Chapter 6

                           Bone Tissue

                          Lecture Notes

BONE/OSSEUS TISSUE:

     ‑ connective tissue in which the matrix is hardened by calciumphosphate and other minerals

     ‑ Osteo‑ = bone

I.     Basic Functions Of Bone and Skeletal System:

A.     Support of soft tissue, especially muscles.

B.     Protection of internal organs.

C.      Movement assoc. with muscle action.

D.     Mineral storage.

E.     Hemopoiesis = production of blood cells in the Red bone           marrow.

F.     Storage of energy.  Lipids are stored in Yellow bone marrow         (Adipose tissue).

II.     General structure of a bone:

A.     Diaphysis ‑ the shaft of a bone (also called the Body)

B.     Epiphysis ‑ the end(s) of a bone

C.     Articular (Hyaline) cartilage ‑ covers the ends of moveable      bones

D.     Periosteum ‑ fibrous membrane that covers the outer portion of        a bone, tendons attach here, highly innervated

E.     Medullary (Marrow) cavity ‑ space inside bone that contains      marrow

F.     Endosteum ‑ the lining of the medullary cavity

G.     Metaphysis ‑ the region between the Epiphysis and Diaphysis in        a mature bone(Epiphyseal plate) ‑ area that           contains hyaline cartilage in growing bone, area that           causes bone to elongate, becomes ossified (bone) in           adults

III.     Histology of bone.

A.     Cells:

1.   Osteogenic cell ‑ a stem cell (mesenchyme) that differentiates          to become an Osteoblast

2.     Osteoblast ‑ cells that lay down collagen fibers and                 matrix to form bone

3.      Osteocyte ‑ mature osteoblast, "bone cells", maintain daily          metabolic needs of bone tissue, sense the various        stressors on bone

4.     Osteoclast ‑ bone dissolving cells that lead to reabsorption,           develop from the fusion of several Monocytes

B.     Matrix:

     ‑ mostly Hydroxyapatite, a calcium phosphate salt whichprovide      much of the hardness of bone, also some calcium carbonate           and other inorganic compounds

     ‑ organic portion is mostly collagen fibers which provide the      basic frame work for a bone

C.     Compact bone

     ‑ densely packed bone containing few spaces

     ‑ usually located in the outer diaphysis of bones

     ‑ is arranged in units called Osteons (Haversian Systems)

     ‑ parts of an Osteon:

1.     Central (Haversian) Canal

     ‑ centrally located passageway that contains vessels andnerves

2.     Concentric lamellae

     ‑ concentric layers of calcified matrix that surround the      Central Canal

3.     Lacunae

     ‑ space that contains Osteocytes that are located between the      lamellae

4.     Canaliculi

     ‑ "tiny canals"

     ‑ thin spaces that connect lacunae

     ‑ contain then processes that project from Osteocytes to allow          for dispersion of nutrients and intercellular           communication between these cells

5.     Osteocytes ‑ bone cells

6.     Perforating (Volkmann's) Canal

     ‑ passage ways that contain vessel and nerves

     ‑ these connect Osteons (and also run between the Periosteum         and Osteons)

7.     Interstitial lamellae

     ‑ located between Osteons

     ‑ fragments of older Osteons

D.     Spongy bone

     ‑ loosely packed bone, usually found inside of the epiphysis

     ‑ arranged in Trabeculae which is an irregular lattice of thin        bone, Trabeculae form along lines of stress

     ‑ contains many spaces that are filled with bone marrow

IV.  Bone Development

A.   Bone Formation

     ‑ Ossification or Osteogenesis = formation of bone

     ‑ Two methods:

1.  Intramembranous ossification

       ‑ produces flat bones, ex. skull, clavicle, bones ofskull

       ‑ mesenchymal cells differentiate into osteoblasts tocreate      centers of ossification, new bone is laid down around         these centers

2.  Endochondral ossification 

      ‑ produces most bones

       ‑ hyaline cartilage matrix is laid down initially, then         ossification occurs replacing this cartilage

