HISTOLOGY (CARTILAGE AND BONE

• Cartilage


  Forms bulk of the skeleton of the fetus in utero
• Cartilage


  • Important for support of soft tissues
  • Important for formation and growth of long bones
  • Important for durability of articular joints
• Cartilage matrix contains


  • Chondroblasts and chondrocytes (cartilage cells)
  • Collagen and in some cases elastin fibers
  • Glycosaminoglycans
  • Proteoglycans, proteoglycan aggregates
  • Water
• Chondrocyte (Cartilage Cell)


  • Found inside the cavities (lacunae) that are scattered in the intercellular substance (cartilage matrix)
  • Young cells are elliptical in shape with long axes parallel to the cartilage surface; Mature- larger and rounded
  • Cytoplasm- finely granular and basophilic
  • Nucleus- ovoid
  • Young cells- capable of mitosis; mature are not. -In mitosis- daughter cells are called ISOGENOUS cells

  
• Cartilage matrix


  • Consist of an amorphous ground substance where EXTRACELLULAR FIBERS are embedded
  • Ground Substance - Mostly made up PROTEOGLYCANS
  • Proteoglycans are large molecules that consist of CORE PROTEINS to which disaccharides (in the form of GLYCOSAMINOGLYCAN-chondroitin sulfate and keratan sulfate) are covalently attached in a radiating pattern
  • Most are bound by "link protein" to HYALURONIC ACID and form as proteoglycan aggregates
  • Capsule of the chondrocyte (50um thickness)- Territorial matrix
  • Area between the territorial matrices- Interterritorial matrix
  • Innermost layer (1-3 um)- Pericellular capsule- protects chondrocyte against mechanical stress

  
• Extracellular fibers in cartilage matrix

  • Collagen fibers
  • Elastic Fibers
• Hyaline cartilage


  • Glistening, smooth, and pearly white in fresh specimen
  • Most abundant type in the body
  • Cartilage type that comprises the epiphyseal plate
  • In adults- persists only in joints as ARTICULAR CARTILAGE, TRACHEA and BRONCHI, parts of LARYNX, NOSE, and EARS and on the sternal end of ribs- COSTAL CARTILAGE
  • Dominant protein component of extracellular matrix is collagen (type 2)

  
• Elastic cartilage


  • More flexible the hyaline
  • Yellowish in fresh specimen
  • Present in the AURICLE and EXTERNAL ACOUSTIC MEATUS of the ear, AUDITORY TUBE, EPIGLOTTIS, and some parts of the LARYNX
  • Morphologically similar to hyaline EXCEPT that its MATRIX is LESS abundant
  • Collagen fibers- made up of collagen type II which contains considerable quantity of ELASTIC fibers- reason for its PLIABILITY and YELLOWISH color of the tissue

  
• Fibrous cartilage


  • Can withstand greater stress than hyaline and elastic
  • White in fresh specimen
  • Cartilage type that makes up INTERVETEBRAL DISCS, ARTICULAR DISCS, and GLENOID and ACETABULAR LABRA
  • Can also be found in the surface layers of TENDONS and LIGAMENTS that are pressed on by bone
  • Collagen fibers- TYPE I- thicker than those in hyaline and elastic
  • Can be regarded as transitional stage between dense regular CT and cartilage

  
• Perichondrium

  • Outer layer- FIBROUS LAYER- that blends with the surrounding tissue
  • Inner layer which is more cellular- CHONDROGENIC LAYER- adheres to the cartilage
  • The cells of the chondrogenic layer are apposed (placed near/ side by side) to the surface of the cartilage are stem cells OSTEOPROGENITOR CELLS- can differentiate to form CHRONDROBLAST or OSTEOBLAST
  • CHONDROBLAST- synthesize the components of the extracellular matrix (ECM)

  
• Chondrogenesis


  Cartilage formation

  
• Interstitial growth


  • Also known as Endogenous Growth
  • Expands (grow) the cartilage from within
  • Possible in relatively young cartilage where intercellular substance is still malleable and chondrocyte can still divide
  • Growth is heralded by the mitosis of young chondrocytes- daughter cells enlarge the cartilage from within by secreting precursors of EC fibers and other component of matrix into the intercellular area
  • As cartilage ages, its intercellular substances become rigid- at some point- interstitial growth stops
  • Also responsible for the growth in length of long bones

  
• Appositional growth


  • Also known as exogenous growth
  • Occurs when the osteoprogenitor cells in the perichondrium multiply and differentiate into chondroblasts
  • A layer of chondroblasts can lay down matrix at the outer edge of a mass of growing cartilage

  
• Degeneration and regeneration of cartilage

  1. Calcification of the matrix- most common form of degenerative process of cartilage

  2. Fatal to chondrocytes because when intercellular substance calcifies nutrition is compromised

  3. Not always abnormal phenomenon- normally occur in the formation of some bones

  4. With age, cartilage loose its translucency and becomes opaque- less cellular, its matrix becomes less basophilic which indicate decrease in GAG content

  5. Regeneration of cartilage is difficult and limited - no blood vessels - rely on perichondrium

  6. Regenerative capacity of cartilage Is greater in children than in adults

  
• Bone


  • One of the hardest substances in the body
  • Important for protecting vital organs
  • Important for serving as levers for muscles
  • Important for storage of Ca and Phosphorus
  • Important as site of blood formation-hematopoiesis
• Functions of bones


