Skeletal System I (tissues and structure)

CARTILAGES location

1. External ear; cartilages of nose
   

2. Articular cartilages (cover ends of most bones at movable joints)
   

3. Costal cartilages (connect the ribs to the sternum)
   

4. Trachea; larynx, including epiglottis
   

5. Intervertebral discs

Cartilage Structure

1. Firm, gel-like matrix contains complex polysaccharides called chondroitin sulfates, which form complexes with proteins, resulting in proteoglycans.
   
   

2. CHONDROCYTES (mature cartilage cells), which occupy cavities (or pits) called LACUNAE (singular = lacuna), are surrounded by an extracellular matrix of ground substance, fibers and water.
   

3. Physical properties of cartilage depend on the nature of the matrix, which vary in water content between 60–80% volume.
   

   • Collagen fibers provide tensile strength.
     

   • Combined characteristics of the extracellular fibers and the ground substance confer flexibility and resilience.

     4. Avascular and NOT innervated
        
        

     5. Cartilage tissue is surrounded by a fibrous perichondrium (outer fibrous layer of dense irregular connective tissue + inner cellular layer).
        
        

   6. Perichondrium resists outward expansion of cartilage under pressure; also functions in the growth and repair of cartilage.
      

Cartilage Types

1. HYALINE CARTILAGE (collagen fibers predominate):
   

   • Provides support through flexibility and resilience
     

   • Most cartilage in the body is hyaline.
     

2. ELASTIC CARTILAGE (elastic fibers predominate):
   

   • Tolerate repeated bending (e.g. cartilage in external ear and in epiglottis).
     

3. FIBROCARTILAGE (thick collagen fibers predominate):
   

Resists strong compression and strong tension forces.

Growth Mechanisms (General)

• Neither growth mechanism occurs in adult cartilages.
  

Most cartilages cannot repair themselves after a severe injury.

Growth Mechanisms— Appositional Growth

• Chondroblasts (cartilage-forming stem cells) in perichondrium undergo repeated cycles of division to produce new cartilage tissue.

• Stem cells of the inner layer differentiate into chondroblasts.
  

• Immature chondroblasts secrete matrix.
  

• As the matrix grows, more chondroblasts are incorporated (become mature chondrocytes) and are replaced by divisions of stem cells in the inner layer.
  

This mechanism gradually increases the outer dimensions of the cartilage; hence, growth from the outside.

Growth Mechanisms—Interstitial Growth

• Chondrocytes within the cartilage divide, and their daughter cells secrete new matrix.

As daughter cells secrete additional matrix, they separate and consequently expand the cartilage from within.

BONES

• FUNCTIONS of BONES (besides contributing to body shape):
  

  1. Support weight of the body and cradles its soft organs.
     

  2. Protection (e.g. The skull houses the brain; vertebrae – spinal cord; rib cage – thoracic organs).
     

  3. Movement: act as levers for muscles to pull on.
     

  4. Mineral storage: mainly calcium; also phosphate.
     

  5. Blood-cell formation:
     

     • Red bone marrow makes the blood cells.
       

Yellow bone marrow does not – it is a site for fat storage.

• CLASSIFICATION OF BONES and EXAMPLES OF EACH (based on shape of the bones

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  1. LONG BONES: elongated shape (not based on overall size);
     

     • e.g. femur (thigh bone); phalanges and metacarpals (hand bones).
       

  2. SHORT BONES: roughly cube-shaped;
     

     • e.g. carpals (wrist bones); patella (also includes sesamoid bones).
       

  3. FLAT BONES: thin, flattened, usually somewhat curved shape;
     

     • e.g. most cranial bones of the skull; ribs; sternum; scapula.
       

  4. IRREGULAR BONES: of various shapes and do not fit into previous categories;

     • e.g. vertebrae; os coxae (hip bones).

GROSS ANATOMY OF BONES

• COMPACT BONE (EXTERNAL layer) – smooth, homogenous appearance.
  
  

• SPONGY BONE or Cancellous Bone (INTERNAL) – honeycomb of trabeculae (singular = trabecula: a connective tissue partition that subdivides an organ) with open spaces in between, which are filled with red and yellow bone marrow.
  
  

• STRUCTURE OF TYPICAL LONG BONE
  
  

  • (a) – DIAPHYSIS: shaft or long axis of the bone.
    
