Figure 3 -.

(A) Schematic representation of an embryonic mouse cerebellum between e12.5-e18.5 (CB) sectioned along the sagittal plane. The cerebellum is derived from the dorsal region of rhombomere 1 (rh1) under the influence of signaling factors from the Isthmic organizer (IsO) and roof plate (RP). (B) A composite of embryonic developmental processes during embryogenesis. The developing cerebellum has two zones of neurogenesis, the ventricular zone (VZ) and the rhombic lip (RL). The cerebellar ventricular zone consists of a lining of radial glia (RG) and gives rise to all cerebellar GABAergic neurons and interneurons. GABAergic cerebellar nuclei neurons are produced first, followed by Purkinje cells and PAX2-expressing cerebellar interneuron progenitors. Bergmann Glia are also derived from the cerebellar ventricular zone. The rhombic lip on the other hand gives rise to the three major glutamatergic neuronal subtypes that populate the cerebellum. Firstly, cerebellar nuclei projection neurons migrate from the rhombic lip into the Nuclear Transitory Zone (NTZ) over the anlage as the rostral migratory stream. As embryonic development proceeds, granule neuron progenitors (GNPs) next migrate out of the rhombic lip between embryonic day 12.5 and 16. These cell progenitors migrate tangentially under the pial surface to establish the EGL of the developing cerebellum in an anterior to posterior manner. The RL also gives rise to unipolar brush cells (UBC) later in development, that migrate into the cerebellar anlage, (C) The EGL is a secondary germinal zone, or transit amplifying center. The EGL is composed of 2 sublayers – a proliferating external zone and an inner differentiating zone. Proliferation of GNPs takes place during postnatal days P0–P14. This proliferation is largely driven by the mitogen sonic hedgehog (SHH) secreted from Purkinje cells which have formed the Purkinje layer (PL) under the EGL. (D) Proliferation of GNPs in the EGL is responsible for the dramatic size increase of the post-natal mouse cerebellum. As granule neurons exit the cell cycle, they migrate tangentially within the inner EGL and then exit the EGL migrating radially inward to settle below the developing Purkinje cell layer to form the internal granule layer (IGL), resulting in the final laminar arrangement of the mature cerebellum. (E) Schematic representation of the multiple cell types that arise in the cerebellar ventricular zone and rhombic lip. Reference : Haldipur P., Dang D. and Millen K.J., (In press) Embryology. In: M. Manto and T.A.G.M. Huisman (Eds.) The cerebellum in children and adults. Elsevier, Amsterdam.