Early scanning electron microscopic studies of hard tissue resorption: their relation to current concepts reviewed.

This paper highlights some observations made by the authors in SEM studies of hard tissue resorption and considers their significance in relation to current concepts. All mammalian mineralised tissues may undergo physiological resorption, the resulting surface reflecting the density of mineralisation and the organic matrix chemistry, organisation and orientation. Resorption-repair coupling may follow the resorption of any tissue, but SEM studies first noted this process in the case of the dental tissues. The difference between fetal and adult bone formation and resorption provided evidence against the concept of osteocytic osteolysis. SEM stereophotogrammetric methods for the quantitation of individual resorption lacunae are now much quicker and have been extended to the study of in vitro resorption by mammalian and avian osteoclasts isolated from bone and seeded into new substrates. Experimental studies using SEM were first conducted on the osteotropic hormonal effects on bones forming in vivo and extended to the in vitro situation. The effects observed underlined the several actions of PTH on osteoblasts and indicated their important role in the control of bone resorption. Immunological marking techniques monitored by SEM first established that osteoclasts had no Fc or C3 receptors, although other cells in the vicinity did. The study of osteoclasts resorbing substrates other than bone in vitro has increased our understanding of the essential components of a resorbable substrate. Experiments growing separated bone cells and marrow cells on calcified substrates have shown that such cells will continue to resorb for at least six weeks.