Figure 3. Chondrocytes have dilated cisternae of ER and the ECM ultrastructure is altered in mutant growth plates. A and B: These representative images were taken from a complete TEM montage of a day 7 tibia growth plate (from resting to mineralization zones) and show four cell chondrons from the proliferative zone. Chondrocytes from mutant mice show enlarged individual cisternae of ER (D) compared to the wild-type mice in which the ER has a typical ribbon appearance (C). E and F: The ultrastructure of the interterritorial matrix at 1 week of age is altered in mut/mut growth plates and is characterized by more prominent appearing collagen ﬁbrillar material. Scale bars are 3.0 μm (A), 2.3 μm (B), or 1.2 μm (E and F). mut/mut, mice homozygous for D469del. mice homozygous for the mutation was uneven and appeared dif- fuse, particularly at the proliferative–hypertrophic boundary (Supp. Fig. S4E and F). Ultrastructural Analysis Conﬁrms An Enlarged rER in Mutant Chondrocytes and Abnormal Changes to ECM Morphology in Mutant Cartilage The tibia growth plates from 7-day-old mice were studied by transmission electron microscopy (TEM) to observe individual chondrocytes and their associated ECM (Fig. 3). While the ER of wild-type chondrocytes appeared within normal limits (Fig. 3A and 224 HUMAN MUTATION, Vol. 33, No. 1, 218–231, 2012 C), most chondrocytes in the growth plates of mice homozygous for the Comp D469del mutation showed enlarged cisternae of ER that was consistent with the retention of mutant COMP and other proteins (Fig. 3B and D). Hypertrophic chondrocytes from wild- type mice were rectangular in shape and the condensation of chro- matin was clearly visible. In contrast, hypertrophic chondrocytes from mice homozygous for the mutation were generally irregular in shape and showed the persistence of protein within remnants of the ER (data not shown). In addition, there were clear changes in the morphology of the ECM in the pericellular region of the cartilage growth plate (Fig. 3E and F). This included more pronounced ap- pearing collagen ﬁbrils, which is consistent with a reduction in the