An electron microscopic study of skeletal cell aging—II. The osteocyte

Abstract

An electron microscopic investigation of the distal femoral bone of mice 5, 8, 26, 52 and 104 weeks of age was undertaken to study osteocyte formation and aging. The following major conclusions were drawn from the observations made at the perichondrial zone of the periosteum and metaphyseal trabeculae. Although newly formed osteocytes resembled their active osteoblast precursors for a time, degenerative ultrastructural changes occurred rapidly. This is not to deny the continuing role of osteocytes or their participation in osteolysis, osteoplasis, proteinsynthesis, mucopoly saccharide regulation or mineral homeostasis, for all these activities may occur coincidental with diminishing activity. Degenerative changes in all osteocytes became significantly obvious with increasing age as progressive, irreversible, debilitating age changes occurred and which subsequently lead to death of osteocytes as an end point of the cell's life cycle. Death of osteocytes was rarely observed in 5 and 8 weeks-old mice and occasionally seen at 26. They were noted at 52 weeks and very commonly observed at 104 weeks of age. In young mice, degenerative age changes were, however, not uncommon. In older mice, osteocytes near the bone surfaces also showed considerable degenerative changes. Osteoclastic resorption of lacunar walls resulted in complete cytolysis of osteocytes, save for their nuclei which appear to be entertained by osteoclasts for further digestion. With increasing age the perilacunar walls revealed evidence of repeated phases of osteocytic-osteoclasis and osteocytic-osteoplasis.