Why some of our cells cease to divide by mitosis

[ajilololo]

Hi, this question has been bugging me off..why some of our cells like heart cells or nerve cells do not divide once they reach the maturity? As we know, our cells constantly divide to replace the old or dead cells so why this happen to these two cells ? And it is true only our nerve,heart and brain do not divide?

 

[Drakkith]

Heart and brain cells are mostly mature cells whose replication machinery has been switched off since it isn't needed. However it isn't true that they cannot divide anymore. Both the brain and the heart are capable of limited repair. Not much, especially in the central nervous system, but some. When injured, some of the cells will divide to replace the dead and damaged cells, though this is never a 100% repair. Nerve cells have connections to thousands of other cells and their axons sometimes run for several feet. A new nerve cell simply cannot reach out and grow to replace all of the connections that the old cell had because of the complicated way nerve cells are connected and because the environment in the central nervous system is fairly hostile to repair (see here). Losing a nerve cell generally means a permanent loss of function. That's why brain degeneration diseases are so devastating.

And it is true only our nerve,heart and brain do not divide?

Nope. As I said, those cells are capable of dividing again in the right circumstances. However there are cells that don't divide at all. Red blood cells contain no DNA and are truly incapable of dividing once they are mature.

 

[ajilololo]

TQ so much for your explanation:-):-)

 

[Andy Resnick]

Hi, this question has been bugging me off..why some of our cells like heart cells or nerve cells do not divide once they reach the maturity? As we know, our cells constantly divide to replace the old or dead cells so why this happen to these two cells ? And it is true only our nerve,heart and brain do not divide?

That's not exactly true- stem cells (progenitor cells) divide and replace dead cells. Most of your tissues consist of 'terminally differentiated' cells- cells that have permanently exited the cell cycle. Some organs have a 'cache' of stem cells (skin, intestine, bone marrow, ) but many others do not. The organ I study, kidney, does not appear to have a cache of stem cells, yet the tubular epithelial cells only live a couple of weeks- it's currently unclear how your kidney maintains itself:

https://www.ncbi.nlm.nih.gov/pubmed/20148680

 

[BillTre]

Elaborating on @Andy Resnick's reply:
In many muscles (speaking of muscles as a tissue, made up of a variety of different cell types) at least, there are in addition to the contractile cells (filled with myofibriles) satellite cells. They are pretty insignificant looking compared to the contractile cells). Here's a picture.

In skeletal muscle, when a muscle is injured, the contractile cells and other damaged cells die and get removed by things like macrophages.
Then some of the satellite cells proliferate (divide) and start forming new contractile cells.

There could be multiple reasons why terminally differentiated cells don't divide:

• The molecular equipment for division (the gene products required to do it) may (for unknown reasons involving how the cell differentiates (takes its final form) be permanently turned off.
  
• Certainly in muscle and nerve cells (new nerve cells come from stem cells or left over neuroblasts) there is a significant cytoskeleton which would be in the way of the cell undergoing division. Either it would have to be dismantled, or the cell body might bleb out of the old cell (ouze out the side of the cell in a little blob of cytoplasm) in order to divide. I don't know of this happening.
  
• Many muscle cells are also multi-nucleate syncytiums, formed by the fusion of separate precursor cells. This (many nuclei) could present an additional problem for the cell to organize a division.

Healthy multi-nucleate cells can undergo a kind of division where they split longitudinally to increase muscle mass when needed. This is more of a response to exercise than a response to injury however.