Which structures are considered the fundamental functional units of compact bone?

The fundamental functional units of compact bone are known as Haversian systems, or osteons. These structures consist of concentric layers of mineralized bone matrix, known as lamellae, surrounding a central canal that contains blood vessels and nerves. The organization of Haversian systems allows for efficient nutrient delivery and waste removal, facilitating bone maintenance and repair.

Osteocytes, while essential for bone maintenance and communication within the bone matrix, are actually located within the lacunae between the lamellae and do not constitute the structural unit of compact bone itself. Cartilage is a different type of connective tissue and serves various supportive roles in the body but does not form bone. Ligaments, which are made of dense connective tissue, connect bones to one another and provide stability to joints but are not involved in the composition of bone tissue. Thus, Haversian systems are indeed the correct answer as they represent the primary structural and functional units of compact bone, crucial for its strength and resilience.