Fig. 5. Decreased precursor proliferation in the brain of Shh null embryos. (A) Morphological appearance of an E18.5 Shh null (right) and a wild-type littermate (left). Arrow points to the position of the forebrain, immediately posterior to the proboscis. (B,C) Details of the ventricular areas of the neocortex of wild-type (B) and in the forebrain of E18.5 Shh null embryos seen in sagittal sections (C) showing the marked decrease in proliferation in the vz/ svz in the mutant. (D,E) Expression of nestin in the vz/svz of both wild-type (D) and mutant (E) cortices. (F,G) Expression of the neuronal tubulin marker Tuj1 (red) in the neocortical plate of a wild-type embryo (F) and in the cortex region of a Shh-null sibling (G). (H,I) The characteristic proliferation of EGL cells in the E18.5 wild-type cerebellum (H) is missing in a sibling mutant brain (I), where no structure is morphologically recognizable as a cerebellum adjacent to the posterior choroid plexus (chp). (J,K) Proliferation of interfollicular cells appears normal in the wild-type (J) and mutant (K) epidermis (ep). Hair follicles (hf) are absent in the mutants. (L) Lateral views of dissected E15.5 wild-type (bottom) and Shh mutant (top) brains showing the existence of a morphologically defined cortex (Ctx), tectum (Tct) and medulla (Me) areas, as well as an elongated proboscis (Pb) anteriorly. The corresponding parts of the mutant brain and the wild-type littermate are indicated by white lines. Note the apparent presence of superior (SC) and inferior (IC) colliculi in both brains, but the apparent absence of the cerebellum (Cb, arrowhead) in the mutant. The olfactory bulb (OB) is also indicated in the normal brain. D, dorsal; Nctx, neocortex; V, ventral. (M) Camera lucida drawings of the outlines of sagittal sections of the brains of an E18 wild-type embryo (left) and a Shh-null sibling (right) at scale. The red lines and arrowheads indicate the source and deduced action of SHH protein in the neocortex, tectum and cerebellum.