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Osteoclasts and osteoblasts are partners in the bone remodeling process – osteoblasts are the bone-building and osteoclasts the bone-resorbing cells (Fig 1-4a and b). Osteoclasts are therefore specialized in the breakdown of calcified tissue. Hematopoietic cells, particularly those of the monocyte lineage, are the pool of progenitors that have the potential to become osteoclasts; otherwise, they develop into macrophages or dendritic cells with a focus on the immune system. The molecular signature to drive osteoclastogenesis was discovered almost two decades ago, with the introduction of the RANKL-OPG system, the agonist, and the respective antagonist.^23,61,118 Mouse models that lack RANKL⁷³ or the respective receptor RANK³⁸ develop severe osteopetrosis, indicated by the lack of a bone-marrow cavity and non-disrupted teeth. In contrast, mice lacking RANKL-OPG acquire a fulminant osteoporosis.^14,111 RANKL was considered the ‘bottleneck’ of osteoclastogenesis. Mature osteoclasts are characterized by the sealing zone that sticks the osteoclasts to the mineralized bone surface, surrounding that extensively folded ‘ruffled border,’ where the protons (to lower the pH) and the proteases (to digest the collagen, mainly cathepsin K) are transported into the space facing the naked bone matrix.¹²¹ Osteoclasts are considered to be of “great beauty”¹⁸ and are not simply “bone eaters”²⁷ as they contribute to bone formation and also interact with the hematopoietic system, including the stem cell niche and adaptive immune cells.

Fig 1-4 Creation of osteons by basic multicellular compartments (BMU). The BMU defines the site of bone remodeling. (a) Tunneling of cortical bone by multinucleated osteoclasts. (b) This image is characteristic for the activity of bone forming osteoblasts with an osteoid layer, rebuilding the concentric structure of osteons. [The image is of pig bone.]

The main physiologic function of osteoclasts is to participate in bone remodeling. Localized in Howship’s lacunae, which represent the active resorption sites on a bone surface, osteoclasts are indicated as multinucleated cells staining positive for tartrate-resistant acid phosphatase. The acidophil cytoplasm contains vacuoles, which indicate resorption. In trabecular bone, osteoclast resorption does not usually exceed 70 µm before a team of osteoblasts fills the space with new bone. Howship’s lacunae are part of the bone remodeling compartment (BRC) canopy.³⁵ In cortical bone, however, the basic multicellular unit (BMU) defines the site of bone remodeling.¹⁰⁶ Here, osteoclasts produce a tunnel in the cortical bone that is closed in concentric layers of new bone by the bone-forming osteoblasts with a blood vessel in the center, culminating in the characteristic histologic picture of the osteons in a transversal section (Fig 1-5). Even though the two remodeling compartments are not identical in structure, there is the common principle of the coupling: when osteoclastic bone resorption has ceased, osteoblastic bone formation is initiated. Preosteoclasts are not only important for bone renewal and remodeling but also for bone revascularization,¹³⁷ thereby possibly supporting the sprouting of blood vessels at the site of bone regeneration.

Fig 1-5 Osteon with osteocytes being connected via their canaliculi. The osteon is a functional bone unit consisting of a central canal filled with soft tissue, with bone lamellae arranged concentrically around it. They can be found in the substantia compacta of the bone. Osteocytes are interconnected via canaliculi. They are in contact via canaliculi with the lining cells in the central channel. [The image is of human bone, from an implant extraction.]