Aging-dependent skull channel reduction and bone marrow expansion. (A, C, and E) Representative images of parietal bone sections from 2-mo-old mice stained with VEGFR3 (red) and Col1a1 (white). White boxes show magnified views of bone marrow and non-bone marrow areas in the lower panels (A1 and A2). Purple arrowheads point to individual bone marrow channels. Scale bars: 500 and 200 µm (inset). (B, D, and F) Corresponding images from parietal bones of 2-year-old mice stained with VEGFR3 (red) and Col1a1 (white). White boxes indicate zoomed-in areas, focusing on the bone marrow and non-bone marrow regions (B1 and B2). Purple arrowheads highlight the channels. Scale bars: 500 and 200 µm (inset). (G and H) Quantification of bone marrow volume and vessel volume in parietal bones of 2-mo-old and 2-year-old mice (young, n = 5 mice; aged, n = 4 mice). (I) Quantification of the number of channels connecting to the periosteum and meninges from young and aged mice (young, n = 5 mice; aged, n = 4 mice). Data are represented as the mean ± SEM, analyzed using ****P = 0.0001 (SMCs) or 0.0001 (SPCs) by two-way ANOVA; **P = 0.045 (blood vessels) or 0.058 (bone marrow) by Student’s t test analysis.