The relationship between extracellular matrix vesicles and the calcifying fronts was examined by studying vesicular diameters and types. Transmission electron microscopy combined with computerized morphometry three weeks after injury to the tibial bone in rats was used. The different vesicle types were defined as: (1) vesicles with electron lucent contents referred to as empty; (2) vesicles with amorphous electron opaque contents, called amorphic; (3) vesicles containing crystalline depositions, called crystalline; and (4) vesicles containing crystalline structures with ruptured membranes, referred to as ruptured. The diameters of most vesicles ranged between 0.07 and 0.17 micron. More than 95% of the vesicles were located less than 2 micron from the calcified front. The vesicles were distributed among the categories as follows: empty, 9.6%; amorphic, 19.3%; crystal, 39.2%; and ruptured, 31.9%, respectively. The diameters of the crystalline and ruptured vesicles were significantly larger than those of the empty and amorphic types. The ruptured type had the largest diameters. The sequence of distances from the calcified front was recorded as follows: ruptured, crystalline, amorphic, and empty, with the ruptured and crystalline types being the closest to the front. This study supports the accepted theory on matrix vesicle mineralization. The cell is responsible for secretion of empty vesicles that accumulate amorphous Ca and Pi to form a hydroxyapatite crystal. This is followed by rupture of the vesicular membrane. The propagation of the process is accompanied by an increase in the vesicular diameter and its approximation to the calcifying front.