Skeleton System

I. Introduction

A. The Skeletal System includes the bones of the skeleton, cartilages, ligaments, and other connective tissues that connect and stabilize

B. Functions of the skeletal system:

1. Structural support for the entire body (framework)

2. Storage of minerals and lipids [98% of the calcium in the body is deposited in the skeleton]

3. Produces blood cells

4. Surrounds and protects delicate tissues

5. The bone functions as levers-they change the magnitude and direction of the forces generated by skeletal muscles

II. Structure of the bone

A. Bone is also called osseous tissue

B. The matrix of the bone, a supporting connective tissue is a solid and sturdy due to deposition of calcium salts

C. Histological organization

1. The Matrix of the bone

   1. Calcium phosphate crystals account for almost two-thirds of the weight of bone

      1. Crystals are strong a relatively inflexible

      2. Can withstand compression

      3. Tend to shatter when exposed to bending, twisting, or sudden impacts

   2. Collagen fibers (and other calcium salts) account for the remaining one-third of the bone weight

      1. Collagen fibers are tough and flexible

      2. Can bend, twist, and stretch

      3. Will bend out of the way when compressed

      4. Hydroxyapatite (crystals) form in the collagen fiber network

2. Cells in osseous tissue

   1. Osteocytes (osteo=bone; cyte=cell)

      1. Mature bone cells

      2. Found in Haversian systems

   2. Osteoblast B for build

      1. Cells that synthesize the organic components of the bone matrix (this material is called osteoid)

      2. Responsible for osteogenesis- the production of new bone

   3. Osteoclast

      1. Cells whose lysosomes contain an acid that breaks down the bone matrix

      2. Responsible for osteolysis - the breakdown of bone matrix

      3. Osteoclast is important as it regulates the concentration of calcium and phosphate ions in body fluids

      4. [Osteoblast and osteoclast activity needs to be in balance. When osteoclast remove calcium salts faster than osteoblast deposit them, bones become weaker; when osteoblast activity predominates bones become stronger and more massive.]

      5. Weaker=fracture Stronger =lack of movement

   4. Osteoprogenitor cells

      1. Mesenchymal cells that can divide into daughter cells that differentiate into osteoblast

      2. Have an important role in fracture repair

Compact and spongy bone

1. Compact bone (also called dense bone)

   1. The osteon (or Haversian) system is the basic functional unit

   2. Relatively dense and solid

   3. Forms the walls of the marrow cavity

      1. Marrow cavity contains bone marrow

      2. Yellow marrow is dominated by adipocytes

      3. Red marrow is dominated by blood cells

2. Spongy Bone (also called cancellous bone)

   1. No osteons in spongy bone

   2. Struts or plates, called trabeculate, form an open branching network

3. Functional differences between compact and spongy bone

   1. A layer of compact bone-whose thickness varies in different regions in the body and different bones in the body-covers the surface of bones

   2. A periosteum-a layer of connective tissue (with an outer layer of fibrous layer and an inner cellular region) that surrounds bone-forms over the superficial layer of compact bone

   3. Compact bone will be thickness where stresses arrive from a limited range of directions

   4. The diaphysis (or shaft) is the long, tubular portion of a long bone. The compact bone is prominent bone

   5. The epiphysis (or heads) is found at the ends of long bone typically where articulation points are located. Spongy bone is typically found here as stresses are applied from many directions

   6. Spongy bone is much lighter than compact bone thus reducing the weight of the skeleton and making it easier for muscles to move bones

E. The Periosteum and endosteum

1. Periosteum

   1. Functions of the Periosteum

      1. Isolates the bone from surrounding tissues

      2. Provides a route for circulatory and nervous supply

      3. Actively participates in bone growth and repair

   2. Periosteal fibers are interwoven with the tendon attached to the bone. The fibers become connected to the bone as it grows

      1. Tendon fibers become part of the structure of the bone