Fig. 1. Labeling of early and late born cortical neurons in wild-type, Vldlr^-/- and ApoER2^-/- mice at P7 using ER81 and Cux2. (A,D,G) In situ hybridization for ER81, a marker of early generated neurons, showed strongest expression in cortical layer V neurons on sagittal brain sections of wild-type animals (A) and Vldlr^-/- mice (D). In ApoER2 mutants (G), labeling in the outer portion of the cortex, normally occupied by layer II-IV neurons, was found, indicating a more superficially located layer V. (B,E,H) In situ hybridization for Cux2, a marker of late generated neurons, revealed expression in the upper cortical layers (II-IV) in wild-type animals (B). In Vldlr mutants (E), the Cux2-positive population showed a similar distribution, but the marginal zone was invaded by many Cux2-positive cells. In ApoER2 mutants (H), in situ hybridization for Cux2 showed two bands, a superficial one underneath layer I, and a deep one, located in the innermost portion of the cerebral wall. A comparison of the expression patterns of ER81 (G) and Cux2 (H) in this mutant suggests that the upper Cux2-positive cells represent layer IV neurons. (C,F,I) The prominent phenotype of ApoER2 mutants when compared to Vldlr^-/- mice is also seen in Nissl-stained sections. Arrow in I indicates cell accumulation in deep cortical layers. Scale bar: 200 µm.