Theory 11/Developmental Biology /Morphogenesis/3 min
Organs Stop at a Stress Set Point
Growth stops when the organ is the right size. The measure of 'right' may be force, not number.
Organ size is set by a mechanical set point rather than by a cell-count programme: growth continues until the tissue's mechanical stress reaches a target, so perturbing tension changes final size without changing the genetic programme.
01The evidence
- Growth-induced stresses have been shown to guide shape changes during morphogenesis, establishing that mechanical state is both a product of growth and an input to it.Seminars in Cell and Developmental Biology
- Models of growing objects have asked whether size can be determined by local controls alone, a question that only arises if the global size is an emergent mechanical property rather than a counted quantity.Local control of growth
02The tension
Size control is explained by counting: cells count divisions, or morphogens set a boundary, or a organ-specific programme runs for a fixed time. Each of these predicts that size should be robust to mechanical perturbation, and size is not robust: compressing a tissue changes its final size, and growing it in a soft matrix changes it too.
03The theory
Cells sense tension through adhesion complexes and respond by proliferating or by stopping. If the target is a stress level rather than a cell number, then growth continues until the tissue relaxes to that level, at which point proliferation stops.
This makes size a mechanical equilibrium. It explains why organs grown in different stiffness environments reach different sizes, and why cutting a tissue can trigger regrowth: the cut changes the stress field and re-opens the set point.
The prediction is that final size should track the set-point stress, not the cell number, and that changing matrix stiffness or applying external load should shift the final size in a predictable direction. It also predicts that tissues with different stiffness should have different size-to-cell-number ratios.
The idea also resolves a puzzle in regeneration: the same tissue can regrow to different sizes depending on the mechanical environment, which a counting model cannot accommodate.
04What this is not
Those models make size a number or a position. This makes it a force, which predicts size changes under mechanical perturbation that counting models cannot explain.
05Predictions
- P1Final organ size should scale with the set-point stress across stiffness environments.
- P2Applying external load during growth should shift final size in the predicted direction.
- P3Partial excision should trigger regrowth to the same stress, not the same cell number.
06What would refute it
Size that is invariant to mechanical environment at fixed genetics would leave counting in control.
07What it would change
It would make tissue engineering a matter of designing the mechanical environment, and explain regeneration limits.
08Evidence base
- How growth-induced stresses guide shape changes during morphogenesishttps://www.sciencedirect.com/science/article/pii/S1084952125000710
- Can the size of a growing object be determined through local controls alone?https://scholar.google.com.vn/citations?view_op=view_citation&hl=vi&user=e6A1gX8AAAAJ&sortby=title&citation_for_view=e6A1gX8AAAAJ:isC4tDSrTZIC