decreased axial mesoderm size axial mesoderm hypoplasia 2013-11-14T11:49:43Z MP:0012273 MPheno.ontology csmith hypoplastic axial mesoderm 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