Cn/+ Electron Microscopy. Images from the normal cn/+ physes from site and age-matched littermates, in relation to those seen in Figure 4, are shown. Upper proliferating cells (i-iii) show flattened chondrocytes with abundant well organized rough endoplasmic reticulum (RER) with dilated cisternae containing a homogenous material indicative of active protein synthesis. The hypertrophic cells (iv-vi) show progressively diminishing RER organelles but these are still present even at the lowest levels at the metaphyseal junction. Arrows point in direction of cells closer to the upper regions of the respective zones. 5v shows persistent mineralization of a longitudinal cartilage septum. Line markings: i) 2.20 μm; ii) 1.55 μm; iii) 2.20 μm; iv) 2.20 μm; v) 2.20 μm; and vi) 2.20 μm.