(A) Generation of Arpc2-null alleles in collagen II–expressing cells. (B) Frequency of pups that survived to genotyping with an expected Mendelian distribution of 1/4 each. (C) H&E-stained ribcage of mutant at P0 compared with control (CTR). Scale bar: 100 μm. n = 1. (D and E) Images of Arp2/3 mutant mice. P6, left panel; P26, right panel; shown next to littermate controls. Red arrows indicate abnormal curvature of the spine. (F) Coronal sections of discs from a P10 animal stained by Safranin O/Fast Green/Hematoxylin. White outlines define the disc compartment. Boxes indicate higher-magnification images in bottom row, showing magnified end plates and growth plates. Yellow arrowheads show chondrocyte columns; white arrowheads show chondrocyte-like cells. Scale bars: 200 μm in top row, 50 μm in high-magnification view. n = 1; 4 discs. (G) Vertebrae at P10. Scale bar: 200 μm. n = 1; 6 vertebrae. (H) H&E-stained sections of tibia from P26 mouse (n = 1). Yellow arrowheads indicate proliferative columns, black arrowheads indicate disorganization of cells, and pink arrowheads indicate chondrocytes with hypertrophic morphology. Scale bar: 100 μm, 50 μm in higher-magnification image. (I) H&E-stained sections of embryonic tibia at E17.5. Scale bar: 100 μm, 50 μm in higher-magnification image. n = 3 embryos. (J) Ki67 IHC on E17.5 tibia. Scale bar: 100 μm. n = 1. (K) The number of proliferative columns containing 3 or more aligned cells were quantified from embryonic tibia. (L) In the same area, the number of cells in the proliferative zone were counted. (M) The heights of the proliferative zone (PZ) and hypertrophic zone (HZ) as shown in I were quantified. NP, nucleus pulposus; AF, annulus fibrosus; EP, end plate; GP, growth plate; VB, vertebral body. Quantitative measurements represent mean ± SD. Significance was determined using unpaired Student’s t test. ***P ≤ 0.01.