  • Provide shape, support, and the framework of the body
  • Protect internal organs
  • Serve as a storage place for minerals such as salts, calcium, and phosphorus
  • Play an important role in hematopoiesis- the formation of blood cells that takes place in bone marrow
  • Provide a place to attach muscles
  • Make movement possible through articulation
• Forms of bone tissue

  • Compact bone - dense and looks smooth and homogenous
  • Spongy bone - spiky, open appearance like sponge

  
• Types of bone (shape)

  • Long bones - longer than they are wide
  • Flat bones - thin flattened and usually curved
  • Short bones - are generally cube-shaped and contain mostly spongy
  • Irregular bones - bones that do not fit one of the preceding categories
  • Sesamoid bones - a small bone commonly found embedded within a muscle or tendon near joint surfaces

  
• Gross anatomy of a long bone

  • Epiphysis - ends of long bones: PROXIMAL & DISTAL
  • Epiphyseal plate - flat plate of hyaline cartilage
  • Diaphysis - shaft; makes up the most of the bone's length and is composed of compact bone, covered by a fibrous connective tissue membrane called the periosteum
  • Periosteum - external surface of the bone
  • Medullary - inner space containing bone marrow
  • Endosteum - lining of the medullary cavity

  
• Sesamoid bones


  Small bones commonly found embedded within a muscle or tendon near joint surfaces

  
• Types of bones


  • Short bones (carpals of the wrist, tarsals of the ankle)
  • Flat bones (skull, sternum, scapula)
  • Irregular bones (vertebrae, pelvis)
  • Sesamoid bones
• Epiphysis


  Ends of long bones: proximal & distal

  
• Epiphyseal plate


  Flat plate of hyaline cartilage
• Diaphysis


  Shaft; makes up the most of the bone's length and is composed of compact bone, covered by a fibrous connective tissue membrane called the periosteum

  
• Periosteum


  • External surface of the bone
  • Outer (fibrous layer): blends with the surrounding tissue
  • Inner (osteogenic layer): more cellular that adheres to the bone

  
• Medullary


  Inner space containing bone marrow
• Endosteum


  • Lining of medullary, marrow and vascular cavities
  • Thinner than periosteum - consists of single layer of osteoprogenitor cells

  
• Important roles of the periosteum and endosteum are nutrition of bone cells and provision of osteoblasts for bone histogenesis and repair
• Osteocytes


  Mature bone cells found in tiny cavities called lacunae, arranged in concentric circles called lamellae around central/haversian canals

  
• Canaliculi


  Tiny canals that radiate outward from central canals to all lacunae
• Volkmann's canals


  Performing canals
• Osteoprogenitor cells


  • Aka osteogenic cell
  • Differentiate from embryonic mesenchymal cells
  • Stem cells that have limited potential
  • Can multiply infinitely but only either osteoblast or chondroblast
  • Form the layer of the periosteum and endosteum (bone) and perichondrium (cartilage)
• Osteoblasts


  • Differentiate from osteoprogenitor cells
  • Immature bone cells that synthesize and secrete the osteoid matrix that will calcify as the bone extracellular matrix
  • Matrix is composed of glycoproteins and collagen
  • Are located on the surfaces of forming bone and are not yet embedded in the calcified extracellular (osteoid) matrix
  • Have cytoplasmic processes that bring them into contact with neighboring osteoblasts, as well as nearby osteocytes
  • Cells are round, polygonal or cuboidal and nucleus is located far from the bone surface

  
• Osteocytes


  • Mature bone cells - osteoblasts that have become embedded in calcified bone matrix
  • They reside in lacunae within the matrix
  • Are in contact with neighboring osteocytes via cytoplasmic processes that extend through small tunnels called canaliculi
  • Contacting cytoplasmic processes are characterized by gap junctions. This allows communication between osteocytes and is important in the transfer of nutrients to these cells since they generally are far removed from blood capillaries
  • The cells are flattened and their internal organelles exhibit the characteristics of cells that have reduced synthetic activity

  
• Osteoclasts


  • Large multinucleate cells (up to 150um in diameter) seen on surface of bone where resorption is taking place
  • Active in bone resorption during specific stages in bone formation and healing, and during the continual reworking of internal bone architecture that occurs throughout life
  • Important in maintaining calcium balance in the body - respond to calcitonin (secreted by parafolllicular cells of thyroid/ultimobranchial bodies - lowers Ca++ concentration in blood)and parathyroid hormone (secreted by parathyroid glands - raises Ca++ concentration in blood)

  
• Modes of bone formation

  • Intramembranous ossification (direct formation of bone structure with no cartilage template, e.g. flat bones of skull)
  • Endochondral ossification (formed from cartilage template that is subsequently replaced by bone, e.g. vertebral column, long bones of limbs)

  
• Bone can only grow by appositional method because its rigid intercellular substance does not allow for interstitial growth or expansion of the tissue from within

  
• Long bones can grow in length, at least until individual is 20 years old or so, because of the epiphyseal plates

  
• Conversion of spongy to compact bone


  1. Bone spicules in spongy are connected together to constitute the framework of a network of spaces that are in a form of irregular tunnels

  2. Successive layers of bone matrix are deposited on the inner surface of the tunnels by the osteoblasts in the endosteum, until small canal (haversian canal) that is occupied by blood vessels remains of the original tunnel

  
• Deficiency of either Calcium or Phosphorous


  Leads to poor mineralization of bones- become brittle and fracture easily