The structural unit of compact bone is called which of the following?

• A. Osteon
• B. Osteoid
• C. Trabecula
• D. Periosteum

Answer: (A)

The main idea here is how compact bone is organized into repeating units that give it strength. The structural unit of compact bone is the osteon, a cylindrical unit also called the Haversian system. Each osteon centers on a Haversian canal that contains blood vessels and nerves, and is surrounded by concentric lamellae of mineralized matrix. Between the lamellae lie tiny spaces called lacunae, each housing an osteocyte, and these osteocytes communicate through tiny channels called canaliculi to exchange nutrients and waste. This organized, parallel arrangement to the long axis is what makes compact bone dense and strong.

Osteoid, by contrast, is the organic, unmineralized matrix produced by osteoblasts before it hardens. Trabeculae are the thin, beam-like elements of spongy (cancellous) bone, not the dense cylindrical units of compact bone. Periosteum is the outer fibrous membrane that covers the bone, not a structural unit within the bone tissue.