    

  • EPIPHYSIS (singular = epiphysis):
    
    

    • The bone ends are called (b) – PROXIMAL EPIPHYSIS and (c) – DISTAL EPIPHYSIS.
      
      

    • Each articulating (joint) surface is covered with articular cartilage.
      
      

  • EPIPHYSEAL LINE: remnant of the epiphyseal plate, which is a disc of hyaline cartilage that grows during childhood (refer to cartilage growth in previous section).
    
    

• BLOOD VESSELS
  
  

  • NUTRIENT ARTERIES and VEINS together run through NUTRIENT FORAMEN (hole in wall of diaphysis) to supply the diaphysis:
    
    

    • The arteries nourish the bone marrow and spongy bone, and their branches extend outward to help supply compact bone.
      
      

  • EPIPHYSEAL ARTERIES and VEINS serve each epiphysis in the same way.
    
    

• MEDULLARY (or MARROW) CAVITY: center of diaphysis
  
  

  • Contains no bone tissue, but filled with yellow bone marrow.
    
    

• SKELETAL MEMBRANES (internal and external bone coverings):
  
  

  • PERIOSTEUM: membrane of connective tissue that covers the entire outer surface of each bone except the ends of epiphyses
    
    

    • Superficial layer of dense irregular connective tissue.
      
      

    • Deep layer with osteoblasts (bone-depositing cells) and osteoclasts (bone-destroying cells).
      
      

    • RICHLY INNERVATED
      
      

    • VASCULARIZED (supplied by branches from nutrient and epiphyseal vessels).
      
      

    • SHARPEY’S FIBERS: thick bundles of collagen that secure the periosteum to the underlying bone.
      
      

      • Also provide insertion points for tendons and ligaments.
        
        

  • ENDOSTEUM: thinner membrane of connective tissue that covers the trabeculae of spongy bone (internal bone surfaces) and lines the medullary cavity.
    
    

Also contains osteoblasts and osteoclasts.

STRUCTURE OF SHORT, IRREGULAR AND FLAT BONES

• Same composition as long bones:
  

  • Compact bone covered by periosteum.
    

  • Spongy bone covered by endosteum.
    

  • NO diaphysis.
    

  • NO medullary cavity.
    

  • Trabeculae of spongy bone is filled with bone marrow.
    

In flat bones: DIPLOË = internal spongy bone.

BONE DESIGN AND STRESS

• Compression stress: loading off center threatens to bend the bone by compressing the bone on one side and stretching it on the other side.
  

• Strong, compact bone tissue in external region resists maximal compression and tension forces.
  

• Compression and tension forces cancel each other out in internal spongy bone region.
  

• Spongy bone and marrow cavities serve to lighten the heavy skeleton and accommodate bone marrow.
  

• Trabeculae of spongy bone align along stress lines in organized pattern to provide support along the stress lines

MICROSCOPIC STRUCTURE OF BONE— compact bone

• OSTEON (Haversian system): basic functional unit
  

  • Long, cylindrical structure oriented parallel to the long axis of bones and to the main compression stresses.
    

  • Also referred to as “weight-bearing pillars.”
    

• General structure of osteon: A central (Haversian) canal is surrounded by concentric lamellae (singular = lamella).
  

  • (a) – LAMELLA: concentric layer of bone matrix in which all the collagen fibers run in a single direction.
    

  • (b) – CENTRAL (Haversian) CANAL: lined by endosteum; vascularized; innervated.
    

  • PERFORATING (Volkmann’s) CANALS: connect blood and nerve supply of periosteum to that of the central canals and the medullary cavity.
    

  • (c) – OSTEOCYTES (osteoblasts surrounded by matrix): spider-shaped mature bone cells which occupy (c1) – lacunae (small cavities or chambers in bone matrix).
    

    • Their “spider legs” processes occupy canaliculi (thin tubes), which run through the matrix and connect lacunae to each other, and to the central canals.
      

  • (d) – Canaliculi (singular = canaliculus): mode of communication and supplying osteocytes with nutrients.
    

Osteocytes are essential for maintaining the bone matrix.

SPONGY BONE structure

• Each trabecula contains several layers of lamellae and osteocytes, but NO osteons or blood vessels of its own.
  

Osteocytes are nourished by capillaries located in the endosteum surrounding the trabeculae.

CHEMICAL COMPOSITION OF BONE TISSUE- Bone cells

• BONE CELLS (osteocytes, osteoblasts and osteoclasts):
  

  • Osteocytes = mature bone cells that are completely surrounded by hard bone matrix; occupy lacunae.
    

