DAILY MISCELLANY

SCIENCE QUESTION

In an embryo, how do the cells begin to specialise? How do they decide, for example, to become brain cells?

This answer applies to mammals only. Other animals can have very different developmental strategies.

In the initial stages, the fertilised egg divides to form a ball of cells. Cell position in the ball gives the cue for the first differentiation step. Cells on the outer surface of the ball become one kind of tissue (which will contribute to the placenta only) and cells on the inside become another cell type, which will give rise to the baby that is born. It is believed that the number of cell contacts is the key feature: outside equals a few cell contacts, inside equals many.

Once you have two cell types, there is further scope for interaction. Some of the “outside” cells are in contact with “inside” cells, and some are not. This leads to another two different types of cell arising. And so it goes on in a cascade of increasing complexity. During that cascade, genes come into the picture. While every cell has the same set of genes, its position (outside/inside) determines which genes are read and used to make the proteins that the genes encode. Depending on which genes the cell uses, even more

varying sets of genes are activated, and so on, leading to an astonishing number of different tissues such as brain cells, bones and others. Just from one fertilised egg.

WORD OF THE DAY

gadzookery [gad-'zoo-kuh-ree]

The overuse of archaic words and expressions

Edmund was despairing of gadzookery in this timeless exchange from Blackadder II:

Blackadder: Tell me, young crone, is this Putney? Young crone: That it be, that it be! Blackadder: ‘Yes it is’, not ‘That it be’. And you don’t have to talk in that stupid voice to me, I’m not a tourist.