On light microscopy of a cross-section of compact bone, which structure represents the functional unit (osteon) and where are the osteocytes located?

• A Trabeculae: irregular spicules of bone with osteocytes in flat lacunae
• B Haversian system: concentric lamellae around a central (Haversian) canal containing blood vessels; osteocytes in lacunae between lamellae, connected by canaliculi ✓
• C Volkmann's canals: horizontal vessels connecting Haversian canals, lined by osteoblasts
• D Periosteum: fibrous layer with osteoprogenitor cells and osteoblasts on its inner surface only

Explanation

The osteon (Haversian system) is the structural and functional unit of compact bone. It consists of concentric lamellae of calcified collagen matrix surrounding a central Haversian canal containing blood vessels, lymphatics, and nerves. Osteocytes reside in lacunae (spaces between lamellae) and communicate with each other and with the central canal via canaliculi (tiny channels). Volkmann's (perforating) canals run transversely/obliquely connecting adjacent Haversian canals. The periosteum has an outer fibrous layer and an inner cellular (osteogenic) layer with osteoblasts and osteoprogenitor cells.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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