Eli5: How do bones and other organs know what shapes to grown into?
In: 3675
Imagine you’re a cell and you have a huge book that serves as a blueprint to build an entire human body. Then one day you divide into 2 cells, so you give each new cell a copy of the blueprint book, but you specifically highlight all of the blueprints to make the upper body in cell 1’s book and you black out the lower body plans. You give the opposite to cell 2. So cell 1 now only really has the instructions for the upper body. But the biggest instruction in that whole half of the blueprint is to divide and give part a of the instructions to one cell and part b to the other cell.
Now the ‘upper body’ cell splits and gives copies of the blueprints that they have to the 2 new cells. One set of blueprints highlights the head and blacks out the torso/arms, while the other cell gets the inverse set of instructions.
So on and so on, every generation the cells keeps getting more and more specific instructions to read from the original book. Eventually you have cells that are just reading an individual chapter. Maybe a chapter that says ‘congratulations. You are the last cell that we need in this location to form the bone. Don’t divide any more, because if you do you will nudge out the lines of the bone and create a deformity.’ So that cell doesn’t divide.
It just lives there as a bone cell. In fact the next sentence in that chapter might read something like ‘collect as much calcium and phosphate as you can from the blood, and pile it around you as densely as you can.’ Now you have a bone cell, keeping that corner of your bone hard, and correctly shaped – at least in his own little corner of your body.
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