Distribution of resorption processes in the compacta and spongiosa of bones from lactating rats fed a low-calcium diet.

An attempt has been made to determine if the structural renewal of bone tissue takes place to a greater extent in the spongiosa than in the compacta and how this is related to the more extensive free surface area in the former. To this purpose, on transverse sections of the femur and 2 degrees lumbar vertebral body of the female lactating rats fed a normal (L group) or a calcium free diet (H group) the area of compact and spongy bone, and the perimeter of the trabeculae were measured. Then TBV, MTPT, MTPD, MTPS, Sv and S/V, according to the Parfitt et al. (1983) method were calculated. In the femurs of rats maintained on a normal calcium diet bone loss takes place to a higher degree in the spongiosa than in the compacta. In the distal metaphysis, moreover, the bone loss is more elevated than in other femoral regions. In the second lumbar vertebral body the bone rarefaction appears to have a similar extent in the compacta and spongiosa and in the three levels inspected. In rats maintained on a restricted calcium intake the osteoporosis is enhanced but the bone is removed predominantly from the compacta in all the femoral regions except in the distal metaphysis and, to a similar degree, from the compacta and the spongiosa of the vertebral body. In the centrodiaphyseal compacta the bone is reabsorbed exclusively at the endosteal level, while in the spongiosa the frameworks subjected to a more extensive resorption occupy a topographic position specific for each single region, and are often formed of thin trabeculae. The dynamic of the trabecular resorption seems to be different in the various spongy frameworks: in each of these it would take place by the removal of the entire trabecula or the thinning of the single trabeculae or by both mechanisms. Lastly, significative correlation was found between trabecular bone loss and trabecular thickness, none with trabecular surfaces. This study shows that bone removal is not regulated by the extent of the trabecular free surface. The mechanisms involved in the control of the distribution of bone removal are strictly connected with those controlling the trabecular thickness. They are probably of a mechanical nature.