Re: Are the cells in the G0 (g zero) phase of mitosis really suspended ?

Thanks for your question!  

The National Center for Biotechnology Information has a fantastic database, 
called PubMed, which is very useful for finding information from biomedical 
literature.  
 http://www.ncbi.nlm.nih.gov/
PubMed/  

I found quite a few articles on the G0 phase of the cell cycle using PubMed.

Growth and division of cells is called the cell cycle. The phases of the 
cell cycle are:

     1.G1  The cell grows in size, and senses the cell environment.
     2.S   DNA synthesis, or replication
     3.G2  The cell checks the new DNA for mutations 
     4.M   Mitosis, the cell divides.

Following mitosis, the daughter cells may re-enter the G1 phase, or a phase 
called "G0", where growth and replication stops. Cells
in G0 are said to be "quiescent". G0 cells may eventually re-enter G1 or 
perhaps die. 

For a more thorough description of the cell cycle, please see my previous 
answer to a similar question:
http://www.madsci.org/posts/archives/oct98/906391245.Cb.r.html

The distinction between G0 and G1 is not a very good one, and to be honest, 
in the literature the terminology is pretty inconsistent.  Often, this phase 
is refered to as G0/G1.  When people use the term G0, it implies the cell 
will not be re-entering the cell cycle (will not be dividing).  The term is 
often used to apply to senescent (old) cells, muscle cells, or neurons.  G0 
cells have low levels of cell cycle proteins, such as cyclin dependent 
kinases, and low levels of growth factor signalling proteins such as ras and 
myc.  These proteins are necessary for the cell to traverse G1 and go on 
through the cell cycle. "G1" cells are primed to go through the cell cycle, 
they are just waiting for an appropriate signal to divide.  This signal can 
be a growth factor or a hormone, for example. "G0" arrested cells CAN enter 
G1 and go through the cell cycle in some circumstances, and this is where 
the terminology gets pretty shaky.

See:  Tianen, M et al. Terminally differentiated skeletal myotubules are not 
confined to G0 but can enter G1 upon growth factor stimulation.  Cell Growth 
and Differentiation 1996 Aug. 7(8): 1039-50


If this is confusing, don't worry. I have found in my cell cycle research 
that it is not critical to define the exact differences between G0 and G1.  
I suspect as we learn more about cells, the distinction will continue to 
blur.  What IS important is that G0/G1 cells are not doing "nothing".  This 
is a very important phase of the cell cycle.  For one thing, the cells have 
to grow.  They have to store up nutrients for the "active" phases of the 
cell cycle.  They perform their normal tissue specific functions. More 
importantly, G0/G1 is the time the cells sense and respond to the 
extracellular environment.  They determine whether it is a good time to 
divide (plenty of nutrients, appropriate hormones etc) or a bad time (low 
nutrients, signals to remain quiescent).  Most cells in the adult remain in 
G1 most of the time. In cancers, this stage of the cell cycle is 
disregulated, and the cells divide and make a tumor even though they are 
recieving signals NOT to divide. 
 
So, I hope I have convinced you that G1 is an interesting and important 
phase of the cell cycle, and that the distinction between G0 and G1 is not 
critical to worry about (at least in most cell types).

Hope that's helpful!

Erin