Biology · Development, Differentiation & Stem Cells
Cell movement, adhesion and tissue morphogenesis
Eight cells form a patch eight units wide and four units tall. Later the same eight cells form a patch four units wide and eight units tall. Nothing has divided. The tissue has become narrower and longer through rearrangement: changing an organism's shape need not begin by adding more cells.
Gene Regulation & Cell Signalling provides receptors, gene regulation and feedback; Cell Architecture, Growth & Division provides adhesion, cell shape, division and cell death. Gene Expression explains how cells can use a shared genome differently. Begin with those ideas rather than memorising a catalogue of embryonic stages. The numerical cases here need addition, multiplication and reading simple coordinates. Rules for the invented models are introduced beside them; no earlier knowledge of stem-cell assays or embryo anatomy is assumed.
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Account separately for cell number, cell geometry and neighbour arrangement when explaining changes in tissue shape.
From cellular activities to organised form
Morphogenesis is the generation and change of biological form. Differentiation asks what properties a cell develops; morphogenesis asks how cells and extracellular materials become arranged into structures. A tissue can contain appropriate cell types yet be incorrectly shaped or connected. Identity and arrangement contribute different parts of a functional organ.
Several activities can change form: cell division, changes in cell shape, movement, neighbour exchanges, extracellular-matrix changes and selective cell removal. Their effects depend on location and orientation. Dividing everywhere is different from dividing along one axis; removing a few cells at a boundary is different from losing an entire layer.
Cells generate and transmit forces through cytoskeletal systems and their connections to neighbours or matrix. Adhesion has to permit both cohesion and suitable remodelling. A drawing of a bent sheet is an outcome to explain, not a measurement of the forces that caused it. Time-resolved observations and appropriately interpreted perturbations help connect molecular activities to geometry.
Narrowing one face can contribute to bending
An epithelial cell has an apical side and a basal side. In our planar cross-section, the first cell is a rectangle four units wide and six units high. The second has an apical width of two, a basal width of six and the same height. Its upper face has narrowed while its lower face has widened, giving a wedge-like profile.
Both drawn areas are 24 square units: 4×6 for the rectangle and (2+6)×6/2 for the trapezoid. Equal cross-sectional area is an assumption of this geometry case. It does not establish equal three-dimensional volume, because the unshown dimension could differ.
Coordinated apical constriction in a sheet can contribute to tissue bending. Its effect depends on the surrounding cells, connections, boundaries and other forces. Research using live imaging of a quail neural plate followed changing junctions and multicellular arrangements during bending; perturbing actin organisation also altered tissue shape. Such evidence links cellular dynamics to morphogenesis without making one wedge shape a complete explanation of every folding tissue.
A narrowed apical face changes a cell's profile
Read the diagram in words
Before: widths 4 and 4, height 6, area 24. After: apical width 2, basal width 6, height 6, area (2+6)×6/2 = 24. The unshown third dimension prevents a volume conclusion.
Before shape change
| Apical edge (Model length units) | 4 |
|---|---|
| Basal edge (Model length units) | 4 |
| Height (Model length units) | 6 |
| Cross-sectional area (Square model units) | 24 |
| Vertex order | Coordinates (x, y) |
|---|---|
| 1 | (0, 0) |
| 2 | (4, 0) |
| 3 | (4, 6) |
| 4 | (0, 6) |
After shape change
| Apical edge (Model length units) | 2 |
|---|---|
| Basal edge (Model length units) | 6 |
| Height (Model length units) | 6 |
| Cross-sectional area (Square model units) | 24 |
| Vertex order | Coordinates (x, y) |
|---|---|
| 1 | (1, 0) |
| 2 | (3, 0) |
| 3 | (5, 6) |
| 4 | (-1, 6) |
Two planar cross-section snapshots of one invented cell. Apical is the upper edge; basal is the lower edge. Model lengths are arbitrary; equal polygon area does not measure or prove conserved three-dimensional cell volume. No tissue curvature or force magnitude is calculated.
- Apical edge
- Basal edge
- Height
- Cross-sectional area
- Model length units
- Square model units
- Vertex order
- Coordinates (x, y)
Rearrangement can extend a tissue
In the eight-cell diagram, each cell remains a two-by-two square. Initially four columns and two rows give width 8 and height 4. Finally two columns and four rows give width 4 and height 8. Total drawn area remains eight cells × four square units = 32 square units.
The final cells have different neighbours. Intercalation describes cells inserting between neighbours; coordinated rearrangements can narrow a tissue along one axis while extending it along another. This process is called convergent extension. Our two endpoint layouts demonstrate the geometric possibility. They do not specify continuous movement paths, the mechanics of each exchange or the speed of a real tissue.
Track labelled cells as well as the outer boundary. A longer patch could also result from growth or division. Conserved labels, unchanged cell size and unchanged cell count are the conditions that make rearrangement the explanation in this particular model.
Neighbour rearrangement can double length without new cells
Read the diagram in words
Before rows are A,B,C,D and E,F,G,H, width 8 and height 4. After rows are A,B; E,F; C,D; G,H, width 4 and height 8. Cell count and individual area stay unchanged.
