Histological and Ultrastructural Findings on Bone Cells

Norio Amizuka, Hiromi Hongo, Kanako Tsuboi, Tomoka Hasegawa, Tomomaya Yamamoto and Mai Haraguchi

Department of Developmental Biology of Hard Tissue, Graduate School of Dental Medicine, Hokkaido University

Abstract: Osteoblasts, bone-forming cells, synthesize a huge amount of type I collagen and mineralize extracellular matrix. Osteoblasts gradually differentiate into osteocytes embedded in bone matrix.  Osteoblasts and osteocytes extend their cytoplasmic processes passing through the narrow passageway referred to as osteocytic canaliculi, consequently forming the functional syncytium. Osteoclasts, multinucleated giant cells, are responsible for bone resorption. Osteoclasts secrete abundant proton and proteolytic enzymes, e.g., cathepsin K and MMP-9 through the ruffled border, deeply folding of the cell membrane facing the bone surface. In a normal state, bone matrix is always metabolized: osteoclasts resorb old bone, while osteoblasts migrate and deposit new bone onto where osteoclasts previously resorbed. The replacement of old bone to new bone is called bone remodeling. Bone remodeling is based on cellular coupling between osteoclasts and osteoblasts.

Key words: osteoblast, osteocyte, osteoclast, mineralized bone matrix, bone remodeling