abnormal midbrain-hindbrain boundary morphology absent midbrain-hindbrain boundary 2013-10-08T15:53:46Z MP:0012133 MPheno.ontology absence of the midbrain-hindbrain domain of the embryonic brain that is comprised of the mesencephalic vesicle and the first rhombencephalic vesicle at early somitogenesis stages; normally, an organizing center located at the boundary patterns the midbrain and hindbrain primordia of the neural plate absent MHB absent isthmic organizer absent isthmomesencephalic boundary absent isthmus absent isthmus organizer absent midbrain-hindbrain organizer csmith