The Guardian, UK
Wed, 27 Feb 2013 14:26 CST
© Cajal/ Wikicommons Media
"Like the entomologist in search of colourful butterflies, my attention was drawn to the gardens of the grey matter, which contained cells with delicate and elegant forms, the mysterious butterflies of the soul, whose beating of wings may one day reveal to us the secrets of the mind" – Santiago Ramón y Cajal.
These drawings by Santiago Ramón y Cajal
show the cellular structure of three different areas of the human cerebral cortex. The cortex is the seat of higher mental functions such as language and decision-making, and contains dozens of distinct, specialised areas. As Cajal's drawings show, it has a characteristic layered structure, which differs somewhat from one area to the next, so that the layers vary in thickness according to the number of cells they contain.
Cells throughout the cortex are arranged in a highly ordered manner. Those in layers 2 and 3, for example, send fibres to the other side of the brain, whereas those in layers 5 and 6 send theirs straight downwards. This organization is under genetic control and, once established, was thought to be fixed. Now, though, researchers at Harvard University report that fully matured neurons in the intact brain can be made to switch identity and re-route their fibres to acquire the characteristics of cells in other layers.
Cortical neurons are generated in vast numbers during the earliest stages of development, when the nervous system is nothing more than a hollow tube running along the back of the embryo. The inner surface of the neural tube is lined with stem cells called radial glia
, which have a single fibre that comes into contact with the tube's outer surface. These cells divide to produce immature neurons, which then climb onto their mother's fibre and migrate outwards. At the front end of the tube, neurons migrate away in waves, and those produced early on form the inner-most layer of the cortex. Subsequent waves of cells migrate past the earlier ones, so that the layers form from the inside out.