RESUMEN
There are many reports on laminar bone in various young animals during their growth periods. One of the concentric laminar bone units around the long-bone marrow periphery consists of three components: bright line, woven bone, and lamellar bone in the long-bone cortex of young calves and pigs. However, the fine structure, especially the uniform bright line or 'hypercalcified line' present in the unit-center, has not been elucidated as yet. The laminar bone of young calves was found to be initially formed from the hypercalcified lines; that is, 'hypercalcified primear' reported previously by the other authors. Such primear lines containing collagen fibrils scattered deposits of rod-like structures containing fine non-collagenous fibrils and globular structures showing no fibrils and some remnants. Osteocytes occasionally existed in the lines as well as adjacent to the lines, although no cells have been reported in the line. The cell lacunae were larger than those of woven bone and lamellar bone. The hypercalcified primear lines contained higher Ca and P content, and the molar ratio (1.78) was similar to 1.75 in the woven bone and lamellar bone. However, the physical and chemical resistances were significantly lower than that of the surrounding bone. Therefore, the hypercalcified primear lines are strongly suggested to show a lower crystallization. Further, fine structural observations of the primear-line forming cells and histochemical and immuno-histochemical investigations of the primear lines will be necessary.