BMP protein is concentrated at the VZ surface.A, Immunofluorescent image of an E16 ventricular zone stained with an antibody specific to BMP-2 and BMP-4 is shown. Cells were not permeabilized with Triton or fixative, and the punctate staining for BMP is limited to and surrounding cells at the VZ surface.B, Preabsorption of the antibody with excess BMP-4 protein eliminates staining at the VZ surface (arrowsindicate the edge of the VZ surface). C, A quantitative analysis of staining shows BMP protein is concentrated at the VZ surface. The histogram shows the number of antibody grains for 20-μm-wide bins, tangential to the VZ surface, through the width of the cortical mantle. The maximum number of antibody grains was in the first 20 μm from the VZ surface. Scale bar, 20 μm.