Bone Histology Overview

Bone Histology

• Bone is a highly vascularized connective tissue, its matrix is calcified making it hard and strong; this hardness aids in carrying body weight.
• Bone is composed of three main components: bone cells, fibers, and hard matrix.
• Bone is anatomically classified as: long (e.g., humerus, femur), short (e.g., carpal, tarsal bones), flat (e.g., skull, ribs), and irregular (e.g., vertebrae).
• Bone functions include support, protection, movement, mineral storage, and blood cell formation.

Learning Objectives

• Classify bone anatomically and histologically.
• Identify locations of compact and spongy bones in the human body.
• Understand the microscopic structure of bone.

Histological Classification

Compact Bone

• Composed of solid, dense lamellae (layers) without cavities.
• Found in the shaft (diaphysis) surrounding the medullary canal, covering surfaces of flat, short, and irregular bones.

Cancellous (Spongy) Bone

• Composed of irregular, branching, and anastomosing bony trabeculae.
• Trabeculae enclose bone marrow cavities.
• Located in the interior of flat, short, irregular bones, and the epiphyses of long bones.

Bone Cells

• Osteogenic cells (osteoprogenitor cells): Precursor cells that develop into osteoblasts.
• Osteoblasts: Bone-forming cells that synthesize osteoid tissue (organic part of bone matrix). Osteoid is the newly formed bone matrix prior to mineralization.
• Osteocytes: Mature bone cells; housed within lacunae; maintain bone's hardness.
• Osteoclasts: Large, multinucleated cells that resorb or break down bone. Situated in Howship's lacunae (shallow depressions) along bone surfaces. These cells work by releasing hydrogen ions (H+) and secreting lysosomal enzymes.

Structure of Compact Bone

• Periosteum: Outer fibrous connective tissue layer covering the bone.
• External circumferential lamellae: Parallel lamellae beneath the periosteum.
• Haversian system (osteon): Cylindrical structural unit containing concentric lamellae surrounding a central canal (Haversian canal); this canal contains blood vessels, nerves, and loose connective tissue.
• Interstitial lamellae: Irregularly arranged lamellae filling spaces between osteons.
• Internal circumferential lamellae: Parallel lamellae surrounding the endosteum.
• Endosteum: Inner layer lining the marrow cavity, composed of a single layer of osteoblasts.

Structure of Spongy Bone

• Composed of branching, anastomosing trabeculae.
• Trabeculae contain bone marrow cavities within them.
• Trabeculae are created by irregularly arranged lamellae enclosing osteocytes.
• Unlike compact bone, spongy bone does not contain osteons.

Reference Material

• Junqueira's Basic Histology, 13th Edition (2013), by Anthony L. Mescher

This quiz delves into the histological structure of bone, focusing on the classification and composition of both compact and cancellous bone. It encourages learners to identify the anatomical locations and functions of different bone types within the human body. Enhance your understanding of bone as a vital connective tissue.