Subject: What exactly is referred to when researchers measure crypt length?

I am trying to decipher research that examines the "guts" of rats.  Using the 
internal organs of rats, researcher compared the guts of rats fed genetically 
modified potatoes to those who were not.  The researchers measured the crypt 
length of the internal organs (in micrometers) and found differences.  I know 
that it is something cellular, but I am not clear as to what the researchers are 
actually referring to. I would appreciate any insight.  Thank you.

Re: What exactly is referred to when researchers measure crypt length?