Before rearrangement
| Row | Column 1 | Column 2 | Column 3 | Column 4 |
|---|---|---|---|---|
| 1 | A | B | C | D |
| 2 | E | F | G | H |
Individual cell dimensions: 2 × 2 (Model length units).
Whole-patch dimensions and area: 8 × 4 (Model length units) = 32 (Square model units).
After rearrangement
| Row | Column 1 | Column 2 |
|---|---|---|
| 1 | A | B |
| 2 | E | F |
| 3 | C | D |
| 4 | G | H |
Individual cell dimensions: 2 × 2 (Model length units).
Whole-patch dimensions and area: 4 × 8 (Model length units) = 32 (Square model units).
Eight labelled square cells occupy two idealised endpoint arrangements. Each square is 2 by 2 model length units. Cells change neighbours and positions with no division, death or change of cell area. These snapshots do not specify continuous movement paths or force laws.
- Four columns and two rows
- Two columns and four rows
- Compare rearranged endpoints
- 8 cells in each panel
- 32 square model units in each panel
- Row
- Column
- Individual cell dimensions
- Whole-patch dimensions and area
- Model length units
- Square model units
Walls change the available mechanics
Plant development also combines regulated division, differentiation and shape change. Meristems provide regions of continuing cell production. Cells with walls can change dimensions through coordinated wall modification and expansion, with turgor contributing the force. They do not generally exchange neighbours by crawling through a sheet as the animal-cell rearrangement picture suggests.
The plant model retains the original four columns, two rows and all neighbour relationships. Cell width stays two units, while cell height doubles from two to four. Tissue width stays eight, height rises from four to eight, and drawn area doubles from 32 to 64. This is expansion of existing cells, a different route to elongation from the constant-area intercalation model.
Expansion can lengthen a patch while neighbours stay the same
Read the diagram in words
Every cell retains its neighbours and width 2 while height increases from 2 to 4. Tissue width stays 8, height doubles from 4 to 8, and drawn area doubles from 32 to 64.
Before expansion
| Row | Column 1 | Column 2 | Column 3 | Column 4 |
|---|---|---|---|---|
| 1 | A | B | C | D |
| 2 | E | F | G | H |
Individual cell dimensions: 2 × 2 (Model length units).
Whole-patch dimensions and area: 8 × 4 (Model length units) = 32 (Square model units).
After expansion
| Row | Column 1 | Column 2 | Column 3 | Column 4 |
|---|---|---|---|---|
| 1 | A | B | C | D |
| 2 | E | F | G | H |
Individual cell dimensions: 2 × 4 (Model length units).
Whole-patch dimensions and area: 8 × 8 (Model length units) = 64 (Square model units).
The same eight cells retain their neighbours in a rectangular plant-tissue teaching model. Each cell's drawn width stays 2 units and height changes from 2 to 4 units. This represents coordinated wall expansion, not intercellular crawling, division or an actual measured growth rate.
- Cell-wall boundary
- 8 cells in each panel
- Neighbour identities retained
- Area 32 square model units
- Area 64 square model units
- Row
- Column
- Individual cell dimensions
- Whole-patch dimensions and area
- Model length units
- Square model units
Worked example
Two ways to make a taller patch
For intercalation, compare height 8/4 = 2 and width 4/8 = 1/2. The patch doubles its height while halving its width, so 4×8 = 32 equals the starting area 8×4. Its eight cells retain their individual four-unit areas.
For wall expansion, height also doubles, but width is unchanged. The final area is 8×8 = 64. Cell number stays eight, so each drawn cell area doubles from four to eight. The identical height change does not identify the same cellular mechanism. Count cells, measure their dimensions and follow their neighbour relationships.
Check your understanding
If an eight-cell patch lengthens, does that establish that its cells divided?
Show reasoning
No. Rearrangement or expansion can lengthen it without division. The intercalation model retains eight cells and constant area, while the plant expansion model retains eight cells and doubles drawn area.
The wedge model keeps cross-sectional area at 24 square units. What prevents a claim that cell volume was conserved?
Show reasoning
The third dimension is unspecified. Equal area in the shown plane alone does not measure equal volume.
What observation would distinguish our intercalation endpoint from our plant-expansion endpoint?
Show reasoning
Intercalation changes neighbour relationships while maintaining the assigned individual areas. Plant expansion retains neighbours and increases individual height and area. Track cell identities and boundaries rather than measuring only the tissue's height.
What to carry forward
Explain a shape change with cell counts, cell geometry, neighbour relationships and the relevant material connections.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2e — Cell structure, development, plant growth and evolutionary comparison (2018; live section text)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStax, Rice UniversityOpen university textbookBiology 2e — Cell structure, development, plant growth and evolutionary comparison (2018; live section text)Plant cells, walls and the central vacuoleMeristems and plant tissue organisationRoot division, elongation and differentiationMature mammalian red blood cellsDevelopment versus population evolution
- A Lifeact-EGFP quail for studying actin dynamics in vivo (Journal of Cell Biology 223:e202404066, 2024)Yanina D. Alvarez and colleaguesOriginal research — selected text and figure captions