Changes in cell, matrix compartment, and fibrillar collagen volumes between growth‐plate zones

To define the contributions of changes in cell, matrix compartment, and fibrillar collagen volumes to longitudinal bone growth, we measured the differences in cell, pericellular/territorial matrix and interterritorial matrix volumes, and fibrillar collagen concentrations between the upper proliferative and lower hypertrophic zones of the proximal tibial physes of six miniature pigs. The mean numerical density of cells decreased from 110,000 cells/mm3 in the upper proliferative zone to 59,900 cells/mm3 in the lower hypertrophic zone. The mean cell volume increased nearly 5-fold (from 1,174 to 5,530 microm3), and the total matrix volume per cell increased 46% (from 8,040 to 11,760 microm3/cell) between the upper proliferative and lower hypertrophic zones. Both the pericellular/territorial matrix volume per cell and the interterritorial matrix volume per cell increased between the upper proliferative and lower hypertrophic zones; the pericellular/territorial matrix volume per cell increased 61% (from 4,580 to 7,390 microm3/cell), whereas the interterritorial matrix volume per cell increased 26% (from 3,460 to 4,370 microm3/cell). The total increase in mean cell volume of 4,356 microm3 exceeded the total increase in mean matrix volume per cell of 3,720 microm3; the total mean pericellular/territorial matrix volume per cell increased more than the total mean interterritorial matrix volume per cell (2,810 compared with 910 microm3/cell). Fibrillar collagen concentration was greater in the interterritorial matrix than in the pericellular/territorial matrix in both zones and increased in both matrix compartments between the upper proliferative and lower hypertrophic zones. The amount of fibrillar collagen per cell also increased in both matrix compartments between the upper proliferative and lower hypertrophic zones (from 1,720 to 3,100 microm3/cell in the pericellular/territorial matrix and from 1,490 to 2,230 microm3/cell in the interterritorial matrix; thus, the total amount of fibrillar collagen per cell increased from 3,210 to 5,530 microm3/cell). Growth rate was inversely related to the cell numerical density in the upper proliferative and lower hypertrophic zones and was directly related to interterritorial matrix volume per cell in the upper proliferative zone and to pericellular/territorial matrix volume per cell in the lower hypertrophic zone. These results show that cell enlargement contributes more to longitudinal bone growth than does increased matrix volume, that increased pericellular/territorial matrix volume makes a greater contribution to growth than does increased interterritorial matrix volume, and that the total amount of fibrillar collagen per cell increases between the upper proliferative and lower hypertrophic zones. The differences between the two matrix compartments in increase in volume, fibrillar collagen concentration, and amount of fibrillar collagen per cell strongly suggest that they differ not only in matrix organization but in rate of matrix accumulation and assembly and that these differences give the two compartments different roles in skeletal growth.