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Skeletal System


 

Content

Skeletal System
Functions of the Skeletal System
Structure of Long Bones
Histology of Bone Tissue
Intramembranous Ossification
Endochondral Ossification
Bone Remodeling
Bone Depostition
Bone Resorption
Repair of Bone Fractures

 

Skeletal System

The skeletal system includes several different organs and tissues

  • Bones (206 bones in an adult skeleton)
  • Cartilages
  • Joints
  • Ligaments

Functions of the Skeletal System

1. Support  and Protection

  • Provides a framework that supports the body.
  • Protects many internal organs from mechanical injury:
    • the rib cage protects the heart and lungs
    • the pelvic girdle protects most of our reproductive and some digestive organs.
    • the vertebral column protects our spinal cord
    • the skull protects our brain;  

2. Movement

  • Muscles attached at their ends to bone (via tendons) can contract and exert a pull on the skeleton.

3. Blood Cell Production (Hemopoiesis)

  • Different blood cells such as red blood cells, white blood cells and other blood elements are made within the red marrow by a process called hemopoiesis or hematopoiesis.
  • Red marrow fills the internal cavities of many bones.

4. Storage of Mineral and Energy Reserves

  • Bones provide a storage site for calcium and phosphate.
  • When calcium and phosphate are needed by the body, some bone connective tissue is broken down and the calcium and phosphate are released into the bloodstream.
  • Calcium is essential for blood clotting and proper functioning of muscles and nerves.
  • Phosphate is needed to make ATP (immediate source of energy).
  • Bones also store lipids in areas of yellow marrow as an energy reserve.

Structure of  Long Bones

The parts of a long bone include the following:

  1. Diaphysis -
    • This is the shaft of a long bone, main portion, composed of compact bone.
  2. Epiphysis
    • This is the extremities or ends of the bone
    • It is composed of spongy bone
  3. Metaphysis
    • This is the region in a mature bone where the diaphysis joins the epiphysis
    • In a growing bone, it is the region that includes the epiphyseal plate - where cartilage is replaced by bone
  4. Nutrient Foramen
    • This is a hole where nutrient arteries supply the diaphysis and metaphysis and epiphyseal arteries supply the epiphysis
  5. Articular Cartilage
    • This is a thin layer of hyaline cartilage covering the epiphysis where the bone forms a joint with another  bone.
  6. Periosteum
    • This a membrane around the surface of a bone not covered by cartilage consists of a layer of connective tissue that surrounds a layer of osteoprogenitor cells
  7. Medullary Cavity
    • This is the space within the diaphysis that contains the fatty yellow marrow.
  8. Endosteum
    • This is the membrane lining of the medullary cavity, the osteogenic layer.

Histology of Bone Tissue

Osseous tissue is composed of a matrix surrounding widely separated cells called osteocytes.  The matrix consist of 25% water, 25% protein fibers (collagen), and 50% calcium salts.

1. Osteoprogenitor cells - unspecialized cells found in the inner portion of the periosteum, endosteum, and canals
2. Osteoblasts - cells that form bone, they secrete collagen and organic compounds .
3. Osteocytes - mature bone cells, principle cells of bone tissue. 
4. Osteoclasts - function in bone reabsorption for development growth, maintenance, and repair.

5. Osteon - the basic unit of structure of adult compact bone, consists of:
    a. Concentric lamellae - rings of hard calcified matrix.
    b. Central canals - canals running longitudinal through the bone located at the center of the osteon, they hold
         the blood vessels and nerves.
    c. Perforating canals - canals that laterally enter from the outside periosteum  into the osteon to the central
        canal.
    d. Caniliculi - small canals that radiate throughout the osteon filled with extracellular fluid and hold the
         processes of the osteocytes.
    e. Lacunae - spaces between the lamella that hold the osteocytes.

