Histology of Cartilage and Bone

Histology of Cartilage and Bone

• Cartilage is a specialized type of dense connective tissue with a firm, flexible matrix
• It is composed of perichondrium (outer layer), a firm intercellular matrix (ground substance and fibers), and cells (chondrogenic, chondroblast, and chondrocytes)
• Cartilage provides support, flexibility, and protection; serves as a model for bone development in embryos.
• It lacks blood vessels and lymph vessels, relying on diffusion for nourishment.
• Cartilage has poor regeneration capacity
• Nourishment occurs through capillaries in adjacent connective tissue or synovial fluid in joint cavities.

Learning Objectives

• Students will be able to describe the histological structure, distribution, and growth of different types of cartilage.
• Students will be able to compare different types of cartilage.
• Students will be able to describe the histological structure, distribution, and growth of different types of bone.
• Students will be able to compare different types of bone.

Cartilage Matrix

• The matrix, a firm, rigid intercellular gel, is secreted by cartilage cells.
• It is composed of proteoglycans (e.g., chondroitin sulfate), glycoproteins (e.g., chondronectin), glycosaminoglycans (e.g., hyaluronic acid), and collagen fibers (primarily type II, with types I, 2, and 11).
• The matrix is arranged in layers; a thin layer of highly basophilic material around the cell membranes is the capsular matrix.
• The bright, basophilic matrix around a cell nest (isogenous group) is called the territorial matrix.
• The lighter areas between cell nests are the interterritorial matrix.

Cartilage Cells

• Chondrogenic cells: stem cells originating from mesenchymal cells. Located in the perichondrium, they differentiate into chondroblasts.
• Chondroblasts: young, actively dividing cells that secrete the cartilage matrix. Located at the periphery of the cartilage, they are small with basophilic cytoplasm and rounded nuclei.
• Chondrocytes: mature cartilage cells residing in lacunae (cavities). These cells are larger than chondroblasts and have condensed chromatin and less basophilic cytoplasm. Chondrocytes reside in groups, called ‘isogenous groups'. They are responsible for maintaining and producing the cartilage matrix.

Perichondrium

• A sheath of vascular connective tissue covering the surface of most cartilage.
• Composed of two layers:
  • An outer, fibrous layer containing type I collagen fibers, fibroblasts, blood vessels, and nerves.
  • An inner, cellular layer containing chondrogenic cells and chondroblasts (no lacunae).
• Plays important roles in cartilage growth and repair through providing nutrients, and removing waste products from cartilage cells.

Types of Cartilage

• Hyaline cartilages are translucent, glassy, and composed mainly of type II collagen.
• Elastic cartilages are flexible and contain elastic fibers alongside collagen type II.
• Fibrocartilages are tough and highly resistant to compression forces, primarily comprising collagen type I.

Bone Tissue

• A specialized, highly vascularized form of dense connective tissue.
• Bones are made of bone tissue, cartilage (at joints), connective tissue (periosteum and endosteum), yellow bone marrow (adipose tissue), red bone marrow (hemopoietic tissue), blood vessels, and nerves.
• It is characterized by a strong matrix of calcium salts, making it rigid while collagen provides flexibility.

Bone Matrix

• The intercellular substance or matrix of bone is made of 25% water, 25% protein, and 50% inorganic mineral salts composed primarily of calcium and phosphorus in the form of hydroxyapatite crystals.
• Organic components include primarily collagen type I (90-95%), and a small amount of glycosaminoglycans and glycoproteins.

Bone Cells

• Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.
• Osteoblasts: Bone-forming cells that produce and secrete the organic matrix (osteoid), including collagen, and other proteins, essential for bone mineralization.
• Osteocytes: Mature bone cells residing in lacunae, maintaining the bone matrix and regulating calcium homeostasis.
• Osteoclasts: Multinucleated, bone-resorbing cells that break down bone tissue through secretion of acids and enzymes, assisting in remodeling and calcium regulation.

Periosteum

• Outer connective tissue covering bone except at joints.
• Consists of an outer fibrous layer and an inner cellular layer.
• Contains osteoblasts, osteoclasts, and osteoprogenitor cells.
• Sharpey's fibers anchor the periosteum to the bone.

Endosteum

• Thin connective tissue lining the inner surface of bone, including the medullary cavity.
• Composed of osteoprogenitor cells, osteoblasts, and osteoclasts.
• Essential for bone growth and repair as it supplies blood and new cells for bone.

Bone Growth

• Appositional growth: Growth in width by adding new bone tissue to the outer surface (periosteum) and removing it from the inner surface (endosteum).
• Interstitial growth: Growth in length by cartilage growth and replacement with bone tissue, occurring primarily at the epiphyseal plate until it fuses and transforms into an epiphyseal line.

Compact Bone

• Lamellar bone or dense bone, composed of osteons, concentric lamellae, interstitial lamellae, and circumferential lamellae.
• Osteons, the structural units of compact bone, form cylindrical structures running parallel to the long axis of a bone.
• Each osteon has a central canal (Haversian canal) that contains blood vessels and nerves.
• Canaliculi connect the lacunae, housing the osteocytes, to the central canal, allowing for nutrient and waste transport.
• Perforating canals (Volkmann's canals) penetrate the compact bone at right angles to the central canals, linking the vascular and nervous supply in different layers.

Spongy Bone

• Porous, trabecular bone, characterized by an interwoven network of thin bony plates or spicules (trabeculae), filled with bone marrow.
• Serves a vital role in providing strength with minimal weight.
• Located in the epiphyses of long bones and the interior of flat bones, with a higher proportion within these parts of the skeleton.

This quiz focuses on the histological features and functions of cartilage and bone. Students will explore the structure, distribution, and growth of various cartilage types, as well as how they compare to bone tissue. Enhance your understanding of these essential connective tissues in the body.