Cartilage and Bone: Histology

• Cartilage and bone lecture covering general and histological characteristics.
• Outlines the cells and fibers found in cartilage and bone.
• Focuses on comparing the types of bone, cartilage as well as the cells found within them.

Cartilage

• Firm, rigid, and rubbery connective tissue lacking blood vessels.
• Composed of cartilage cells (chondroblasts and chondrocytes), connective tissue fibers (collagen and elastic), and matrix.
• Three types exist: hyaline, elastic, and fibrocartilage which are differentiated by fiber type.

General Histological Characters of Cartilage

• Cells are widely separated by abundant extracellular matrix (ECM).
• Covered by perichondrium, except at articular surfaces of joints and white fibrocartilage.
• Avascular, receiving nutrients from the perichondrium or synovial fluid.
• Formed of cells, matrix, and fibers.

Perichondrium

• Connective tissue sheath covering the cartilage surface.
• Consists of two layers: an outer fibrous layer (collagen type I, fibroblasts, blood vessels) and an inner cellular (chondrogenic) layer (chondroblasts).

Cartilage Cells

• Chondroblasts reside in the inner layer of the perichondrium, are flat or oval with basophilic cytoplasm, and contain a central, pale nucleus.
• Chondroblasts form cartilage matrix, create collagen, and differentiate into chondrocytes.
• Chondrocytes originate from chondroblasts, reside in lacunae (spaces) inside the matrix, and are typically present in cell nests (2 to 8 cells per lacuna).
• Chondrocytes are less active than chondroblasts with less basophilic cytoplasm and darkly stained nuclei.
• Chondrocytes secrete cartilage matrix and fibers, maintaining the cartilage.

Hyaline Cartilage

• The most common cartilage type.
• Characterized by a homogenous and basophilic matrix with type II collagen fibrils.
• Found at the ends of long bones, ribs, trachea, and bronchi.

Elastic Cartilage

• Contains an abundance of elastic fibers in addition to type II collagen fibrils and is flexible.
• Located in the ear pinna, external ear, and Eustachian tube.

Fibrocartilage

• Lacks a perichondrium.
• Has chondrocytes in lacunae present in parallel rows between bundles of collagen fibers type I.
• Present in intervertebral discs and the mandibular joint.

Bone

• Specialized supportive connective tissue with a hard (solid) matrix.
• Covered externally by periosteum and lined internally by endosteum.
• Composed of bone cells and a calcified solid ECM.
• Highly vascular.
• Protects and supports vital organs, stores calcium and phosphate, and contains bone marrow which acts as a hemopoietic organ.

Periosteum and Endosteum

• Periosteum is a connective tissue sheath covering the outer bone surfaces, formed of 2 layers
  • Outer fibrous layer, dense layer of CT, collagen fibers type 1, fibroblasts
    • Rich in bl vs
  • Inner cellular layer, divides and differentiates into osteoblasts
• Endosteum lines bone marrow cavities and consists of a single layer of osteogenic cells and osteoblasts in a delicate layer of loose connective tissue.

Bone Matrix

• Bone matrix has organic and inorganic components.
• Organic components include collagen fibers (type I) and proteins.
• Inorganic components mainly consist of calcium phosphate.
• The calcified matrix is organized into layers (lamellae) of calcified collagen bundles embedded in calcified ground substance.
• Small cavities (lacunae) situated between lamellae contain osteocytes.

Bone Cells

• Osteogenic Cells
  • Present within the inner layer of periosteum and endosteum
  • Undergo cell division and differentiate into osteoblasts during bone growth and bone fractures
• Osteoblasts
  • Located side by side on bone surfaces.
  • Cuboidal or oval cells with basophilic cytoplasm and a negative Golgi image.
  • Responsible for the synthesis of bone matrix (bone-forming cells).
  • Over time they transform to osteocytes
• Osteocytes
  • Located within bone lamellae, singly inside lacunae.
  • They have less basophilic cytoplasm, are dark and have many processes extending through canaliculi to connect with other cells
  • Responsible for maintenance of the bone matrix.
• Osteoclasts
  • Present on the endosteum surface
  • They are large, multinucleated (5-20 nuclei) giant cells, and present with acidophilic cytoplasm and a ruffled border
  • Bone-resorbing cells releasing substances to break down bone.

Bone Tissue Types

• Compact Bone: Bone lamellae are regularly arranged forming Haversian systems (osteons), which are structural units with concentric lamellae around a central Haversian canal with osteocytes in lacunae, and have one bone marrow cavity.
  • Located in the shafts of long bones.
• Spongy Bone: Bone lamellae are irregularly arranged forming bone trabeculae lacking Haversian system with multiple bone cavities containing osteocytes.
  • Found at the ends of long bones, vertebrae, and short/flat bones (ribs and skull).