hayflick numbers

[jmsdean at pipeline.com]

some of you have seen my posts.  i'm just a lay person with a large degree
of curiosity in cellular ageing.  i'm also just getting started.  i went
through a lot of Medline abstracts today so i'm learning. 
 
my question is:  is the Hayflick number always 50 +/- 10 ? 
                              or is it different for each type of cell? 
                               what happens when a cell reaches its
hayflick number? 
 
any replies are much appreciated.   
 
luke