Nervous tissue

The regenerative capacity of nerve tissue is very limited compared to other tissues, especially since nerve cells are no longer able to divide.

In early embryonic development, the nervous system anlage is for some time the region with the highest rate of cell divisions, and fetally, at peak times in humans, several thousand young neurons are produced per second. But these neurons are no longer capable of cell divisions afterwards, postmitotic. And not all of them live as long as the organ of the organism in whose tissue they seek their place (see selective apoptosis).

In the adult brain, only in a few regions undifferentiated neural precursor cells remain, which can continue to divide and are capable of forming neuroblasts and young neurons (see adult neurogenesis). In humans, for example, young neurons can also be formed alongside glial cells, for example in regions of the hippocampus or in the subventricular zone, to replace neurons in the olfactory bulb and olfactory mucosa. To do this, these young neurons must migrate into that brain region and search for a place (with chemotaxis or haptotaxis), extend extensions (axogenesis), form transmission sites (synaptogenesis), establish contacts in the found network of other neurons, receive signals and send signals, and finally also those with which the excitation state of certain individual other cells can be changed (excitation or inhibition).

On the way there and in the process along the way, a neuron differentiates - to take a place in a cellular environment with specific connections. If it does not succeed, the neuron does not survive for long. If it succeeds, the neuron occupies a special place in the neuronal network - and can only be replaced in this place by young neurons that follow a similar differentiation process. But these cannot be formed from mature neurons by cell division. For this, the neurons would have to round off, the processes would regress, lose their contacts and thus become non-functional. The replacement of differentiated and functional neurons within a neuronal network is therefore limited by the complexity of the neuronal connections.

In the peripheral nervous system, on the other hand, after damage to a nerve fiber, the extension of a neuron can grow back into the canal of the medullary sheath as an axon - if it is still present - at about the same rate as hair grows.