The Pediatric Posterior Fossa: an Embryologist's View

The paediatric posterior fossa is a complex structure made up of both neural elements and skeletal pieces. The first structures to be formed are the neural elements followed later by bones moulded around the already patterned neural elements. The nervous structures derive from the neural tube at the exact level of the rhombencephalon (also called the hindbrain). This neural tube is segmented into neuromeres called the rhombomeres. These structures form compartments in which cell migration is restricted. Furthermore, these compartments display specific gene expression and if the expression of a gene is forced into an aberrant rhombomere, a severe defect is determined in its own development. The cerebellum, that is one of the major neural components at this level, derives from rhombomere 1. It grows in a rostral to caudal direction like a curtain because its rostral progression is prevented by the cerebellar tentorium. The cerebellum arises as two distinct primordia that unite on the midline thanks to migration of the primordium of this structure. An impairment of this structure will generate a defective fusion at the midline. It is important to note that the different anatomical parts of the cerebellum are not regulated by the same molecular processes leading to a heterogeneity of this organ. Lastly, the bony structures of the posterior fossa derive from the mesoderm. We will review the exact contribution of both the somites and the cephalic mesoderm in such a context.