Tipos de Células Ósseas: Osteoblastos (Formação) e Osteócitos (Manutenção)

Bone remodeling is orchestrated by distinct cell lineages:

Osteoblasts

• Origin: Mesenchymal stem cells → pre-osteoblasts → osteoblasts (Runx2, Osterix transcription factors).
• Function: Synthesize osteoid (type I collagen, proteoglycans, glycoproteins); regulate mineralization (alkaline phosphatase, osteocalcin, matrix vesicles).
• Fate: Become osteocytes (entrapped), lining cells (quiescent), or undergo apoptosis.
• Regulation: PTH (intermittent → anabolic), BMPs, Wnt/β-catenin, IGF-1; inhibited by sclerostin (SOST), glucocorticoids.

Osteocytes

• 90–95% of bone cells; terminally differentiated, dendritic, networked via canaliculi (gap junctions).
• Mechanosensors: Detect strain → signal osteoblasts (formation) / osteoclasts (resorption) via sclerostin, RANKL/OPG, prostaglandins, NO.
• Mineral homeostasis: Perilacunar remodeling (acidic resorption, re-mineralization) — rapid Ca²⁺/PO₄³⁻ buffering.
• Endocrine: Produce FGF23 (phosphate regulation), sclerostin (Wnt inhibition).

Osteoclasts (Third Key Cell)

Hematopoietic origin (monocyte/macrophage); RANKL/RANK/OPG system; resorb bone via ruffled border (H⁺-ATPase, cathepsin K).

Coupling

Resorption → reversal → formation (BMU — basic multicellular unit). Imbalance → osteoporosis, osteopetrosis, fibrous dysplasia.