What is the structural unit of compact bone that is a cylindrical length of matrix aligned with the bone's axis?

• A. Lamella
• B. Osteon
• C. Lacuna
• D. Canaliculi

Answer: (B)

The cylindrical length of matrix aligned with the bone’s axis is the osteon. In compact bone, osteons are the basic structural units that run parallel to the long axis of the bone. Each osteon centers on a Haversian canal (central canal) that houses blood vessels and nerves, and around this core are concentric rings of mineralized matrix called lamellae. Osteocytes sit in small cavities between lamellae called lacunae, and slender processes extend through tiny channels called canaliculi to connect these lacunae with each other and with the central canal, allowing nutrient and waste exchange. The other terms are parts of this structure: lamellae are the rings of matrix, lacunae are the spaces that house osteocytes, and canaliculi are the little channels linking lacunae.