A living sheet one cell thick lines your gut. New cells are born in the pits and pushed out along
the surface, where old ones are squeezed out into the gut. Births and removals must match, or the
barrier fails.
Mean cell strain
strain 1.00births/s 0.0extrusions/s 0.0cells 0born 0 / out 0
The blue is the inside of your gut. Food being broken down, digestive juices, and enormous
numbers of bacteria. The single row of cells is the only thing separating all of that from the
tissue below, so the row must never break open.
Green cells at the bottom of each pit are stem cells. They divide to make new cells, which
are then pushed up and out along the surface.
Cells go darker when they are stretched thin. A crowded sheet has short fat cells; a
stretched sheet has thin dark ones.
A red cell has been picked for removal. It first weakens in place, then squeezes out into
the gut while its neighbours seal up underneath it, so the barrier never opens.
Your job: keep the needle in the green band. Set how fast cells are born and how fast they
are removed. Too few births and the sheet is stretched dangerously thin. Too many and the cells
are crushed together.
The blue is the inside of your gut. Food being broken down, digestive juices, and enormous
numbers of bacteria. The single row of cells is the only thing separating all of that from the
tissue below, so the row must never break open.
Now the cells decide for themselves. Every cell has its own size and its own springiness, so
when the sheet is pulled tight the floppy ones stretch furthest. They lose the tug of war, thin out
and darken — and a thin cell is the one most likely to be thrown out next.
Green stem cells feel the strain too. When the sheet is stretched they divide faster and glow
brighter green; when it is crowded they slow down. That is the body's own thermostat, and it is
what keeps the barrier intact without anyone setting the rates.
Two dials set the strength of each effect.Extrusion stretch selectivity is how much a
thin cell is punished. Division response is how hard the stem cells push back. Turn the first
up far past the second and the thermostat loses: throwing out thin cells stretches the rest, which
makes them thinner still, and the whole sheet runs away.
Your job: keep the needle in the green band — but now by tuning the rules rather than the
rates.
Under the hood. The sheet is a chain of springs in series, so tension is uniform along each
crypt–villus–crypt domain and cell width follows w = rest + T·compliance.
Extrusion is a per-cell hazard h = λ·sk and division a local response
λ₀·sm. Linearising about the fixed point gives a stable sheet when
m > k − 1; the defaults sit at k = m = 4.