Is there a limit to neuron multiplication in the hippocampus?

[gianeshwar]

Hi Friends!
We know that nucleus of neurons remains like a closed blueprint!Is there any evolutionary pressure for cells not to multiply?
Also in hippocampus they can multiply in only a highly limited number.Why different behaviour in hippocampus?
Thanks in advance!

 

[BillTre]

Neurons are (mostly at least) considered to be terminally differentiated cells. This means they turn into neurons and do not divide any more.
I don't really know why neurons can not divide (normally), but it may be due to their vast and intricate cytoskeleton (including microtubules) that underlies their unusual cell shape and function. Dividing cells would have to remodel all that. The shape and therefore function of the cell would be presumably lost.

The new cells in the vertebrate hippocampus (and olfactory bulb) are made by non-neuron cells (neuroblasts or neuronal stem cells), which can divide. The resulting preneurons can then migrate to where they will end up and differentiate into neurons. Neuroblasts are only found in certain places in adult animals.

During insect metamorphosis, the nervous system is extensively remodeled (to deal with the vastly different body that results from changing from a (often worm-like) larvae into an adult insect (often with legs and wings). During this many new neurons are formed from neuroblasts that have been kept around in the larvae. Some old neurons are retained and can completely change their shape. These cells are not know to divide. Since this is done during the behaviorally quiescent period of metamorphosis, their temporary loss of function is not detrimental.

There maybe unusual conditions where neurons might de-differentiate (losing their cell shape) and divide. There were some reports of this in some regeneration conditions, but I believe this turned out to be dividing glia (another non-neuron neural tissue cell type).

 

[gianeshwar]

Thanks BillTree!

 

[Pythagorean]

Good post BillTre. I'll just add that their structure is very delicate and their microtubule pathways to the dendritic system are complicated and always reacting to changes in the brain and channel density in the membrane is constantly being rearranged. In terms of evolution, the intricate structure of neurons is what allows for the mechanisms of plasticity and adaptation to occur in nervous systems - cell division would completely disrupt this process.

Even giving neural cells the basic requirement of rest and maintenance is a difficult task as shutting down neurons would erase their physical memory mechanisms, so organisms with complex nervous systems evolved to sleep. Sleep involves a lot of methodical ways to keep the brain stimulated during unconsciousness, while simultaneously giving groups of neurons a turn to rest intermittently.

 

[gianeshwar]

Thanks Pythagorean!