basisphenoid bone hypoplasia MP:0004461 hypoplastic basisphenoid bone underdevelopment or reduced size, usually due to reduced cell number, in part of the base of the cranium between the basioccipital and the presphenoid, which usually ossifies separately in the embryo or in the young, and becomes a part of the sphenoid in the adult; in many animals it persists as a separate bone between the basioccipital bone and the presphenoidal bone decreased basisphenoid bone cellularity MPheno.ontology small basisphenoid bone