Pathogenesis
Currently there is no identified cause of acalvaria. The primary presumed pathogenesis is problematic migration of the membranous neurocranium with respect to the normal positioning of the immature ectoderm. When an embryo develops normally, the anterior neural pore closes about the fourth week. After this occurs, mesenchymal tissue migrates under the ectoderm. This ectoderm underlies where the cerebral hemisphere will eventually be. When a fetus has acalvaria, the embryonic ectoderm is in its correct place, but the mesenchymal migration does not occur correctly. Therefore, acalvaria is considered to be a postneurulation defect. Because it is a postneurulation defect, it must develop after embryonic stage 11, between 24 and 26 days after conception.
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