Bone is a specialized form of the connective tissue characterized by impregnation of the extracellular matrix with salts of calcium and phosphate. Mineralization is connected mainly with the apposition of hydroxyapatite crystals onto both components of the ground substance and collagen fibrils and leads to a special strength and stability of the tissue, making it particularly qualified for providing support and protection for the body and its organs. Another important function of bone is to establish a reservoir for calcium and phosphate ions. Bone is highly vascularized and plays a crucial role in the regulation of blood calcium levels. Although it appears rigid and inflexible, it is metabolically active and sensitive to functional alterations and changes in load, which result in a reorganization of the tissue and reconstruction of the bone skeleton. Bone undergoes continuous remodeling associated with resorption, new production of mineralized matrix, and neovascularization not only during development but also in the adult bone skeleton according to functional conditions. Undisturbed secretory and endocytic trafficking in osteoblasts and osteoclasts, including secretion of lysosomal enzymes and transcytosis of endocytic materials, play a pivotal role in bone remodeling.

Panels A–C show segments of bone tissue derived from distal femur and proximal epiphyses of the mouse. In panel A, a vascular channel is shown, leading a capillary (Cap) embedded in loose connective tissue. Fibroblasts and osteoprogenitor cells are assembled. Active osteoblasts (Obl) are lined up, forming an epithelial-like monolayer of polarized cells, which produce and deposit osteoid, the nonmineralized organic bone matrix consisting mainly of type I collagen fibrils, to a lesser extent of type V collagen, and a range of ground substance components including glycosaminoglycans, and glycoproteins such as osteocalcin, osteopontin, and osteonectin. Osteoid is shown in panel A, forming a light zone around the osteoblast layer. The neighboring, extremely electron dense masses represent already mineralized matrix.

The osteoblasts in panel C show characteristics of highly active secretory cells with large nucleoli and the cytoplasm stuffed with rough endoplasmic reticulum (ergastoplasm). One of the osteoblasts has already left the epithelial-like osteoblast layer and has lost its polar organization. It is in part surrounded by its product, the osteoid (O), and forms thin projections, one of which (arrow) comes in contact with one of the processes of the “young” osteocyte (arrow) visible in the lowermost part of the figure. This young osteocyte is already surrounded by mineralized matrix (M), but it is still highly active, as can be seen by the abundance of rough endoplasmic reticulum. In this feature, it contrasts with the osteocyte shown in panel B. This “older” osteocyte is located in a small cave and is already completely embedded in calcified bone matrix. The small cytoplasm around the nucleus contains a reduced number of organelles. A thin cell process projects into a fine channel (arrow). The mineralized matrix is interspersed with numerous such channels (canaliculi), providing a communicating nutritive system within the hard bone tissue. Nutrients diffuse from the blood vessels in the vascular channels through the canaliculi into the osteocyte caves. The channels also lead osteocyte processes, by which the cells communicate via gap junctions.