  Numerous studies of the tumor microenvironment, interspecies xenografting,
and limb regeneration suggest the existence of a tissue spatial code [TSC] that
controls tissue structure in a quasi "epigenetic" fashion. Epigenetic is an
inadequate label, because this information does not act directly upon DNA
molecules like methylation. A broader term is needed to capture the diversity
of three-dimensional spatial codes in biology. One such term is Heritable
Nongenetic Information [HNI], which encompasses the TSC. The term heritable is
appropriate because this information is passed onto offspring, otherwise it
would have disappeared during evolution. Another reason for the heritability of
HNI is that the spatial information observed in tissues is not reducible to the
laws of physics, meaning structures like epithelial tubes or neural circuits do
not spontaneously form in an aqueous solution. Pre-existing physiological
information in the microenvironment is necessary.
