Figure 6.

Electron micrographs illustrate epiphyseal hypertrophic chondrocytes. i, ii and iv illustrate metatarsal epiphysis, 1 week of age, solution C (including PFA 1%), mOsm 511; iii is from distal newborn femur, solution B, mOsm 283. (i) At peripheral region adjacent to central hypertrophic area parts of 2 active chondrocytes are seen. The RER is present but less densely packed than in the most peripheral regions (not shown), nuclear sections are round with membranes intact and cell membranes are immediately adjacent to cartilage matrices. Bar=500 nm. (ii) Parts of four cells are all intact (cell membrane against matrix). In each hypertrophic cell, the RER is immediately adjacent to the cell membrane, a pattern which is not seen in the physeal hypertrophic cell. RER is not randomly dispersed within the epiphyseal cell cytoplasm but is either part of the nucleocytoplasmic mass or packed against the peripheral cell membrane and continuous with the nucleocytoplasmic mass. As the cytoplasm lacks RER presence, it often contains moderately electron-dense bodies consistent with intracellular fat. Bar=2 μm. (iii) Electron micrograph illustrates an adjacent epiphyseal hypertrophic chondrocyte at a more advanced stage. Note nucleocytoplasmic continuity across central cell diameter. Peripheral regions of cytoplasm are empty of RER or other organelles. Bar=500 nm. (iv) Metatarsal epiphysis, showing advanced stages of the epiphyseal hypertrophic chondrocytes continuous with image in ii with characteristic organellar patterns. There is RER packing against cell membrane at periphery and intracytoplasmic material consistent with fat. Bar=2 μm.