Certain vertebrates, including some strains of mice can regenerate complicated structures like limbs. How do the cells know when the finger is long enough, or that a joint goes there, for example? What if the injury is a gnarled complicated mess, and not just a clean cut? Will the cells know what needs fixing, and what doesn’t? Any experts out there?
Frankly, it is not very well understood, and what is understood about it (some of has to do with concentration gradients of certain biochemical substances) is complicated and confusing. This is at the frontiers of research, and has been for thirty of forty years, because the problem is so difficult. There is progress, but it is slow and incremental, and the problem is huge. (If we could answer your question satisfactorily, we could probably cure cancer.)
I guess not much has changed since the last time I read up on this…many years ago! I just can’t imagine a mechanism by which this kind of precise self-assembly occurs.
It happens when a lot of individual entities each follow a few simple rulesw. Complex patterns can emerge from this. The individual entities are not working towards the design of the complex system, it just happens.
The entities might be cells in a body, or, for example, termites.
A termite mound is a complex structure, with a sophisticated system of circulating air among other features. Yet it is not built to a design, it just happens when a lot of termites each follow a simple set of rules.
Find some earth and chew it with saliva to create a pellet.
Wander around at random.
When you find a raised area, drop the pellet.
If possible, drop your pellet on top of another pellet, forming a pillar.
Yes, but the termite example shows the termites using judgement. Cells don’t have judgement. How do they know how long a finger is and when to make a joint? It’s easy for us to say “build the section until it’s 2 cm long” but cells can’t “know” what’s long enough. The only thing that makes sense is the aforemetioned discovery that the developing cells secrete some kind of substance, and when there’s enough of whatever chemical gets secreted, they stop multiplying and start making something else. But that would also mean that the cells are detecting these gradients in 3 dimensions (thickness, depth, length), which is quite a feat.
If you do some reading on embryology, you’ll start to get an idea how the body’s put together in the first place. Regenerating lost bits is a similar process. I don’t pretend to know very much about it, just enough to really want to go learn more by buying a textbook and taking a class. It has a lot to do with chemical gradients and cell signalling, as you mentioned. Certain genes may need a molecular trigger to be switched on, and some genes will need a stronger signal, so if you have a single source of a signalling molecule, cells further from it will express different genes than those close to it.
The cells aren’t “detecting” the gradients per se, they’re just living in their environment and transcribing their genes, but that transcription will be affected by the nature of their environment, and cells a little ways apart will have a different enough environment to tweak transcription and end up with a differently active genome.
If you want to consider it quite a feat, don’t let me stop you. I’m all for people being blown away by the complexity and wonder of biology. I know I am. But you should understand that this a very basic and fundamental process underlying all embryonic development in pretty much any creature more advanced than a sponge, and I’d have to double check on the sponge. There are hundreds of millions of years of evolution behind that feat, and the networks of signals, detectors, and responses are incredibly complicated and vast.
What you’ve got is a 2-dimensional grid of square cells. Each cell has 8 neighbours.
Cells are born, survive, or die according to a simple set of rules.
1) Any live cell with fewer than two live neighbours dies, as if caused by under-population.
2) Any live cell with two or three live neighbours lives on to the next generation.
3) Any live cell with more than three live neighbours dies, as if by overcrowding.
4) Any dead cell with exactly three live neighbours becomes a live cell, as if by reproduction.
From these rules, complex patterns of behaviour emerge.
Still Life, i.e. stable patterns of cells that don’t change.
Spaceships, i.e. patterns of cells that move across the screen, keeping their original shape.
oscillators, which change back and forth between 2 or more states.
If you start with a random pattern, of just a few live cells, what you will see is that the pattern will grow, initially it’ll seem like a random mess, then still life, spaceships and oscillators will emerge spontaneously. Eventually the pattern is likely to stop growing.
Question - How does the pattern “know” to stop growing?
Answer - It doesn’t. There is no central authority telling it to stop. It stops because eventually every living cell is part of a still life, spaceship, or oscillator. Nothing tells the cells that they should stop, they just do.