Life Unfolding: How the human body creates itself by Davies Jamie A

Author:Davies, Jamie A. [Davies, Jamie A.]
Language: eng
Format: epub
Publisher: Oxford University Press, USA
Published: 2014-01-09T23:00:00+00:00

13

WIRED

Only Connect! … Live in fragments no longer.

E. M. Forster

Of all of the organs that the human foetus develops, the central nervous system is surely the most remarkable. When finally complete, it consists of tens of billions of individual cells, each of which can be connected so densely that a single cell can be connected to well over a thousand others. There are areas of the human brain in which there are around one hundred million connections in every cubic millimetre. For comparison, the microprocessor at the heart of the computer on which this book is being written contains far fewer connections than the brain. If we compare the number of nerve cells (neurons) in the brain with the number of transistors in that chip—a very conservative comparison because each neuron is, in terms of its function, more like a full microprocessor than a simple transistor—we find that the brain has about three million neurons for every transistor. In cooperating to develop such an organ, the simple cells of the embryo have to achieve feats of assembly that are massively more complicated than those we have so far achieved with our most advanced engineering. Though we have very much more to learn about how they do this, recent decades have produced at least the first glimmerings of an understanding.

The central nervous system develops from the neural tube of the early embryo, which is, in turn, formed by the in-folding of the neural groove along the back of the embryo (Chapter 5). The part of this tube that will be in the future head enlarges to become the brain and the rest becomes the spinal cord. Once the basic tube of the central nervous system has formed, the next phase of its development is dominated by cell proliferation. This runs faster than would be needed for the length of the neural tube to keep up with the general growth of the embryo, and this leaves ‘extra’ cells that are used to thicken the walls of the tube. The process of wall thickening involves a somewhat complicated choreography of cell movements, and movements of nuclei within cells, but the result is simple enough: production of a series of layers. Broadly speaking, the left and right sides of the neural tube are thickest, while the very top and the floor are thin.

Previous chapters have already considered patterning along two dimensions of the neural tube. Patterning along the tube was achieved by the chromosome-opening processes that activated different combinations of HOX genes at different levels of the head–tail axis (Chapter 6). Patterning across the whole tube was achieved by gradients of signalling molecules that spread up from the floor plate and down from the roof to instruct cells to develop into different types according to their position on that axis (Chapter 7). The formation of layers in the side wall of the neural tube adds a third, radial, direction of patterning (Figure 64). Cells finding themselves in a layer next to the cavity at

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