Pore:  This is a freeze fracture
electron microscope image of a
NUCLEAR ENVELOPE and cytoplasm.  
Note the moon-like appearance of
the nucleus.  Each one of these
pits represents a pore.  The pores
allow for communication between the
nuclear and cytoplasmic
compartments.  The number of pores
is so large that an additional
network of fibers called the lamina
is needed to hold the nucleus
together.  The hole is only about
9nm wide.  Molecules of under 5000
mol wt can cross quickly.  In
addition ribonuclear proteins
including mRNA cross.  The barrier
is selective since we know that
only mRNA with poly A tails cross
and the hnRNAs do not.   