Re: HOW LONG DOES THE AVERAGE ANIMAL CELL LIVE?

	To tackle the question of how long a cell lives we should define 
the lifespan of a cell.  When we think of life spans, we usually mean a 
period of time that begins with birth and ends with death.  Cells, 
however, are not really born the way humans are born.   Cells reproduce by 
mitosis.  Mitosis is a biological process that involves dividing into 2 
parts. Each time a cell divides, it makes a copy of itself and then 
divides in half.  The idea of cells coming from other cells is credited to 
Rudolph Virchow a German scientist who lived in the 1800s.  Virchow is 
famous for the phrase "omnia cellula e cellula" which means all cells from 
cells.  Because cells are created from the division of cells, any cell 
that is alive today came from another cell that went through mitosis.  And 
that other cell came from another cell which came from another cell... ad 
infinitum...until you trace back to the first ancient cells-- a topic that 
challenges scientists today.   
	Cell death is also a fascinating topic that thousands of 
scientists across the world are working to learn more about.  Cells can 
die by either necrosis or apoptosis.  Necrosis is what happens if a cell 
gets severely damaged, as by infection or trauma. For example, if you get 
an ulcer or a bruise, injured cells will die.  Apoptisis is a form of auto-
destruction that occurs when a cell is worn out or mildly stressed.  
	Both intrinsic cell factors and the external environment of a cell 
contribute to careful regulation of the cellular reproductive cycle.  
Chemicals,  proteins, gene expression, and even changes in temperature or 
pressure act as signals that influence cell growth, mitosis, and 
apoptosis.  A carcinogen is a substance that causes cancer by changing the 
regulation of the reproductive cycle of a cell to cause unregulated growth-
- either by stimulating mitosis or interfering with apoptosis.  Scientists 
have been able to keep cancer cells alive in cell cultures for years after 
the people who donated the cells died.  While cancer cells may be able to 
live forever, they are not average animal cells.
	The lifespan of average animal cell depends on the type of cell in 
question.   In thinking about cell replication and tissue renewal, 
scientists divide adult mammalian cells into three groups: Continuous 
replicators, Discontinuous replicators, and Nonreplicators.  
	Continuous replicators include blood cells and cells that line 
your small intestine.  They are cells with high turnover that undergo 
apoptosis after a relatively short period of time.  Red blood cells live 
for about 120 days.  Cells that line the gut live for 3-5 days.   Certain 
B cells, part of the immune system, have a half life of 5-6 weeks, while 
others live for only a few days.  The continuous replicators undergo 
regular mitotic activity.
	Discontinuous replicators include cells of cartilage, smooth 
muscle, pancreas, kidney tubules, liver, fibroblasts, and bone.  These are 
cells that are part of stable cell populations.  They last for days, 
months or years, depending on how much they get used.  For example, cells 
of the pancreas live for about a year.  Endothelial cells that line blood 
vessels can live for months to years, but if a vessel is injured, they can 
grow in reproduce in days to repair it. Discontinuously replicating cells 
undergo mitosis to rebuild stable cell populations after injury and 
wearing down.  If a tissue is injured and cells are killed by necrosis or 
apoptosis, growth factors stimulate neighboring cells to reproduce.
	Nonreplicators are the cells that live the longest, including 
neurons, heart muscle cells, renal glomeruli, and some cells in the lens 
of the eye.  These cells are "postmitotic."  This means that after a short 
time of development and differentiation, the cells don't divide anymore.  
These cells last for a person's whole life without ever being replaced.
For more information you can read these references:

1. Alberts, B, Bray, D, Lewis, J, Raff, M, Roberts, K Watson, J.  
Molecular Biology of the Cell. Third Ed. 1994. Baltimore: Garland 
Publishing. Co. Chapter 22, especially. 

2. Cotter, TG; al-Rubeai, M. Cell death (apoptosis) in cell culture 
systems. Trends in Biotechnology, 1995 Apr, 13(4):150-5.

3. Fulcher, DA; Basten, A. B cell life span: a review.  Immunology and 
Cell Biology, 1997 Oct, 75(5):446-55.

4. Post, J; Hoffman, J. Cell renewal patterns. New England Journal of 
Medicine, 1968 Aug 1, 279(5):248-58

5. Ross, M H. Romrell, L J. Kaye, GI. Histology: AText and Atlas Third ed. 
Baltimore: Williams and Wilkins.1995.