In human skeletal tissue, what is the basic structural and functional unit of compact bone?

Introduction / Context:
Compact bone is the dense, hard outer layer of bones that provides strength and support. This question focuses on the microscopic organization of compact bone and asks about its basic structural unit. Understanding this structure helps explain how bones can be strong yet allow channels for blood vessels and nerves.

Given Data / Assumptions:
• Long bones have an outer compact bone region and inner spongy bone. • Compact bone is organized into repeating cylindrical units. • The question asks for the name of this repeating unit.

Concept / Approach:
The fundamental unit of compact bone is called the osteon, also known as the Haversian system. Each osteon is a cylindrical structure running parallel to the long axis of the bone. It consists of concentric rings of calcified matrix called lamellae surrounding a central Haversian canal containing blood vessels and nerves. Osteocytes, or bone cells, lie in small spaces called lacunae between lamellae and communicate via tiny canals called canaliculi. Recognizing this description allows you to distinguish the osteon from other bone related structures.

Step-by-Step Solution:
Step 1: Recall that compact bone is not solid all the way through but is organized into many microscopic cylindrical units. Step 2: Remember that these units have central canals with blood vessels and nerves and concentric lamellae of bone matrix. Step 3: These cylindrical units are called osteons or Haversian systems in standard anatomical terminology. Step 4: Identify the option that uses the term osteon or Haversian system to describe the structural unit.

Verification / Alternative check:
Diagrams in anatomy textbooks show a cross section of compact bone with circular structures labeled osteons. The central Haversian canal is in the middle, surrounding lamellae form rings, and small dark spots represent osteocytes. This confirms that the osteon is the structural unit, whereas spongy bone and periosteum are larger scale structures and not the repeating microscopic unit.

Why Other Options Are Wrong:
Option B: Spongy bone is a different type of bone tissue with trabeculae and marrow spaces, not the structural unit of compact bone. Option C: Lamellae are part of an osteon but are not the complete unit by themselves. Option D: Periosteum is a fibrous membrane that covers the outer surface of bone, not the structural unit within compact bone. Option E: The bone marrow cavity is a larger space inside long bones, mainly filled with marrow, and is not a microscopic repeating unit.

Common Pitfalls:
Students sometimes confuse macroscopic and microscopic structures. They may choose spongy bone or marrow cavity because they are familiar terms, but those refer to larger regions, not the repeating units of compact bone. Another mistake is to assume that lamellae alone are the structural units, forgetting that an osteon includes lamellae, a central canal, and associated cells. Focus on the complete cylindrical system to avoid these errors.

Final Answer:
The basic structural and functional unit of compact bone is the osteon or Haversian system.