"Developmental biology" (The Encyclopaedia of Ignorance)

Date:
1975-1976
Reference:
PP/CRI/H/5/18
Catalogue details

Licence: In copyright

Credit: "Developmental biology" (<i>The Encyclopaedia of Ignorance</i>). Source: Wellcome Collection.

    12/60
    strongly and specifically by their contacts with neighbouring cells. We can see that they often form special junctions, of several different types, when they come into contact and we know some molecules can move from cell to cell across some of the junctions. It is a measure of our ignorance that we do not know whether 'positional information' is mediated by small molecules, by cell to cell contact, or by some other process such as signals, analogous in a loose sense to the signals passing down a nerve cell, passing over the tissue. Then again, for a tissue to grow to a fixed size and shape, there must be some control of the divisions of its cells and in many cases over the direction of these divisions. Here again, when we realize what a complicated job has to be done, we can see that our ignorance far exceeds our knowledge. At least we know that it is not always done by a strict lineage mechanism. In some simple organisms - for example a soil nematode - it has been shown that a particular cell in a developing animal will always produce exactly the same descendants. It will divide twice; one of the four grand-daughters will always die and the other three each develop in a special way. If this mother cell is killed early in development, all the three special grand daughter cells will be absent. No other cell will take over the function of the dead cell. But in other organisms - the fruit fly, for example - killing one cell of a group, at an early stage in development, will merely mean that the others divide rather more often so that in the adult the final number of descendants of that group will be roughly the same as usual. Perhaps the most challenging problem in the whole of developmental biology is the construction of the nervous system of an animal. Many years ago it was shown by Sperry that if a newt's eye was removed, so
    digitised image 1
    digitised image 2
    digitised image 3