the equatorial cortex — that the molecular biology of the 1990s–2000s (identification of RhoA activation zones, centralspindlin, ECT2) later explained mechanistically.

RhoA activity zone visualized directly (Bement, Miller, and colleagues, 2000s–2010s). Using FRET-based biosensors for GTP-bound RhoA expressed in living echinoderm and Xenopus embryos, researchers directly imaged a narrow, dynamic zone of active RhoA appearing at the equatorial cortex in anaphase, tightly co-localized with the position of the forming furrow and dependent on astral/midzone microtubule signals — confirming at the molecular level what Rappaport's manipulations had predicted decades earlier, and showing that the RhoA zone is actively maintained by a balance of local GEF (ECT2) recruitment and GAP-mediated inactivation, giving the zone sharp, self-correcting boundaries.

Genetic and RNAi dissection of centralspindlin and ECT2. Depletion of MKLP1, RacGAP1/MgcRacGAP, or ECT2 by RNAi in C. elegans embryos and cultured mammalian cells abolishes or severely displaces the equatorial RhoA zone and prevents furrow ingression, while the central spindle itself fails to form properly, directly demonstrating the causal chain from midzone motor complex → GEF recruitment → RhoA activation → contractile ring assembly.

Visualizing the cell plate and phragmoplast (light and electron microscopy, from the 1950s onward; live-imaging with GFP-tubulin in the 1990s–2000s). Classical electron microscopy of dividing plant cells (e.g., in onion root tip meristems, a long-standing model system) revealed the tubulovesicular network at the cell equator maturing into a flattened cisterna, directly visualizing centrifugal growth of the cell plate from center to periphery. Later, live imaging of GFP-tagged tubulin and Golgi markers in tobacco BY-2 cells and Arabidopsis root tips showed in real time that phragmoplast microtubules continuously depolymerize centrally and are renewed at the expanding leading edge, directly demonstrating the centrifugal expansion mechanism inferred earlier from static EM snapshots.

KNOLLE and KEULE mutants (Lukowitz, Mayer, Jürgens, 1990s, Arabidopsis). Mutations in the KNOLLE gene (encoding a cytokinesis-specific syntaxin) and in KEULE (a Sec1/Munc18 regulator) produce Arabidopsis embryos with grossly enlarged, multinucleate cells because vesicles delivered to the phragmoplast midzone fail to fuse properly, leaving incomplete or absent cell plates. This genetic evidence established that plant cytokinesis, unlike animal cytokinesis, depends on a dedicated membrane-fusion (SNARE) machinery rather than on contractile force, and pinpointed the exact molecular step — vesicle fusion at the midplane — as the plant-specific analog of the animal contractile ring's constriction step.

Preprophase band ablation and displacement experiments. Laser ablation or drug-induced disruption of the preprophase band in plant cells before mitosis causes the subsequent phragmoplast/cell plate to grow in an aberrant, sometimes oblique, orientation and can fail to properly fuse with the parental wall at the originally marked site, demonstrating that the PPB (or its persisting molecular mark, the cortical division zone) actively guides the destination of the expanding cell plate, independent of spindle orientation itself.

Example 1 — Predicting furrow position in an experimentally deformed egg (Rappaport-style reasoning). Imagine a fertilized sea urchin egg compressed into a dumbbell shape, with two separate mitotic spindles, one in each lobe, oriented so that their long axes are parallel to each other and perpendicular to the neck of the dumbbell (i.e., the spindles are side by side, not end to end). Each spindle has two asters and a central spindle. Question: how many cleavage furrows form, and where? Reasoning: The furrow-inducing signal arises specifically at regions of cortex that lie between two facing asters (regardless of which spindle they belong to) — this is the "double-aster stimulation" rule Rappaport established. In this dumbbell geometry with parallel spindles, each spindle has its own pair of facing asters within its own lobe, generating its own equatorial signal. Additionally, cortex in the neck region lying between the nearest asters of the two different spindles (the two inner asters, one from each spindle) can also receive an aster-derived signal if those asters are close enough, potentially producing an extra furrow in the neck. Outcome observed experimentally by Rappaport: two furrows form, one within each lobe at each spindle's own equator, and — critically — if the two spindles are close enough together that their inner asters overlap sufficiently in the neck, a third, unpredicted furrow can appear in the neck region even though no chromosomes are anywhere near it. This confirms the rule is purely geometric/microtubule-based (astral or central-spindle overlap), not chromosome position, and shows the furrow-inducing signal obeys a quantitative rule based on aster proximity and overlap, not qualitative "presence of DNA."

