Learning Objectives
- Identify the structural unit of compact bone: the osteon (Haversian system).
- Recognize the spatial arrangement of osteocytes within lacunae and canaliculi.
- Distinguish between Haversian canals and Volkmann canals.
- Understand the composition of the bone matrix and its metabolic control.
Overview of Cortical Bone Structure
This high-power view of an undecalcified ground bone section reveals the intricate micro-architecture of mature adult bone. Unlike stained sections, ground bone preparations allow for the visualization of the mineralized matrix and the delicate network of channels that support cell life.
Characteristic Features
- Osteons: The cylindrical structural units of compact bone, consisting of concentric lamellae (rings) surrounding a central canal.
- Osteocytes: Mature bone cells trapped in small spaces called lacunae. Their dark nuclei are visible at the center of the rings.
- Canaliculi: Microscopic, thread-like processes that radiate from the lacunae. These allow osteocytes to communicate and receive nutrients from the central blood supply.
- Haversian Canal: The central channel of each osteon, containing the bone’s blood vessels and nerve fibers.
- Volkmann Canals: Transverse channels that connect adjacent Haversian canals, allowing for a coordinated vascular network throughout the bone.
Matrix Composition & Metabolism
- Mineral Component: Most of the bone matrix is composed of hydroxyapatite crystals (hydrated calcium phosphate).
- Hormonal Control: Bone metabolism is tightly regulated by Serum Calcium, Vitamin D, and Parathyroid Hormone (PTH).
Clinical Relevance: Bone Integrity
- Renal Osteodystrophy: In chronic kidney disease, imbalances in calcium and phosphate lead to abnormal bone remodeling and weakening of the osteon structure.
- Osteomalacia: In adults, a deficiency in Vitamin D leads to poorly mineralized osteoid, meaning the hydroxyapatite cannot properly deposit around the osteons.
- Stress Fractures: Overuse can lead to micro-cracks that disrupt the canaliculi network; if remodeling (via new osteon formation) cannot keep up, a clinical fracture may occur.
