Comparative histology of the laminar bone between young calves and foals

Laminar bone or primary plexiform tissue, not Haversian bone, shows an alternative concentric pattern of laminar-bone units or plates around the bone marrow periphery of long bones, although the laminar bone is gradually replaced by osteons during the growth period. One laminar-bone unit is constructed with a hypercalcified line in the center, woven bone on both sides of the line, and lamellar bone with laminated appositional lines. Such a laminar bone showing a homogeneous calcification has been reported in young calves and some young large animals, but it has not been reported in foals although a previous report proposed that the bone structure was distinguishable from plexiform tissue. In this study, we compared young calves with foals by backscattered electron imaging mainly of transverse ground sections of mid-diaphysis. Foals had many hypercalcified lines arranged concentrically around the bone marrow periphery, which were similar to those of young calves. However, rows of cylindrical osteon-like structures with Haversian canal-like canals running along the long-bone axis were arranged between the concentric hypercalcified lines. Each Haversian canal-like structure was enclosed with laminated appositional rings of lamellar bone deposited on the woven bone. In the developing period, the bone units containing the concentric hypercalcified lines were basically equal to the laminar-bone units. The osteon-like structures or 'pseudo-osteons' were gradually replaced by 'true osteons' during the growth period. The blood vessels in the Haversian canal-like canals of foals ran along the long-bone axis, whereas the blood vessels in the concentrically prolonged bone cavities of young calves ran transversely to obliquely against the long-bone axis. Thus, the long-bone cortex of foals showing an alternative concentric pattern of a row of the osteon-like structures arranged between the hypercalcified lines will be histologically classified into a variety of laminar bone caused by the different arrangement of blood vessels. Such a laminar bone may have a biomechanical structure against physical stress, especially the modified laminar bone of foals with osteon-like structures, when compared with the typical concentric laminar bone of young calves and also Haversian bone possessing variously calcified numerous osteons caused by bone remodeling.