Cytokinesis

Every cell division ends with a physical act — the splitting of one cell into two. This is cytokinesis: the process that partitions the cytoplasm, organelles, and plasma membrane between the two daughter cells that mitosis has produced. It is easy to think of cytokinesis as a simple mechanical afterthought following the drama of chromosome segregation. In reality, it is one of the most mechanically demanding and molecularly precise events in cell biology. A ring of actin and myosin must assemble in exactly the right place on the cell cortex, generate enough force to constrict a cell that may be tens of micrometres in diameter, all without rupturing the membrane it is pulling inward — and then, once two daughter cells are left connected only by a thread, a completely separate molecular machine must arrive and sever that thread with the precision needed to avoid trapping chromosomes or leaving the daughters permanently joined.

Getting cytokinesis wrong has serious biological consequences. Cytokinesis failure generates tetraploid cells — cells with double the normal chromosome content — that can fuel the chromosomal instability driving tumour evolution. Errors in division plane specification produce daughters of unequal size with asymmetric organelle distribution that disrupt tissue organisation. In asymmetric cell division — a fundamental mechanism in development and stem cell biology — the precise control of cleavage plane position is the mechanism by which two cells with different identities are produced from a single mother. Understanding cytokinesis is therefore not simply about knowing the steps of cell division — it is foundational to understanding development, cancer, evolution, and the origin of multicellularity itself.

Cytokinesis — Definition, Position in the Cell Cycle, and the Logic of Physical Division

Cytokinesis is defined as the division of the cytoplasm following nuclear division (karyokinesis), producing two daughter cells each containing a complete nucleus and a complement of organelles. It is the concluding event of the M phase of the eukaryotic cell cycle — following prophase, prometaphase, metaphase, anaphase, and telophase in which chromosomes are separated — and the act by which one cell physically becomes two. In the standard cell cycle model, cytokinesis begins during anaphase (when the cleavage furrow or cell plate starts to form) and concludes with the complete separation of the two daughter cells, typically one to two hours after anaphase onset in mammalian cells under normal culture conditions.

The conceptual challenge of cytokinesis is understanding how a cell that has just successfully segregated two complete chromosome sets — an impressive feat of molecular precision — then manages to divide its cytoplasm in a way that puts one chromosome set in each daughter. The answer is that the same mitotic spindle that segregates chromosomes also specifies where cytokinesis occurs, using spatial signals from the spindle midzone and the astral microtubule arrays to position the contractile ring or cell plate precisely at the cell equator — between the two separating chromosome masses. This coordination ensures that cytokinesis never proceeds before chromosome segregation is complete, and that the division plane never bisects the chromosomes themselves.

A point of biological diversity worth establishing at the outset: cytokinesis does not occur by the same mechanism in all cell types. Animal cells use an actomyosin contractile ring. Plant cells use a cell plate built from within by the phragmoplast. Prokaryotes use a Z-ring made of the tubulin homologue FtsZ. Fungi use a combination approach. Even within animal cells, some divisions are symmetric (producing two equal daughters) and others are asymmetric (producing daughters of different sizes or fates through deliberate repositioning of the cleavage plane). The molecular principles of force generation and membrane remodelling are conserved, but the structural solutions are remarkably diverse — a reflection of the different constraints imposed by cell wall presence, cell shape, cell size, and developmental context.

Division Plane Specification — How the Mitotic Spindle Tells the Cell Where to Cut

The most consequential decision in cytokinesis is made before the contractile ring assembles a single actin filament: where, precisely, to place the division plane. In most animal cells, the answer is determined by signals from the mitotic spindle during anaphase. Getting this position wrong — dividing off-centre, or at the wrong angle — would produce daughter cells of unequal size, with unequal chromosome complements, and potentially with incorrect identities in developmental contexts where asymmetric division is programmed. The spatial information provided by the spindle is therefore translated into a cortical actomyosin signal with remarkable precision.

The RhoA Signalling Pathway — Translating Spindle Position Into Cortical Contractility

RhoA is a small GTPase — a molecular switch that is active when bound to GTP and inactive when bound to GDP — that serves as the central regulator of contractile ring assembly. Its local activation at the equatorial cortex is both necessary and sufficient to direct cleavage furrow formation: ectopic RhoA activation at any cortical site can generate an ectopic cleavage furrow, even far from the spindle. Understanding the RhoA pathway — its activation by ECT2, its downstream effectors, and the spatial regulation that concentrates its activity at the equator — is the molecular core of animal cell cytokinesis.

The Contractile Ring — Architecture, Dynamics, and Constriction Mechanism

The contractile ring is the molecular engine of animal cell cytokinesis. Despite its central importance, it remains one of the most technically challenging cellular structures to study — it is transient (assembling and disassembling over a period of minutes to hours), thin (approximately 0.2 μm in cross-section), and highly dynamic (filaments and proteins are continuously turning over even as the ring constricts). What is known about its architecture and mechanics comes from a combination of electron microscopy, super-resolution fluorescence imaging, and quantitative biophysical measurements of cortical tension and furrow ingression rate.