B.   Bone Growth and Remodeling.

1.     Interstitial Growth

       ‑ growth in length

       ‑ the diaphysis elongates at the Epiphyseal plates which is      made of Hyaline Cartilage    p. 172

       ‑ the Epiphyseal plate eventually becomes bone           (ex.metaphysis)

2.     Appositional Growth

       ‑ growth in thickness

       ‑ Osteoblasts lay down new bone on the outer surface of a           pre‑existing bone

       ‑ the Medullary cavity is enlarged by the activity of        Osteoclasts

V.     Homeostasis of Bone

A.     Remodeling

     = The change of the size and shape of bones to accommodate the         changing forces applied to the skeleton.

1.     Resorption (Reabsorption) ‑ destruction of the existing matrix         so the raw materials can be reused, assoc. with           Osteoclastic activity

B.     Fracture Repair

1.     Fracture

     = any break in a bone (a disruption in the continuity ofa          bone)

     ‑ there are various types of fractures based on their          location, degree of damage, forces involved, etc....

          see p. 175

2.     Steps involved with fracture repair.

     ‑ fracture repair occurs best when the bone ends are in        alignment

     a.     Formation of a Hematoma (clot) around the fracture site.

          (1)      Capillaries grow into the clot.

          (2)      Phagocytes and osteoclasts move into the area to                 resorb debris and bone fragments.

     b.     Formation of a Fibrocartilage Callus.

          (1)     Fibroblasts move into the area and begin laying down                     collagen fibers.

          (2)      Osteogenic cells also move into the area.  These                    cells develop into chondrocytes and begin to                lay down the fibrocartilage which forms the                     callus.  (Callus = a mass of repair tissue                   surrounding the fracture site)

     c.     Bony Callus Forms

          (1)     Osteoblasts form and begin laying down spongy bone,                    replacing the fibrocartilage.

     d.     Bony Callus Remodels

          (1)     Cortex becomes compact bone.

Note:  Bone is one of the few tissues where the damaged area is        stronger after repair.

C.     Calcium and Phosphate Homeostasis.

     ‑ Bone stores 99% of the body's calcium

     ‑ Normal range of blood calcium = 9.2 to 10.4 mg/dL

     ‑ Hypocalcemia ‑ low blood calcium, muscle is unable to relax,         tetany

     ‑ Hypercalcemia ‑ high blood calcium, nerves and muscles are      slow to respond; weakness, sluggish reflexes, poss.          cardiac arrest

     ‑ Hormone regulation:

1.     Calcitriol

     ‑ a form of vitamin D, is stimulated by absorption of UV light

     ‑ this hormone raises blood calcium by:

       a.  Increasing absorption in the Small Intestine.

       b.  Stimulates calcium removal from bones (Osteoclast).

       c.  Causing the Kidney to retain calcium in the blood.

2.     Calcitonin

     ‑ released by the Thyroid gland

     ‑ this hormone lowers calcium level in the blood by:

       a.  Inhibiting Osteoclast.

       b.  Stimulate Osteoblast (places calcium into bone).

3.     Parathyroid hormone (PTH)

     ‑ released by the Parathyroid glands

     ‑ this hormone raises the blood calcium level by:

       a.     Stimulating Osteoclast.

       b.     Causing the Kidneys to retain calcium in the blood.

       c.     Stimulates the production of Calcitriol.

       d.     Decreases Osteoblast activity.

VI.  Bone Disorders:

1.     Osteoporosis     p. 180

     ‑ Loss of bone mass, making them more susceptible to fracture.            Associated with aging.

     ‑ Calcium absorption in the Small Intestine increases withage.            Also, protein synthesis decreases which reduces collagen      fiber production.  In addition with the normal decrease      in hormones, bone tissue decreases in density asone ages.

2.     Osteoarthritis (O.A)

     = Degenerative Joint Disease (D.J.D)

     ‑ the most common type of Arthritis ("Joint Inflammation")

     ‑ a progressive degeneration of synovial joints associatedwith          the normal "wear and tear" on joints

     ‑ this process is accelerated by injury to joints