Cerebellar and brainstem development: an overview in relation to Joubert syndrome.

An overview of cerebellar and brainstem development is provided as a foundation for suggesting hypotheses about developmental defects in Joubert syndrome. Although neuropathologic studies of Joubert syndrome are rare, and the spectrum of brain pathology is not yet known, consistent findings include agenesis of the cerebellar vermis and hypoplasia or fragmentation of several brainstem nuclei (including dentate nuclei, inferior olives, and basis pontis), nuclei and tracts of cranial nerve V, solitary nuclei and tracts, and nuclei gracilis and cuneatus. Two aspects of cerebellar development might be important in the pathogenesis of Joubert syndrome: First, cerebellar development is regulated by a critical region of the embryo called the "midbrain-hindbrain organizer," and both mesencephalic and metencephalic elements take part in normal cerebellar development. While the metencephalon gives rise to the cerebellar hemispheres, the vermis is derived almost exclusively from the mesencephalon. This suggests that Joubert syndrome could involve an abnormality in formation of the pontomesencephalic junction (rhombomere 1). Second, the histogenesis of cranial nerve nuclei and brainstem structures derived from the embryonic rhombic lip (such as the inferior olives, neurons of the basis pontis, and arcuate nuclei) involves the formation, migration, and reorganization of nuclei and tracts during a critical period of development (6 to 8 weeks' gestation). Because these structures are abnormal in Joubert syndrome, an understanding of factors that regulate the proper formation and migration of cells that give rise to them could provide important clues about the pathogenesis of this disorder.