Overview
This lecture covers the microscopic structure (histology) of bone tissue, focusing on the differences between compact and spongy bone and their structural features.
Compact Bone Structure
- Compact bone forms the hard, outer layer of bones and resists stress and strain.
- Compact bone is organized into cylindrical units called osteons.
- Each osteon consists of 4 to 20 concentric rings called lamellae (singular: lamella).
- Collagen fibers in the lamellae run in alternating directions in each layer for added strength.
- The central canal (Haversian canal) runs down the center of each osteon and contains blood vessels and nerves.
- Osteocytes (bone cells) live in small cavities called lacunae (singular: lacuna) within the lamellae.
- Lacunae are connected by tiny canals called canaliculi, allowing osteocytes to communicate and share resources via gap junctions.
- Bone is dynamic, with osteoclasts breaking down bone and osteoblasts building new bone, leaving behind interstitial lamellae between osteons.
- Circumferential lamellae are found on the outer and inner surfaces of compact bone, adding overall strength.
- Perforating (Volkmann's) canals run perpendicular to central canals, connecting blood vessels from the periosteum to osteons.
Spongy (Trabecular) Bone Structure
- Spongy bone is found inside bones and is less dense than compact bone, providing support against multidirectional stress.
- Spongy bone consists of a network of struts called trabeculae, not osteons.
- Trabeculae are covered by endosteum, which contains osteoblasts for bone formation.
- Although lamellae and lacunae with osteocytes exist in spongy bone, there are no central or perforating canals.
- Spongy bone receives its blood supply from vessels in the bone marrow.
Key Terms & Definitions
- Osteon — Cylindrical structural unit of compact bone.
- Lamella(e) — Concentric rings of bone matrix found within osteons.
- Central Canal (Haversian canal) — Channel in the center of an osteon containing blood vessels and nerves.
- Lacuna(e) — Small cavities in bone that house osteocytes.
- Canaliculi — Tiny canals connecting lacunae for cell communication.
- Interstitial Lamellae — Fragments of old osteons found in spaces between new osteons.
- Circumferential Lamellae — Rings of bone matrix surrounding the entire bone beneath the periosteum.
- Perforating (Volkmann's) Canal — Canals running perpendicular to central canals to connect blood supply.
- Trabecula(e) — Struts forming the framework of spongy bone.
- Periosteum — Outer membrane covering bones, containing blood vessels and nerves.
- Endosteum — Membrane lining internal bone surfaces and covering trabeculae.
Action Items / Next Steps
- Review diagrams of compact and spongy bone structures in your textbook.
- Study the functions of each bone component for upcoming quizzes.