CHANGES IN CELL, MATRIX COMPARTMENT, AND FIBRILLAR COLLAGEN VOLUMES BETWEEN GROWTH-PLATE ZONES

To define the contributions of changes in cell, matrix compartment, an d fibrillar collagen volumes to longitudinal bone growth. we measured the differences in cell, pericellular/territorial matrix and interterr itorial matrix volumes, and fibrillar collagen concentrations between the upper proliferative and lower hypertrophic zones of the proximal t ibial physes of six miniature pigs. The mean numerical density of cell s decreased from 110,000 cells/mm(3) in the upper proliferative zone t o 59,900 cells/mm(3) in the lower hypertrophic zone. The mean cell vol ume increased nearly 5-fold (from 1,174 to 5,530 mu m(3)), and the tot al matrix volume per cell increased 46% (from 5,040 to 11,760 mu m(3)/ cell) between the upper proliferative and lower hypertrophic zones. Bo th the pericellular/territorial matrix volume per cell and the interte rritorial matrix volume per cell increased between the upper prolifera tive and lower hypertrophic zones; the pericellular/territorial matrix volume per cell increased 61% (from 4,580 to 7,390 mu m(3)/cell), whe reas the interterritorial matrix volume per cell increased 26% (from 3 ,460 to 4,370 mu m(3)/cell). The total increase in mean cell volume of 4,356 mu m(3) exceeded the total increase in mean matrix volume per c ell of 3,720 mu m3; the total mean pericellular/territorial matrix vol ume per cell increased more than the total mean interterritorial matri x volume per cell (2,810 compared with 910 mu m(3)/cell). Fibrillar co llagen concentration was greater in the interterritorial matrix than i n the pericellular/territorial matrix in both zones and increased in b oth matrix compartments between the upper proliferative and lower hype rtrophic zones. The amount of fibrillar collagen per cell also increas ed in both matrix compartments between the upper proliferative and low er hypertrophic zones (from 1,720 to 3,100 mu m(3)/cell in the pericel lular/territorial matrix and from 1,490 to 2,230 mu m(3)/cell in the i nterterritorial matrix; thus, the total amount of fibrillar collagen p er cell increased from 3,210 to 5,530 mu m(3)/cell). Growth rate was i nversely related to the cell numerical density in the upper proliferat ive and lower hypertrophic zones and was directly related to interterr itorial matrix volume per cell in the upper proliferative zone and to pericellular/territorial matrix volume per cell in the lower hypertrop hic zone. These results show that cell enlargement contributes more to longitudinal bone growth than does increased matrix volume, that incr eased pericellular/territorial matrix volume makes a greater contribut ion 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 stro ngly suggest that they differ not only in matrix organization but in r ate of matrix accumulation and assembly and that these differences giv e the two compartments different roles in skeletal growth.