decreased axial mesoderm size axial mesoderm hypoplasia MPheno.ontology MP:0012273 2013-11-14T11:49:43Z csmith underdevelopment or reduced size, usually due to a reduced number of cells, of the part of the middle of the three primary germ layers of the embryo which includes the prechordal mesoderm and the chordamesoderm and gives rise to the prechordal plate and to the notochord hypoplastic axial mesoderm