Embryology of Skull Base Malformations

Introduction: The human skull base is a developmentally complicated structure which is formed, like a mosaic, by different parts. Principally, a prespinal section can be differentiated from the spinal one composed by the hypoglossal segments. Malformations of the skull base can be divided into major groups, often associated with syndromes and minor entities such as ossification defects between the different components. These minor defects are clinically important because they may cause celes and recurrent meningitis. Some of these defects are very small and difficult to detect, so that thorough anatomical and embryological knowledge is necessary for finding them. Furthermore, variations of the craniocervical junction should not be overlooked. These osseous variations are often delicate but make this region more vulnerable due to increased mobility or stiffness. The variations of the craniocervical junction can be diagnosed by x-rays or CT (Prescher et al, 1996).

Material and Methods: Using several anatomical specimens and histological series of embryological material, the development of the human skull base will be reviewed.

Results: The anterior part is mainly shaped by the sphenoid and ethmoid complex. The sphenoid bone is a complicated osseous structure formed by the presphenoid, the basisphenoid, the alisphenoid, the orbitosphenoid, and the pterygoid. Between these different components exist synchondroses, which ossify and vanish during normal development. Furthermore, there are some contacts to neighboring dermal bones, for example, the os frontale. According to the scheme of the frequently occurring celes (Cela frontoethmoidalis, transethmoidalis, sphenoethmoidalis, spenopharyngealis, spenoorbitalis, and occipitalis) by Lang (1985), it becomes obvious that these defects are located at the former synchondroses of the skull base or at the contact zones of chondral and dermal parts of the skull base. Therefore, these troublesome regions must be thoroughly investigated if occult celes or defects are looked for. The posterior medial part of the skull base is mainly derived from the hypoglossal segments, which were incorporated into the skull base. For a long time the number of the incorporated segments was unknown, but recently Müller and O'Rahilly (2003) were able to show that the incorporation of four segments is really correct. If more segments are incorporated the assimilation of the atlas will be formed; if parts of the last segment are not incorporated the manifestation of the occipital vertebra will result. Another important structure is the hypochordal blastema of the proatlas. From this structure different osseous variations can be derived depending on which part persists