ULTRASTRUCTURAL AND LANTHANUM TRACER EXAMINATION OF RAPIDLY RESORBINGRAT ALVEOLAR BONE SUGGESTS THAT OSTEOCLASTS INTERNALIZE DYING BONE-CELLS

Glutaraldehyde-formaldehyde fixed undecalcified alveolar bone from 7-d ay-old rats was prepared for light and electron microscopy. Colloidal lanthanum was used as an ultrastructural tracer, and both random and s emi-serial sections were examined. Lanthanum penetrated the infoldings of the ruffled border and some nearby vacuoles and vesicles. The majo rity of vacuoles and vesicles were lanthanum-free. Some osteoclast pro files contained a large vacuole with a cell enclosed in its interior. The enclosed cell exhibited an irregular nucleus containing condensed peripheral chromatin, intact cytoplasmic organelles, conspicuous rough endoplasmic reticulum and large blebs on the cell surface. These feat ures are characteristic of osteoblasts or bone-lining cells or immatur e osteocytes which may be undergoing apoptosis or necrosis. The observ ation of remnants of cellular structures within internalized osteoclas t vacuoles, together with the above results, suggests that osteoclasts engulf and probably degrade dying osteoblasts/bone-lining cells or im mature osteocytes.