|MadSci Network: Cell Biology|
Mike: Your teacher asks a good question, let me try to answer to some extent, its not as simple as you might think. There are roughly 200 different types of human cells (depending how you define “type”). Most cell types divide in very young children as they grow, but in the adult human, there are cells that (1) do not divide, (2) can be forced to divide but usually do not, and (3) cells that are constantly being renewed at various rates… then there are the unique cases of cancer cells and stem cells for example. In the most general terms, nerve and brain cells and heart muscle cells don’t grow new. If these cells should die, that’s it, you don’t get more. This is why spinal injuries, heart attacks, and strokes are that much more scary and difficult to treat in the hospital. The second group of cells that usually don’t divide are cells like liver cells and kidney cells. Liver is perhaps the best example of cells that do not divide or replicate in an adult human, however a surgeon can remove part of your liver and transplant it to someone else and the rest of your liver will begin to grow back. The third group of cells makes up tissues where brand new cells are always being made to replace “old” cells. Skin cells, the cells in your gut lining, and blood cells are renewed. Blood cells for example are produced from stem cells in the bone marrow and the average adult produces 3-10 billion cells per hour. Oddly enough, one type of neuronal cell that does renew is the olfactory sensory neuron, which actually dies; new are replenished from neuronal stem cells or precursor cells and actually re-wire to the brain. So why? Well, in the simplest terms, many proteins and molecules contained in cells or in a cell’s environment (or both) work to regulate if and/or when a cell divides. There are huge numbers of proteins involved in often complex signaling pathways within a single cell that regulate how the cell functions, what the cell produces, whether it lives or dies, how it communicates with its neighbor cells, and yes… tells it whether to divide or not. Interestingly, it’s the malfunction of these same checks and balances and feedback mechanisms within cells that result in cancers. Mutations of certain genes result in deactivating proteins that inhibit cell division and often over-activate proteins that cause cell division. Many kinds of cells can be grown in the lab, outside the body in artificial atmospheres under a variety of conditions that scientists have or can work out (known as tissue culture or cell culture). This is exciting because this means cells largely all have a capacity to divide. In fact, there are ongoing studies to implant neuronal cells grown this way into stroke patients at several hospitals in the US. The same kind of theory holds true for stem cell research and figuring out how these premature cells “decide” what kind of cell they will become and how they get to and integrate into their environment. Often the key to this kind of science is not how to get the cells to divide, but how to control this division. Keith Anderson Brigham & Women's Hospital, Boston
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