Figure 2. Bhlhb5 is Expressed with Both Lamina and Area Specificity.

(A–D) Triple immunolabeling of Bhlhb5 (A) with CTIP2 (layer V) (B), and Tbr1 (layers II, III, VI and subplate) (C), reveals Bhlhb5 expression in layers II, III, IV, and V at P4.

(E–F) Bhlhb5 is not expressed by a majority of Tbr1+ neurons (layer VI and subplate) at E13.5 (E) and E15.5 (F).

(G) Bhlhb5 is expressed in a subset of CTIP2+ neurons (layer V; more superficial) and layers superficial to the CTIP2-expressing layer at E15.5.

(H–J) In situ hybridization (H), immunocytochemistry (I), and X-Gal staining (counterstained with eosin) (J) on adjacent sagittal sections from a Bhlhb5^lacZ/+ heterozygote brain, reveals faithful recapitulation of Bhlhb5 mRNA and protein expression by Bhlhb5-lacZ reporter.

(K–P) Dorsal views of X-Gal histochemistry on whole mount brains. (K) At E12.5, Bhlhb5 is expressed highest medially in cingulate cortex and weakly in neocortex. (L) At E15.5, Bhlhb5 is expressed in a high caudomedial to low rostrolateral gradient in neocortex. (M,N) The Bhlhb5 gradient transforms into a sharp border (N; arrowhead) between the rostral motor (M) and sensory (S) cortical domains by P0. (O and P) Bhlhb5 expression is further restricted to primary sensory areas at P4 (O) and P7 (P) such as the primary somatosensory cortex (S1) and primary visual cortex (V1).

VZ, ventricular zone; SVZ, subventricular zone; CP, cortical plate; A, anterior; P, posterior; B, vibrissal barrel field. Scale bars: 50 µm (A–G), 500 µm (H–J), 1 mm (K–P).