Essays on Science and SocietyGE PRIZE ESSAY

The Molecular Basis of Size Differences

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Science  04 Dec 2009:
Vol. 326, Issue 5958, pp. 1360-1361
DOI: 10.1126/science.1184444

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Size differences account for a great deal of the diversity found in the animal kingdom, but we still have much to learn about how sizes are programmed. Generally, the cells of different animals are comparable in size, and all animals begin as a single cell. This leaves the number of cells accumulated as the main determinant of animal size. We can reasonably expect the genes controlling cell number to be conserved among animals. So it seems that size-determining genes must be deployed in the elephant in such a way that it amasses several hundred thousand times more cells than the mouse. Which are these genes and how do they control size? I asked this question in a more experimentally tractable context: How do body parts of a single animal become different sizes? Fingers, toes, and ribs are sets of structures whose members are similar in form but differ in size. Although we know that Hox transcription factors specify the identity of individual fingers, toes, and ribs, little is known about how their individual sizes are programmed.