## Lacunae and Osteocytes in Mature Bone **Key Point:** Lacunae are small, lens-shaped cavities within the mineralized bone matrix that house osteocytes — the mature, differentiated form of osteoblasts. ### Lacunar Structure and Function - **Definition:** Small spaces (10–20 μm) embedded within mineralized bone matrix - **Occupant:** Osteocytes (one cell per lacuna) - **Appearance:** Appear as empty spaces in standard histological sections (cells are lost during decalcification) - **Distribution:** Arranged in concentric layers around Haversian canals in compact bone ### Osteocyte Characteristics 1. **Origin:** Differentiated osteoblasts that became trapped in their own matrix 2. **Morphology:** Stellate (star-shaped) cells with multiple cytoplasmic processes 3. **Function:** - Maintain bone matrix integrity - Sense mechanical stress via canaliculi - Regulate mineral homeostasis - Communicate via gap junctions through canaliculi 4. **Lifespan:** 10–20 years (longest-lived bone cell) ### Canaliculi - **Definition:** Small channels radiating from lacunae - **Content:** Osteocyte processes and extracellular fluid - **Function:** Allow nutrient diffusion and cell-to-cell communication - **Appearance:** Visible as fine lines in ground sections under light microscopy **Mnemonic:** **LOC** — **L**acunae contain **O**steocytes with **C**analiculi extending outward. **High-Yield:** Osteocytes are the most abundant bone cells (~90% of total bone cells) despite being less metabolically active than osteoblasts. 
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