Chapter 5 - Bone Tissue
 
Functions
1. Support
     The skeletal system provides structural support and a framework for attachment of soft tissues and organs.
2. Storage of Minerals 
     Bones maintain a large reserve of calcium and phosphate ions. 
3. Blood Cell Production   
     The hollow spaces in some bones contains red marrow where blood cells are produced.
4. Protection
     In many places bone protects soft tissues and organs.
5. Leverage
     The articulation of bones creates levers that change the magnitude and direction of the forces exerted by muscles.
Structure of Bone
     Bone is similar to other connective tissue in having specialized cells and a matrix with ground substance and fibers. It is unique in that calcium salts are deposited around the fibers to make bone less compressible.
Histological Organization of Mature Bone
Matrix:
     The matrix combines the properties of compressional strength due to the presence of hydroxyapatite crystals and tensile strength due to collagen fibers.
1. Hydroxyapatite
     Hydroxyapatite crystals result from the interaction between calcium phosphate [Ca3(PO4)] with calcium hydroxide [Ca(OH)2].
     These crystals may also incorporate other ions including sodium, magnesium and fluoride.
2. Collagen Fibers
     Collagen fibers provides a framework for the deposition of mineral crystals and contribute to about one-third of the weight of the bone.

Cells of Mature Bone:
1. Osteocytes
     These cells are mature bone cells that maintain the protein and mineral content of the surrounding bone matrix.
     Osteocytes occupy spaces called lacunae that are sandwiched between mineralized layers of matrix called lamellae. Osteocytes communicate with each other by cytoplasmic processes that extend through narrow passageways in the bony matrix called canaliculi. Canaliculi permit the exchange of nutrients and waste products between  the osteocytes and blood vessels  and provide a means for signals to be transferred from one osteocyte to another.
2. Osteoblasts
     Osteoblasts are precursor cells that are found on the inner and outer surfaces of bone. These cells secrete the organic components of bone matrix called osteoid. The osteoid then becomes mineralized to form bone.
     The process of bone formation is called osteogenesis. When the osteoblast is surrounded by bone matrix it differentiates into an osteocyte.
3. Osteoprogenitor Cells
     These cells are mesenchymal, or stem, cells that can divide and differentiate into osteoblasts.
4. Osteoclasts
     Osteoclasts are giant cells with 50 or more nuclei and are derived from the stem cells that also produce monocytes and macrophages. They secrete acids that dissolve bone matrix and release calcium and phosphorus into the body fluids.
     The process of bone erosion by the activities of osteoclasts is called osteolysis.
Osseous Tissue Forms Two Types of Bone:
1. Compact Bone
     Compact bone is dense and solid and forms the walls of bone. 
     The basic functional unit of compact bone is the osteon (Haversian system). The osteon consists of concentric lamellae of bone matrix arranged around a central (Haversian) canal containing blood vessels. The osteons are cylindrical and aligned parallel to the long axis of the bone. 
     Blood vessels travel from the surface to the central canals and the bone marrow by means of perforating (Volkmann's) canals that run roughly perpendicular to the osteons and the bone surface.
     The collagen fibers spiral within each lamella and the orientation of the collagen fibers within each lamella changes to create a crisscrossing pattern that increases the strength of the entire osteon.
     Lamellae of bone form on the inner and outer surfaces of bone and form what are called circumferential lamellae. Interstitial lamellae are found between  osteons and represent lamellae that were once part of an entire osteon or circumferential lamellae.
2. Spongy Bone
     Spongy bone is found in the interior of normal bones.   
     In this type of bone parallel lamellae form struts and thin branching plates called trabeculae. The trabeculae surround spaces  that contain bone marrow. Bone marrow is loose connective tissue that is dominated either by adipocytes (yellow marrow) or hemopoietic tissue (red marrow). 
     Osteons are generally not found within spongy bone unless the trabeculae are large. Without blood vessels in a central canal to supply osteocytes nutrients reach the osteocytes by means of canaliculi that open onto the surface of the trabeculum.
     Spongy bone is found wherever bone is not heavily stressed or where the stresses come from many directions. Spongy bone reduces the weight of the bone and the open trabecular framework provides support and protection for cells of the bone marrow.
Periosteum and Endosteum
     The outer and inner surfaces of hollow bones are covered by periosteum and endosteum, respectively.
Periosteum
     The outer surfaces of bone are covered by periosteum everywhere except where one bone articulates with another. Periosteum consists of:
a. fibrous outer layer and a
b. cellular inner layer that contains osteoprogenitor cells. 
   The periosteum:
1. Isolates and protects the bone from the surrounding tissue.
2. Contains and supports the blood vessels and nerves that supply bone.
3. Participates in bone growth and repair. 
4. Attaches bone to deep fascia.
     Fibers in the periosteum are interwoven with the tendons that attach muscles to bone.
     Fibers of tendons become incorporated into the bone as it grows and strengthens the attachment of the tendon to the bone. These fibers are called perforating or Sharpey's fibers.
Endosteum
   The inside surfaces of the bone are lined by a cellular endosteum.
   These inner surfaces include the linings of the central and perforating canals and the surfaces on the trabeculae. 
   The endosteum is an incomplete cellular lining and osteoprogenitor cells, osteoclasts and osteoblasts are present on the exposed surfaces.