histo

• Cartilage and bone


  Special type of connective tissue, particularly dense regular connective tissue
• Cartilage and bone


  • Few cells and abundant intercellular substance
• Cartilage


  Intercellular substance is gelatinous
• Bone


  Intercellular substance is calcified, rigid, and hard
• Cartilage

  • Important for support of soft tissues
  • Important for formation and growth of long bones
  • Important for durability of articular joints
• Components of cartilage

  • Chondroblast and chondrocytes (cartilage cells)
  • Collagen and in some cases elastin fibers
  • Glycosaminoglycans
  • Proteoglycans, proteoglycans aggregates
  • Water
  • Glycoprotein - chondronectin
• Chondroblast


  Immature cartilage cells that secrete extracellular matrix
• Chondrocytes


  Mature cartilage cells embedded in the extracellular matrix
• Qualities of different types of cartilage

  • Differences in the type of collagen and concentration of collagen and elastin fibers in the extracellular matrix
• Hyaline and elastic cartilage

  • Surrounded by a connective tissue capsule called the perichondrium
• Cartilage is devoid of blood vessels
• Nutrition of cells within the cartilage matrix is dependent on the diffusion of nutrients from blood capillaries in perichondrium and/or adjacent tissues
• Three types of cartilage

  • Hyaline Cartilage
  • Elastic Cartilage
  • Fibrous Cartilage
• Hyaline Cartilage


  • Dominant protein component of extracellular matrix is collagen (type 2)
  • Translucent to bluish-white in life
  • Important in embryonic formation and later growth of long bones
  • In adult, mainly found lining outer wall of respiratory passages, ventral ends of ribs, and on surfaces of bone joints where it is called articular cartilage
• Perichondrium

  • Vascularized connective tissue sheath surrounding cartilage (except in case of articular cartilage)
  • Rich in collagen
  • Main cell type - fibroblast
  • Inner layer contains cells that can differentiate to form chondroblasts
• Chondroblast


  Immature cartilage cells that secrete extracellular matrix, but are not yet imprisoned in a lacuna
• Chondrocytes


  Mature cartilage cells embedded in the extracellular matrix, residing in small spaces within the matrix called lacunae
• Hyaline cartilage can undergo calcification and can act as the template for bone formation and growth
• Elastic Cartilage

  • High concentration of elastin fibers in extracellular matrix
  • Ground substance is yellow in color (due to elastin content)
  • Chondrocytes are more closely packed, no isogenic groups can be identified
  • Chondrocytes exhibit less accumulation of glycogen and lipids than in hyaline cartilage
  • Does not calcify
• Fibrous Cartilage (fibrocartilage)


  • An irregular, dense, fibrous tissue with thinly dispersed, encapsulated chondrocytes
  • Contains many very large bundles of collagen fibers (type 1)
  • Resists compression and shear forces, has durability and high tensile strength
  • Found at connection of tendons to bone and in intervertebral discs and some joints
  • No perichondrium
• Interstitial Growth (Endogenous growth)

  1. Mesenchymal cells will aggregate and differentiate into closely knit clusters of chondroblasts

  2. Chondroblasts will secrete collagen and mucopolysaccharide matrix

  3. Matrix secretion will cause the chondroblasts to be pushed apart

  4. Cartilage cells will undergo divisions and continue to secrete matrix

  5. Ground substance becomes more rigid and the cartilage cells (now chondrocytes) become trapped in lacunae
• Appositional Growth (Exogenous growth)

  1. New cartilage is added to the surface of existing cartilage

  2. Chondroblasts in the inner layer of the perichondrium divide and secrete new cartilage matrix
• Endogenous growth


  The growth of cartilage from within
• Endogenous growth


  • Responsible for the growth in length of the long bones
  • The one responsible for the interstitial growth of the cartilage are the young chondrocytes
• Interstitial growth of cartilage

  Daughter cells and large cartilage form from within by secreting precursors of extracellular fibers and other organic components of the matrix into the surrounding intercellular area
• New cartilage cells produced during interstitial growth

  • Arise from the division of chondrocytes within their lacunae
  • Possible only because the chondrocytes retain the ability to divide and the surrounding matrix is distensible, thus permitting further secretory activity
  • Initially, the daughter cells occupy the same lacuna
  • As new matrix is secreted, a partition is formed between the daughter cells
  • With continued secretion of matrix, the cells move even farther apart from each other
  • The overall growth of cartilage results from the interstitial secretion of new matrix material by chondrocytes and by the appositional secretion of matrix material by newly differentiated chondroblasts
• Appositional Growth


  • A layer of chondroblasts can lay down matrix at the outer edge of a mass of growing cartilage
  • The process that forms new cartilage at the surface of an existing cartilage
• Exogenous growth

  Also referred as appositional growth
• Appositional growth


  • The responsibility of the perichondrium
  • Osteoprogenitor cells in the perichondrium multiply and differentiate into chondroblast
  • Chondroblast add to the cartilage matrix on the outer surface of the developing cartilage through their secretions
• New cartilage cells produced during appositional growth

  • Derived from the inner portion of the surrounding perichondrium
  • Resemble fibroblasts in form and function, producing the collagen component of the perichondrium (type I collagen)
  • Undergo a differentiation process guided by an expression of the transcription factor SOX-9
  • Cytoplasmic processes disappear, the nucleus becomes rounded, and the cytoplasm increases in amount and prominence, resulting in the cell becoming a chondroblast
• Bone is one of the hardest substances in the body
• Bones


  • Comprise the bulk of adult skeleton and form a rigid framework for the body
  • Protect the vital organs
  • Serve as levers for muscles
  • Store or house calcium and phosphorus
• Bone


  • Consists of bone tissue and other connective tissues, including hemopoietic tissue, fat tissue, blood vessels, and nerves
  • If the bone forms a freely movable joint, hyaline cartilage is present
  • The ability of the bone to perform its skeletal function is attributable to the bone tissue, ligaments and, where present, the articular (hyaline) cartilage
• Surfaces of bones

  • Except articulating surfaces and surfaces where muscles, tendons, and ligaments are attached, are either lined or covered by dense irregular connective tissue with osteogenic potency
• Periosteum


  The special connective tissue with osteogenic potency that covers the external surfaces of bone
• Endosteum


  The special connective tissue with osteogenic potency that lines all the internal surfaces and cavities in the bones
• Periosteum

  • Outer layer is very fibrous
  • Inner layer is more cellular and vascularized
  • Consists of outer fibrous layer and inner osteogenic layer
  • Osteogenic layer consists mainly of osteoprogenitor stem cells
  • Fibers (Sharpey's fibers) penetrate the calcified bone matrix and bind periosteum to bone
  • Provides cells for bone histogenesis during development and in the healing of fractures
• Endosteum

  • Lines the internal surfaces of bones – medullary marrow and vascular cavities
  • Similar in general function to the periosteum, but much thinner
  • Does not exhibit two distinct layers, only made up of single layer of osteoprogenitor stem cells
• Roles of periosteum and endosteum

  • Nutrition of bone cells
  • Provision of osteoblasts for bone histogenesis and repair
• Spongy bone

  Consists of numerous inter-connecting bone fragments known as the spicules or trabeculae