Example 2 — Consequence of cytokinesis failure: from binucleation to tetraploidy to aneuploidy. Suppose a cultured mammalian cell line is treated with a low dose of a myosin II ATPase inhibitor (blebbistatin) sufficient to block contractile ring constriction in a fraction of cells without blocking mitosis itself. Question: trace the karyotype consequences over two subsequent cell cycles, assuming the starting cell is a normal diploid (2n = 46 in humans) and cytokinesis fails completely in cycle 1 but proceeds normally thereafter. Reasoning, step by step: 1. Normal mitosis in cycle 1: chromosomes replicate (S phase) and correctly segregate into two separate nuclear masses at anaphase/telophase — karyotype content is now 2 × (2n) = effectively two full diploid genomes present in one cell, i.e., 4n = 92 chromosomes worth of DNA, but organized as two separate nuclei (binucleate cell) because cytokinesis (the physical division step) failed while mitosis (chromosome segregation) succeeded. 2. This binucleate cell may subsequently undergo mitosis again (cycle 2). If nuclear envelope breakdown occurs normally, the two sets of already-duplicated chromosomes (now doubled again after S phase to 8n replicated content across the whole cell) can be captured by a single, often multipolar or a merged bipolar spindle. 3. A single bipolar spindle attempting to segregate what is now a tetraploid chromosome complement (4n = 92 chromosomes before the second S phase, or effectively the products of 4n replicated in S phase) frequently mis-segregates chromosomes to the two poles unevenly, because there are twice the normal number of kinetochores competing for capture by two centrosome-derived poles — producing daughter cells with unequal, non-46 chromosome numbers: aneuploidy. Conclusion: a single cytokinesis failure event converts a diploid cell into a tetraploid intermediate (binucleate, 4n), and the subsequent, error-prone division of that tetraploid cell is a well-documented route to aneuploid, chromosomally unstable daughter cells — a mechanism implicated in the origin of chromosomal instability in many solid tumors. This example shows why cytokinesis fidelity is not a cosmetic detail of cell division but a genome-stability checkpoint in its own right.

Ray Rappaport (1922–2010) was an American cell biologist who spent most of his career at Union College and, in the summers, at the Marine Biological Laboratory in Woods Hole, Massachusetts — the same institution long associated with classic embryology using marine invertebrates. Working almost entirely with simple, inexpensive tools (glass needles, compression chambers, and a light microscope) on the eggs of sand dollars and sea urchins, Rappaport built a decades-long, remarkably rigorous experimental program asking a single question with obsessive precision: what physically tells a dividing cell where to put its furrow? His approach was resolutely mechanical and quantitative rather than molecular — he manipulated cell shape and spindle geometry directly and measured furrow position and timing with a precision that outpaced the molecular tools of his era by decades. The molecular identity of the signal he inferred (Rho-GTPase activation via centralspindlin and ECT2) was not established until the 1990s–2000s, well after most of his key manipulative experiments, meaning his conclusions about "a stimulus generated at the interaction zone between two asters" were derived purely from careful physical experimentation and stood essentially unmodified once the molecules were finally identified.

Cancer biology and chromosomal instability. Failed or incomplete cytokinesis generates tetraploid intermediates that are a documented precursor state for the aneuploidy and chromosomal instability (CIN) seen in a large fraction of solid tumors. Drugs and biomarkers targeting Aurora B kinase (a chromosomal passenger complex component essential for both correct chromosome segregation and central spindle/midbody function) are in active clinical development and use as anti-cancer agents, partly because tumor cells are often more dependent on robust mitotic/cytokinetic checkpoint machinery than normal cells.

Antifungal and antiparasitic drug targets. Because fungal and some protozoan cytokinesis machineries (e.g., septin-based ring assembly in budding yeast, distinct from but related conceptually to the animal actomyosin ring) differ from human mechanisms, components of these divergent cytokinesis pathways are explored as selective antimicrobial/antifungal drug targets.

Plant biotechnology and crop architecture. Because the plane of cell division set by the preprophase band and cortical division zone directly shapes tissue and organ architecture, manipulating division-plane control genes (e.g., TANGLED, POK kinesins) is used in plant developmental biology to understand and engineer leaf shape, root branching patterns, and stomatal patterning — traits of direct agronomic relevance to crop yield and structure.

Reproductive and developmental biology. Errors in oocyte cytokinesis (asymmetric division producing the polar body) are a major source of egg aneuploidy and are linked to age-related increases in human trisomies (e.g., Down syndrome), directly connecting the mechanics described here to clinical reproductive genetics and to the broader topic of Meiosis Versus Mitosis.

Stem cell and tissue biology. Asymmetric cytokinesis — where the contractile ring or cell plate is deliberately positioned off-center — is a key mechanism generating daughter cells with different fates from a single stem cell division, a mechanistic bridge to the study of Stem Cells and Cellular Potency.

Ask the tutor: How exactly does the chromosomal passenger complex (Aurora B) coordinate the transition from chromosome segregation to central spindle/midbody formation?