STRUCTURAL COMPONENTS
Actin (F-actin):        ~35% of ring protein mass · unbranched filaments · formin-nucleated
Non-muscle myosin IIA/B: Motor domain · bipolar filaments · sliding force generator
Anillin:               Scaffold · crosslinks actin + myosin + septins + membrane
Septin collar:         Filamentous ring · diffusion barrier · membrane curvature sensor
α-actinin:             Actin crosslinker · stabilises antiparallel actin bundles in ring
Cofilin / ADF:        Actin severing · promotes ring thinning during constriction
Tropomyosin:           Stabilises actin filaments · modulates myosin II binding

DIMENSIONAL PROPERTIES (Human HeLa cell)
Initial diameter:      ~20 μm (equatorial circumference ~63 μm)
Final diameter:        ~1–2 μm before abscission
Ring width:           ~0.2 μm (cross-section) · ~4 μm cortical band (total)
Ingression rate:       ~1–3 μm/min in mammalian cells
Force generated:      ~5–10 nN (estimated from cortical tension measurements)

DYNAMICS
Actin turnover half-life:  ~20–30 seconds in the ring (FRAP measurements)
Myosin II turnover:        Similar rapid exchange · ring is not a static structure
Key insight:  The ring constricts while continuously incorporating new material
             Density per unit length is maintained as circumference decreases

How the Ring Constricts — The Molecular Sliding Mechanism

The force that drives cleavage furrow ingression is generated by the same fundamental mechanism as skeletal muscle contraction — sliding of actin filaments by myosin II motor proteins — but in a geometrically very different arrangement. In muscle, actin and myosin are organised into sarcomeres with defined polarity. In the contractile ring, actin filaments are arranged in a mixed-polarity bundle around the cell equator, with myosin II bipolar filaments cross-linking adjacent antiparallel actin segments. When myosin II hydrolyses ATP and undergoes its power stroke, antiparallel actin filaments are slid past each other, reducing the ring’s circumference. This is not a simple contraction — the ring must simultaneously disassemble as it contracts to avoid piling up actin and myosin at the centre, which would block furrow ingression. Cofilin and other actin-severing proteins help manage filament length and ring mass throughout constriction.

Membrane Addition During Furrow Ingression

As the contractile ring constricts and the cleavage furrow deepens, the total surface area of the two daughter cells being created is substantially greater than the original surface area of the mother cell — because two roughly spherical cells have more surface area combined than one cell of the same volume. This membrane deficit must be compensated. Vesicles from the recycling endosome pathway and from the Golgi are delivered to the cleavage furrow by Rab11-positive recycling endosomes, providing the additional membrane required to accommodate expanding daughter cell surfaces. The exocyst complex — a tethering factor for exocytic vesicles — concentrates at the cleavage furrow and is required for efficient furrow ingression. Blocking vesicle delivery to the furrow causes ingression stalling, establishing membrane trafficking as an active requirement for cytokinesis rather than a passive consequence of ring constriction.

Cleavage Furrow Ingression — From Equatorial Constriction to Intercellular Bridge

Cleavage furrow ingression is the visible process of cell constriction — the progressive inward movement of the equatorial plasma membrane driven by contractile ring contraction. It begins during anaphase, as soon as contractile ring assembly is established at the equatorial cortex, and proceeds at approximately 1–3 μm per minute in human tissue culture cells until the diameter of the cytoplasmic connection narrows to approximately 1–2 μm. At this stage, the two daughter cells remain physically connected by the intercellular bridge, but their cytoplasms are already substantially separated and their individual identities established.

A critical feature of furrow ingression is that it must be coordinated with chromosome segregation to ensure that no chromosomal DNA is present in the equatorial plane when the furrow closes. The spindle assembly checkpoint ensures chromosomes are fully segregated before anaphase begins, but even after anaphase onset, chromosomes must clear the equatorial zone as the furrow progresses. The abscission checkpoint (discussed in detail later) provides a final safeguard — detecting chromatin bridges in the intercellular bridge and delaying abscission until they are resolved. Failure of this coordination produces chromosome bridges severed by the abscission machinery, generating double-strand breaks and contributing to the chromosomal rearrangements seen in cancer genomes.

The Midbody — Structure, Signalling Platform, and Developmental Functions

When cleavage furrow ingression has narrowed the cytoplasmic bridge to approximately 1–2 μm in diameter, the remaining antiparallel microtubule arrays from the central spindle compact into a highly organised structure called the midbody (also called the Fleming body, after Walther Flemming who first described it in 1891). For decades, the midbody was considered simply a structural remnant of the mitotic spindle awaiting disposal. Its recharacterisation as an active signalling platform — concentrating over 150 proteins involved in abscission, cell polarity, cilia formation, and stem cell fate — is one of the more remarkable conceptual revisions in recent cell biology.

