Compact vs. Spongy Bone

Compact Bone Tissue: contains few spaces, forms the external layer of ALL bones, and the bulk of diaphyses of long bones. It provides protection and support and resists stress.

Structural Unit: Osteons (Haversian Systems) consist of the following structures:

• Central (Haversian) Canals- longitudinal canals running through bone.
• Concentric Lamellae- rings of hard, calcified matrix around the central canals.
• Lacunae- small spaces between the lamellae that contain osteocytes.
• Canaliculi- small channels that radiate from lacunae and are filled with ECF. They connect lacunae with each other and with central canals, provide routes for nutrients and oxygen to reach the osteocytes, and for wastes to diffuse away.
• Perforating (Volkmann's) Canals- canal through which blood vessels, lymphatic vessels, and nerves penetrate compact bone. 

Spongy Bone Tissue: makes up most of the bone tissue of short, flat, and irregular bones. It does NOT contain osteons!

• Structural Unit: Trabeculae- an irregular latticework of thin columns of bone. Inside the trabeculae are osteocytes, lacunae, and canaliculi.
• Sometime contains red bone marrow, which is the only site of hematopoeisis.
• Difference from compact bone? Spongy bone is lighter than compact bone, and supports and protects red bone marrow.