Figure 22–38 The continuing production of neurons in an adult mouse brain. the brain is viewed from above, in a cut-away section, to show the region lining the ventricles of the forebrain where neural stem cells are found. these cells continually produce progeny that migrate to the olfactory bulb, where they differentiate as neurons. the constant turnover of neurons in the olfactory bulb is presumably linked in some way to the turnover of the olfactory receptor neurons that project to it from the olfactory epithelium, as mentioned earlier. in adult humans, there is a continuing turnover of neurons in the hippocampus, a region specially concerned with learning and memory. (adapted from b. barres, Cell 97:667–670, 1999. with permission from elsevier.)
Another example of the use of stem cells is in the repair of the skin after extensive burns. By culturing cells from undamaged regions of the burned patient’s skin, it is possible to obtain epidermal stem cells quite rapidly in large numbers. These can then be used (through rather long and complicated procedures) to repopulate the damaged body surface.
neural Stem Cells Can be manipulated in Culture and used to Repopulate the Central nervous System
The central nervous system (the CNS) is the most complex tissue in the body, at an opposite extreme from the epidermis. And yet fish and amphibians can regenerate large parts of the brain, spinal cord, and eyes after they have been cut away. In adult mammals, however, these tissues have very little capacity for self-repair, and stem cells capable of generating new neurons are hard to find—so hard to find, indeed, that for many years they were thought to be absent.
We now know, however, that neural stem cells that generate both neurons and glial cells do persist in certain parts of the adult mammalian brain (Figure 22–38). Neuronal turnover occurs on a dramatic scale in certain songbirds’ brains, where large numbers of neurons die each year and are replaced by newborn neurons as part of a process by which the birds refine their song for each new breeding season. In the adult human brain, there is a continuing turnover of neurons in the hippocampus, a region specially concerned with learning and memory. Here, plasticity of adult function is associated with turnover of a specific subset of neurons. About 1400 fresh neurons in this class are generated every day, giving a turnover of 1.75% of the population per year.
Fragments taken from self-renewing regions of the adult brain, or from the brain of a fetus, can be dissociated and used to establish cell cultures, where they give rise to floating “neurospheres”—clusters consisting of a mixture of neural stem cells with neurons and glial cells derived from the stem cells. These neurospheres can be propagated through many cell generations, or their cells can be taken at any time and implanted back into the brain of an intact animal. Here they will produce differentiated progeny, in the form of neurons and glial cells.
Using slightly different culture conditions, with the right combination of growth factors in the medium, the neural stem cells can be grown as a monolayer and induced to proliferate as an almost pure stem-cell population without attendant differentiated progeny. By a further change in the culture conditions, these cells can be induced at any time to differentiate to give either a mixture of neurons and glial cells (Figure 22–39), or just one of these two cell types, according to the composition of the culture medium.