Figure 5. Bhlhb5 Genetic Lineage Tracing Demonstrates Differential Bhlhb5 Expression in Subcerebral Projection Neuron Subtypes along the Rostro-Caudal Axis.
(A–C) Schematic of the areal distribution of CSMN and CTPN. Medially, along the rostrocaudal axis, fewer CSMN (green) are located in rostral motor cortex (M(R)), more CSMN are located in caudal motor cortex (M(C)), and CTPN (red) are located in occipital cortex (O).
(D and E) Bhlhb5 colocalizes with SCPN marker, CTIP2, at E15.5 but not P4 in caudal motor cortex (arrows).
(F) Microarray expression profiling of Bhlhb5 in purified CSMN shows steep decline in expression between E18 and P14.
(G) Schematic of the genetic lineage tracing strategy to permanently mark cells of Bhlhb5+ lineage via continued GFP reporter production.
(H) Dorsal view of whole mount Bhlhb5cre/+; Z/EG/+ heterozygote adult brain reveals cumulative spatiotemporal expression of Bhlhb5 with high GFP fluorescence caudally and low fluorescence rostrally in the neocortex. Yellow circle delineates approximate location of caudal motor cortex.
(I and J) Double immunolabeling of GFP (Bhlhb5) and CTIP2 on P5 sagittal section of Bhlhb5cre/+; Z/EG/+ heterozygote brain. Bhlhb5 is expressed by a majority of presumptive CTPN in occipital cortex, CSMN in caudal motor cortex but not CSMN in rostral motor cortex. (J) Magnified view (arrowhead in I) shows very few GFP+ CTIP2+ neurons in rostral motor cortex.
F, Frontal cortex; P, Parietal cortex, T, Temporal cortex; O, Occipital cortex; M1, primary motor area; M2, secondary motor area; M(R), rostral motor cortex; M(C), caudal motor cortex; S1, primary somatosensory area; S2, secondary somatosensory area; A1, primary auditory area; V1; primary visual area; OB, olfactory bulb; CB, cerebellum. Scale bars: 200 µm (D,E), 1 mm (H), 500 µm (I), 50 µm (J).