The development of the cerebellum

Although cerebellum differentiates early during embryogenesis, it only reaches its final configuration several months after birth (Koop et al., 1986 ; Lechtenberg, 1993). The neural tube is initially composed of a pair of neural folds which will fuse in the midline dorsally. The embryonic neural groove closes to become the neural tube at four weeks of gestation. The fusion proceeds from the most rostral region to the most caudal. When the rostral neuropore closes, three brain vesicles can be identified: the forebrain, the midbrain, and the hindbrain (three-vesicle stage). The hindbrain is also called rhombencephalon. At five weeks, the forebrain and the hindbrain both subdivide (five-vesicle stage). The hindbrain is generating the metencephalon rostrally and the myelencephalon caudally. Metencephalon and myelencephalon are separated by the pontine flexure of rhombencephalon. Cerebellum was thought to originate exclusively from metencephalon, but it has been shown that caudal mesencephalon also participates in the genesis of the rostral parts of cerebellum. The superior vermis begins to be formed at about seven to eight weeks of gestation, and the fusion of the inferior vermis continues up to about 18 weeks. The superior rhombic lip and the adjacent parts proliferate to generate the rudiment of the cerebellum (Fig. 1.1A). The central cavity of rhombencephalon becomes the fourth ventricle. In the following weeks, cerebellar development is characterized macroscopically by an expansion in four directions: rostrally, caudally, dorsally, and laterally.