Cephalic axial skeletal-neural dysraphic disorders: embryology and pathology

Three fundamental types of cephalic axial skeletal-neural dysrapic disorders are analyzed, including: cranioschisis aperta with encephaloschisis (anencephaly and/or exencephaly), cranioschisis occulta with occipital encephalocele, and the Chiari malformation (occipital bone hypoplasia) with compression, deformation and displacement of hindbrain, cerebellum, and medulla. Both clinical and experimental (vitamin A induced) examples of these malformations are used. The study establishes that these are not simple neurological (neural tube defects) disorders as it has been generally assumed, but complex developmental malformations affecting primarily the formation of the axial basicranium (causing skeletal defects) and the elevation of the neural folds and neurocranium (causing neural defects), and, secondarily, the topography of the facial skeleton or viscerocranium (causing oropharyngeal defects). The pathology of these skeletal, neural, and oropharyngeal defects is analyzed, their embryonic origin explored, and their developmental interrelationships discussed. The study proposes that an early paraxial mesodermal insufficiency may be the original anomaly common to all the different malformations that constitutes this heterogeneous group of dysraphic disorders. At any time during the segmental formation of the embryonic skeletal-neural axis a simple reduction in the number of paraxial mesodermal cells produced by the Hensen node/primitive streak complex, could impair the formation of the axial skeleton as well as the elevation of the neural folds thus interfering with their closure. The final type of malformation is determined by variations of the degree, time of occurrence, and duration of the paraxial mesodermal insufficiency.