I have recently experienced what I believe to be this artefact while slicing
sections of squirrel brain.  I know that this was not properly sunk and also that
it was likely in too little volume of paraformaldehyde (4%) fixative.  The person
who gave me the specimen is not very experienced, but wanted to slice it anyway.
My question is, does insufficient cryoprotection from sucrose cause ventetian
blinds?  I use a cryostat set at -20C chamber temperature and -18C object
temperature, and Tussue Tek OCT medium for embedding the specimen.