  • Osteoblasts = immature, bone-forming cells; synthesize osteoid via the process of osteogenesis.
    

  • Osteoclasts = large, multinucleated cells that help dissolve the bony matrix through the process of osteolysis; regulate calcium and phosphate concentrations in body fluids.
    

(Osteoprogenitor cells = mesenchymal cells that play a role in the repair of bone fractures)

CHEMICAL COMPOSITION OF BONE TISSUE—ECM, ORGANIC PART, & INORGANIC

• EXTRACELLULAR MATRIX of connective tissue, collagen fibers, ground substance, water and mineral crystals – consists largely of crystals of hydroxyapatite.
  

• ORGANIC PART is comprised of collagen fibers, which provides tensile strength.
  

  • OSTEOID: the organic part of matrix BEFORE it mineralizes or calcifies.
    

INORGANIC PART is comprised of crystals of CALCIUM PHOSPHATE SALTS, which precipitate in the matrix and make the bone hard and able to resist compression.

BONE DEVELOPMENT (OSTEOGENESIS or OSSIFICATION) 1. INTRAMEMBRANOUS OSSIFICATION

• FORMS MEMBRANE BONES (which occur in flat bones of the skull and the clavicles).
  

• Begins when osteoblasts differentiate within a mesenchymal or fibrous connective tissue, at an ossification center.
  

• Ultimately produces spongy or compact bone.
  

A network of bone tissue woven around capillaries (woven bone tissue) first appears and is then remodeled into a flat bone.

BONE DEVELOPMENT (OSTEOGENESIS or OSSIFICATION) 2. ENDOCHONDRAL OSSIFICATION

• This process FORMS ALL OTHER BONES = endochondral or cartilage bones.
  

• Begins with the formation of a cartilaginous model.
  

• Hyaline cartilage model is gradually replaced by osseous tissue.
  

• The length of a developing bone increases at the epiphyseal cartilage (or epiphyseal plate), which separates the diaphysis from the epiphysis.
  

• New cartilage is added at epiphyseal side.
  

• Osseous tissue replaces older cartilage at diaphyseal side.
  

• Time of closure of epiphyseal cartilage varies among bones and among individuals.
  

Bone diameter grows via appositional growth at the outer surface of the bone.

ANATOMY of the EPIPHYSEAL GROWTH AREA

• The chondrocytes of the growing cartilage of the fetal epiphyses and the postnatal epiphyseal plates are organized into several zones, which allow rapid growth.
  

• Organization of the zones within the epiphyseal cartilage (epiphyseal to diaphyseal side):
  

  • o Zone of Resting cartilage
    

  • o Zone of Proliferating cartilage
    

  • o Zone of Hypertrophy
    

  • o Zone of Calcification

OSSIFICATION

the formation of bone tissue

CALCIFICATION

the deposition of calcium salts within a tissue.

POSTNATAL GROWTH of ENDOCHONDRAL BONES

1. Endochondral bones lengthen during growth through the growth of epiphyseal plate cartilages, which close in early adulthood.
   

2. Bones increase in width through appositional growth (similar to appositional growth mechanism of cartilage).
   

Periosteum adds bone tissue to its surfaces, while the endosteum’s osteoclasts remove bone from the internal surface of the diaphysis wall.

BONE REMODELING—General

• New bone tissue is continuously deposited and reabsorbed in response to hormonal (parathyroid hormone or PTH) or mechanical stresses.
  

• Mineral turnover and recycling allow bone to adapt to new stresses.
  

• Calcium is the most common mineral in the body (>98% of it located in the skeleton), and is an important mineral for bone health.
  

• Spongy bone in human skeleton is entirely replaced every 3–4 years; compact bone – every 10 years.

BONE REMODELING—In adult skeleton

• BONE DEPOSITION: OSTEOBLASTS SECRETE OSTEOID on bone surfaces and CALCIUM PHOSPHATE SALTS CRYSTALLIZE within this osteoid.

• BONE REABSORPTION: OSTEOCLASTS break down the bone by secreting acid (which dissolves the mineral part of the matrix) and lysosomal enzymes (which digest the organic part of the matrix)

  • This process releases calcium and phosphate into the blood

  • PTH inc reabsorption in response to decreased calcium levels in body fluids.

• Compression forces and gravity acting on the skeleton help maintain bone strength, as bones thicken at sites of stress