6. Interstitial lamella - area between the osteon filled with fragments of older osteons.

7. Trabeculae - lattice or network of plates and rods, found in spongy bone.  

 

Intramembranous Ossification

  1. The flat bones of the skull, lower jawbone (mandible), and collar bones (clavicles) develop directly on or within fibrous connective tissue membranes:
  2. At the site where the bone will develop,  embryonic cells become vascularized, cluster, and differentiate into osteoblasts.  The site is called a center of ossification.  Osteoblasts secrete the bone matrix until they are completely surrounded by it.
  3. The cells, now called osteocytes, lie in lacunae with canaliculi that radiate out in all directions.  Later, the tissue hardens or calcifies.
  4. The bone matrix develops into trabeculae.  The trabeculae formed from various ossification centers fuse with one another to create spongy bone.  Eventually the spaces between trabeculae fill with red bone marrow.
  5. Eventually , a  periosteum develops on the outside of the bone,  some of the surface layers of the spongy bone are replaced by compact bone.  Much of this newly formed bone will be remodeled (destroyed and reformed) so the bone may reach its final adult size and shape.

Endochondral Ossification

The replacement of cartilage by bone is called endochondral (intracartilaginous) ossification.  Most bones of the body are formed in this way.

The site of bone formation takes the form of a hyaline cartilage model of future bone.  The  cartilage model is surrounded by a membrane called the perichondrium.

The cartilage model grows in length by continual cell division of chondrocytes and secretion of cartilage matrix by the daughter cells.  This growth in length is called interstitial growth.

Growth of the cartilage in thickness is mainly due to the addition of more matrix to its periphery by new chondroblasts that develop form the perichondrium.  This growth in diameter is called appositional growth.

As the cartilage model continues to grow, chondrocytes in its mid-region hypertrophy (increase in size) eventually bursting and changing the pH of the matrix, which triggers calcification.

Once the cartilage becomes calcified, nutrients can no longer diffuse through the matrix, and the cartilage cells die.  The lacunae of the dead cells are now empty, and the thin partitions between them break down.

Concurrently, a nutrient artery penetrates the perichondrium and then the bone through the nutrient foramen.  This occurs in the mid-region stimulating osteoprogenitor cells in the perichondrium to differentiate into osteoblasts.  The cells lay down a thin shell of compact bone under the perichondrium called the periosteal bone collar.  When perichondrium starts to form bone, it is known as the periosteum.

The primary ossification center develops when capillaries of the periosteum grow into the disintegrating calcified cartilage.  Blood vessels, and the associated osteoblasts, osteoclasts, and red marrow cells form the periosteal bud. 

In the primary ossification center, osteoblasts begin to deposit bone matrix over the remnants of calcified cartilage, forming spongy bone trabeculae.

As the center enlarges toward the ends of the bone, osteoclasts break down the newly formed spongy bone trabeculae, leaving a cavity, the medullary (marrow) cavity.  Eventually, the hyaline cartilage diaphysis (shaft), is replaced by compact bone.
The secondary center of  ossification develops when blood vessels enter the epiphysis.  In these centers, bone formation is similar to that in the primary except  spongy bone is retained and no cavity is formed, also the hyaline cartilage remains covering the epiphysis.   

Bone Remodeling

Bone is active tissue – small changes in bone architecture occur continuously

  1. 5 to 7% of bone mass is recycled weekly
  2. spongy bone is replaced every 3-4 years
  3. compact bone approximately every 10 years

Remodeling Units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces

Bone Depostition

  • Occurs when bone is injured or extra strength is needed
  • Requires a healthy diet - protein, vitamins C, D, and A, and minerals (calcium, phosphorus, magnesium, manganese, etc.)

Bone Resorption

  • Accomplished by Osteoclasts  (multinucleate phagocytic cells)
  • Resorption involves osteoclast secretion of:
  • Lysosomal enzymes that digest organic matrix
  • HCl that converts calcium salts into soluble forms
  • Dissolved matrix is endocytosed and transcytosed into the interstitial fluid → the blood

Repair of Bone Fractures

  1. Hematoma (blood-filled swelling) is formed
  2. Break is splinted by fibrocartilage to form a callus
  3. Fibrocartilage callus is replaced by a bony callus
  4. Bony callus is remodeled to form a permanent patch

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This material is based upon work supported by the Nursing, Allied Health and Other Health-related Educational Grant Program, a grant program funded with proceeds of the State’s Tobacco Lawsuit Settlement and administered by the Texas Higher Education Coordinating Board.