Abscission — ESCRT-III, Membrane Fission, and the Final Severance

Abscission is the membrane scission event that severs the intercellular bridge and completes cytokinesis. It is mechanistically distinct from all the preceding steps of cytokinesis — it is not driven by actomyosin contraction, does not involve force generation by molecular motors, and requires an entirely different protein machinery: the ESCRT system. The ESCRT (Endosomal Sorting Complexes Required for Transport) pathway was originally characterised for its role in forming intraluminal vesicles within multivesicular bodies (MVBs) — and the analogy between that process and cytokinetic abscission is precise: both involve membrane constriction and fission from the cytoplasmic side (rather than from inside the vesicle or from outside the cell), a topology in which the classical dynamin-mediated fission machinery does not operate.

Plant Cell Cytokinesis — Phragmoplast Assembly and Cell Plate Construction

Plant cells cannot divide by cleavage — their rigid cellulosic cell walls cannot be constricted by an actomyosin ring. Instead, plant cells build a new dividing wall — the cell plate — from scratch within the dividing cell, growing it outward from the cell centre until it reaches the existing parental cell wall, at which point the cell is divided into two. This construction process is guided by the phragmoplast, a plant-specific structure composed of antiparallel microtubule arrays and actin filaments that delivers the building materials for the cell plate by directing vesicle trafficking from the Golgi apparatus.

Binary Fission — FtsZ, the Divisome, and Prokaryotic Cell Division

Prokaryotic cell division — binary fission — proceeds by a mechanistically simpler but conceptually parallel process to eukaryotic cytokinesis. The structural heart of bacterial cytokinesis is the Z-ring: a polymer of FtsZ protein assembled at the cell midpoint that generates or organises the constriction force needed to divide the bacterial cell. FtsZ is a GTPase that is structurally homologous to eukaryotic tubulin — establishing a deep evolutionary connection between the cytoskeletal machinery of chromosome segregation in eukaryotes (microtubules) and cell division in bacteria.

Asymmetric Cytokinesis — How Division Plane Position Creates Cellular Diversity

In symmetric cytokinesis, the cleavage plane bisects the cell equally, producing two daughters of the same size and composition. In asymmetric cytokinesis — a fundamental mechanism in development and stem cell biology — the division plane is deliberately offset, or the spindle is oriented non-centrally, producing daughters of different sizes, with different organelle distributions, and often with different cell fates. Asymmetric division is not a failure of symmetric division: it is a precisely controlled developmental mechanism that generates cellular diversity from a single progenitor.

Cytokinesis Failure — Tetraploidy, Genomic Instability, and Cancer Connections

When a cell completes mitosis — segregating its chromosomes to two poles — but then fails to physically divide its cytoplasm, the result is a binucleate, tetraploid cell: a single cell containing two complete nuclei and four times the haploid chromosome content. Cytokinesis failure is not merely a cell biology curiosity — it is increasingly recognised as a significant source of genomic instability that contributes to cancer initiation and progression, and as a physiologically programmed event in certain specialised cell types where polyploidy is a feature rather than a bug.

Physiological Cytokinesis Failure — When Polyploidy Is the Point

Not all cytokinesis failure is pathological. Several cell types undergo programmed cytokinesis failure as part of their normal differentiation. Megakaryocytes — the precursors of platelets — undergo repeated rounds of karyokinesis without cytokinesis (endomitosis), becoming polyploid cells with DNA content reaching 32N or even 128N. This extreme polyploidy equips them to produce thousands of platelets by shedding cytoplasmic protrusions (proplatelets) without requiring additional rounds of cell division. Cardiac myocytes in post-natal mammals complete mitosis but frequently fail to complete cytokinesis, producing binucleate cells that persist throughout adult life. Trophoblast giant cells in the placenta similarly become polyploid through repeated cell fusion or failed division, acquiring the biosynthetic capacity needed for their enormous hormone-secreting activity.

Pathological Cytokinesis Failure — Routes to Aneuploidy

In cancer contexts, cytokinesis failure can arise through multiple mechanisms: treatment with taxol (stabilises microtubules, preventing mitotic exit and furrow initiation), inhibition of Aurora B kinase (disrupts the CPC and central spindle signals for cleavage plane specification), expression of oncogenic RAS (drives premature mitotic exit before cytokinesis completes), cell-cell fusion events, or mutations in cytokinesis genes themselves (RACGAP1/MgcRacGAP, RHOA, DIAPH3, ECT2). The resulting tetraploid cells are unstable: their four centrosomes predispose them to multipolar mitoses in subsequent divisions; their excess genetic material may deregulate dosage-sensitive tumour suppressors; and their enlarged size may impair normal tissue homeostasis. According to research published by the National Cancer Institute, chromosomal instability — to which cytokinesis failure contributes — is a hallmark of many aggressive cancers and a major driver of drug resistance through tumour heterogeneity.

Cytokinesis Checkpoints — The NoCut Pathway and Abscission Control

Cytokinesis has its own checkpoint system — distinct from the spindle assembly checkpoint of mitosis — that ensures abscission does not proceed until it is safe to sever the intercellular bridge. The NoCut pathway (also called the abscission checkpoint) monitors the integrity of the intercellular bridge for the presence of chromatin — chromosomal DNA that has not cleared the bridge — and delays abscission until any chromatin bridges are resolved, preventing their mechanical severance by the abscission machinery.

Frequently Asked Questions About Cytokinesis