Fig. 2. Nmyc is expressed in proliferating precursors of the developing cerebellum. (A-C) In situ hybridization for Nmyc, (A) cyclin D1 (B) and Gli1 (C) mRNA transcripts was performed on adjacent parasagittal frozen sections of PN 7 mouse cerebella. (A) Nmyc and (B) cyclin D1 expression co-localize to regions of Shh pathway activity and proliferation of CGNPs. (D-F) Nmyc expression is observed during the proliferative phase of cerebellar development at PN1 and PN7, but is undetectable in the PN15 or adult (not shown) mouse cerebellum. (D,E) Low power views of PN1 and PN7 cerebella shows the distribution of Nmyc mRNA transcripts primarily in the EGL region (white arrows). Additionally, a small Nmyc-expressing population is observed within deeper layers of the cerebellum at PN1 (arrowheads, D). (F) At PN15, Nmyc transcripts are undetectable in all cerebellar layers. DIC image at right allows discrimination of EGL and molecular layers. Arrowheads (top) indicate meninges that fail to express Nmyc. (G) At high magnification, Nmyc expression at PN7 clearly segregates to the EGLa, or proliferative compartment of the EGL. The EGLb comprises largely immature CGNPs that have recently left the cell cycle. By contrast, neither Myc (H) nor Lmyc (I) are expressed in the EGL at PN7. ant, anterior; post, posterior; EGL, external granule layer; Pur, Purkinje cells; Mol, molecular layer; Mng, meninges.