Basic Tissues: Bone

Basic Tissues

Bones

Classifications of Bones

1. Developmental Classification

• Intramembranous

• Intacartilagenous

• Membranocartilagenous

2. Regional Classification:

• Axial Bones

• Appendicular bones

3. Classification according to size and shape:

• Long bones

• Long-Short bones

• Short bones

• Flats Bones

• Irregular bones

4. Miscellaneous Classification

• Sesamoid bones

• Supernumery bones

• Accessory bones

• Pneumatic bones

• Wormian bones

5. Structural classification:

1. Compact (haversian) bone:

• Well developed Haversian lamellae (cylindrical lamellae) are present

• Lamellae consist of collagen fibers, lying in a calcified material

• Bone cells lie scattered between the lamellae

• Examples: Shafts of long bones

outer and inner table of skull bones

b. Spongy (cancellous) bone:

• Lamellae are arranged flat, Haversian system is not seen.

• Examples: Ends of long bones

Diploe of skull

Parts of a developing long bone:

1. Diaphysis

• Shaft of long bone.

• Ossification center is called primary ossification center.

• Ossification center appears in 7th week of intra-uterine life.

B. Epiphysis:

• Ends of long bones

• Ossification center is called secondary ossification center.

• Ossification center appears after birth at the age of 1 or 2 years except lower end of femur where is appears in 9th month of IU life.

Types of epiphysis:

1. Pressure epiphysis:

• Develop in the vicinity of site of articulation at 1-2 years of age.

• Example: ends of long bones of limbs

2. Traction epiphysis:

• Develop at the site of attachment of certain tendons at puberty

• Example: Tuberosities of humerus

• Trochanters of femur

3. Atavistic epiphysis:

• Are independent bones phylogenetically which become part of other bones

• Example: coracoid process of scapula

C. Metaphysis:

• Actively growing part of diaphysis (shaft) close to epiphyseal cartilage

• Highly vascular

• Responsible for growth in length

4. Epiphyseal Cartilage:

• Cartilaginous tissue present between epiphysis and diaphysis

• After the completion of growth, it becomes ossified and is represented by epiphyseal line in adult bone

• Completion of growth of long bone usually occurs at between 17-20 years of age.

Blood Supply of Bones

1. Adult long bones:

Most of the pores present in the bones are for veins.

Arterial supply comes from:

1. Periosteal arteries:

• Are numerous.

• Supply the compact bone of shafts and spongy bone of the ends.

b. Nutrient artery:

• enters the bone through the nutrient foramen.

• Gives a number of branches which are ascending and descending. The branches are called “nutritiae”.

• Its branches also pass through Volkman’s canal and haversian canals

• It anastomoses with periosteal and end blood vessels

• It supplies a little more than bone marrow.

c. Arteries at the ends of long bones:

These may arise from:

• Peri-articular anastomsis (juxta epiphyseal arteries of Lexer). These pierce the bone at metaphysis.

• From the artery passing over that region. These pierce the bone at epiphyseal cartilage.

2. Immature long bones:

• Have additional epiphyseal and metaphyseal arteries

• Arise from periosteal arteries present at the ends of long bones.

3. Short bones:

• Only periosteal arteries.

4. Flat bones:

• Periosteal arteries

• Nutriens artery

• Anastomose with each other

5. Irregular bones:

Supplied by periosteal arteries

6. Skull bones:

Supplied by:

• periosteal arteries
• Middle meningeal artery

Histologic Structure

• Two components:

  • Cells

  • Intercellar substance

    • Fibres

    • Ground substance

Bone cells

1. Osteoblasts:

• Are bone forming cells which synthesize and secrete unmineralized bone matrix called “Osteoid”.

• Also secrete an enzyme “Alkaline phosphatase” which causes mineralization of osteoid.

• Active osteoblasts

  • are cuboidal in shape, each having a large spherical nucleus, usually eccentric in position.

  • Cytoplasm is markedly basophilic

  • On E/M, basophilia is due to presence of large quantity of RER.

  • Cytoplasm also contains a large golgi apparatus located in the central region of cell.

  • Active osteoblasts also have many finger like cytolasmic processes that bring them in contact with neighbouring osteobalsts

• Inactive osteoblasts (aka bonelining cells)

  • are fusiform (spindle shaped), with slightly basophilic cytoplam and small darkly staining nucleus.

2. Osteocytes:

• Are mature bone cells

• Are derived from osteoblasts which have secreted bone around them.

• Are flat almond shaped cells

• Have faintly basophilic cytoplasm, which contains small amount of RER and a small golgi apparatus.

• Osteocytes lie within small cavities, the Lacunae, in the bone matrix.

• Neighboring lacunae communicate with each other by narrow channels called Canaliculi.

• Fine cytoplasmic processes from each osteocyte extend for some distance into canaliculi and make contact with similar processes from neighboring cells.

• Gap junctions are present where these cytoplasmic processes meet. These junctions allow intercellular flow of ions and small molecules, providing a mechanism by which nutrients and metabolites can be exchanged between blood vessels and distant osteocytes

3. Osteoclasts:

• Are bone-resorbing cells

• Are multinucleated gaint cells, 50-150μm in diameter.

• Each cells has about 30 or more nuclei

• Are mostly found close to bone surface, where they are located in shallow concavities called Howship’s Lacunae. These cavities are produced by erosion of underlying bone

• Nuclei are away from bone surface

• E/M shows that cytoplasm of an osteoclast contains many golgi complexes, abundant mitochondria and many lysosomes.

• The surface of osteoclast which lies in contact with bone surface shows deep infoldings of plasma membrane with formation of irregular cytoplasmic processes. This surface is called “Ruffled border”.

• Surrounding the ruffled border is a cytoplasmic zone called Clear Zone which lacks cytoplasmic processes and is devoid of oraganelles

• Clear zone contains actin filaments which enable the osteoclasts to anchor itself to bony surface.

• In this way, a closed Subosteoclastic compartment is created between the ruffled border and the bone that is undergoing resortion. Osteoclasts seceret H+ ions and hydrolytic enzyms into this compartment.

• Resorption of bone matrix by osteoclasts involves 2 processes:

a. Initial dissolution of minerals: H+ ions secreted by osteoclasts create acidic mircoenvironment which increases the solubility of bones mineral salts resulting in release in re-entry of bone minerals (mainly Ca, PO4) back into blood stream.

b. Subsequent enzymatic degradation of collagen: by lysosomal hydrolases released by osteoclasts into subosteoclastic compartment.

Functions of osteoclasts:

1. Resorption bone matrix causing bone remodelling during bone growth and bone repair.

2. Calcium hemostasis

   1. Calcitonin lowers blood calcium level because it suppresses the process of osteoclastic resorption.

   2. Parathormone accelerates this process, so they raise blood calcium level.