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BONE

September 29, 2011  by: ANIL PATEL  Points: 12   Category: Medical  Earning $0.15   Views: 1209

It is a highly vascular mineralized connective tissue; consisting of cells & intercellular matrix in which fibers are embedded.

         

DEFINITION:




It is a highly vascular mineralized connective tissue;
consisting of cells & intercellular matrix in which
fibers are embedded.

. Bone tissue forms most of the skeleton, the framework that protects and supports our organs and allows us to move.
.Throughout life it is continuously being broken down and re-formed.
. OSTEOLOGY is the study of bone structure and treatment of bone disorders.
. It is the most highly specialized form of connective tissue
. The basic organizational unit of bone is the microscopic " harvesian system "
. The longest bone in our body is the femur (thigh bone)

The smallest bone is the (stepedius) inside the ear
. I Based on shape
? Long Bones :
. eg; Humerus, radius, ulna, femur, tibia
? Short Bones
. eg;carpal & tarsal bones
? Flat Bones :
. eg; scapula,sternum,ribs,skull bones

. Irregular Bones :
eg; vertebrae,mandible
. Pneumatic Bones :
eg; maxilla,sphenoid,ethmoid etc
. Sesamoid Bones :
eg; patella,pisiform etc
. Accessory / Supernumerary Bones :
eg; sutural bones
II Regional classification
. Axial Skeleton
skull, ribs, sternum,
vertebral column,
hyoid bone
. Appendicular skeleton
Bones of upper &
lower limbs,
shoulder, pelvis
III Developmental Classification
. Membranous

. Cartilaginous / Endochondral
. Membrano - Cartilaginous

MICROSTRUCTURE OF BONE :-



Bone contains:

1) Extracellular Mineralized Matrix
2) Different Cell Types : - Osteoblasts
- Osteocytes
- Osteoclasts
- Osteoprogenitor cells
- Cells of its vascular &
nervous supply
3) Components of the periosteum, endosteum &
marrow
In the early stages of bone formation before
mineralization , the matrix is termed Osteoid.

COLLAGEN :
- Collagen fibers are mainly type - I
- Synthesized by osteoblasts
- Polymerized from tropocollagen extracellularly
- Unlike collagen in general connective tissue -
its molecular structure is more strongly covalently
cross-linked internally

NON COLLAGENOUS ORGANIC COMPONENTS :-

Glycoproteins:-

i) Osteonectin:
- phosphorylated glycoprotein synthesized by osteoblasts
- it binds to collagen & hydroxyapatite
- may play a role in initiating hydroxyapatite
crystallization
ii) Osteocalcin:
- synthesized by osteoblasts
- it binds to hydroxyapatite & calcium
- required for bone mineralization
iii) Osteopontin & Thrombospondin:
- mediate osteoclast adhesion to bone surfaces via
binding to osteoclast integrins

Proteoglycans:
- Biglycan & Decorin
- shorter core proteins & fewer side chains
- may bind transforming growth factor-? (TGF- ? )

TGF- ?:- secreted by osteoclasts as well as osteoblasts
- activated in the acid conditions of the ruffled
border zone of the osteoclast
- may be a coupling factor for stimulating new
bone formation at resorption sites
parts of bone
. Hyaline cartilage Cartilage covers the ends of the bones. The smooth surfaces stops the bones rubbing together and absorbs shock
. .Epiphysis This is the name for the extremity of the bones.Cancellous bone This is sometimes called Spongy Bone and stores the red bone marrow where blood cells are manufactured.
. Diaphysis The shaft of the bone.
. Compact bone The word "compact" suggests a hard part of the bone. It surrounds the yellow bone marrow in the diaphysis and gives strength to the hollow part of the bone.
. Periosteum the periosteum covers the surface of the bone. Ligaments and tendons are attached to the periosteum.
. Medullary cavity This space inside the diaphysis contains the yellow bone marrow.
. microscopic structre of bone
. Haversian systems / Osteons
. Primary Osteon / Atypical Haversian System
. Secondary Osteon / Typical Haversian System

FUNCTIONS OF BONE

. SUPPORT - forms a supporting framework, giving shape & rigidity to the body.
. Locomotion - forms a system to which the voluntary muscles are attached.
. PROTECTION - serves to protect the soft & delicate tissues of the body. E.g. skull protects the brain.
. MANUFACTURING OF BLOOD CELLS - RBC are manufactured in red bone marrow which is situated in spongy tissue at the ends of long bones.

BONE FORMATION:

The process of bone formation is called Ossification.
Ossification begins around the sixth or seventh week of embryonic life and continues throughout adulthood.
Bone formation follows one of three patterns:
1. Endochondral ossification
When cartilage is replaced by bone
2. Intramembranous ossification
Occurs directly within the mesenchyme
3.Sutural BONE GROWTH
INTRAMEMBRANOUS BONE FORMATION
1. Development of cartilage model
. Mesenchymal cells crowd together in the shape of the future bone and differentiate into chondroblasts which produce the cartilage matrix.
. A membrane called the perichondrium develops around the cartilage model.
2. Growth of Cartilage model
. It grows in length by continual cell division of the chondrocytes along with secretion of the matrix by daughter cells. This pattern of growth of increase in length is called interstitial growth (growth from within).
. Growth in thickness is due to addition of more matrix to its periphery by new chondroblasts that develop from the perichondrium. This growth pattern of cartilage is called appositional growth.
. Chondrocytes in the midregion hypertrophy. Some of these hypertrophied cells burst, thereby releasing there contents, which changes the Ph of the matrix.
. This change in the Ph triggers calcification. Once the cartilage calcifies, other chondrocytes die because adequate nutrition does not reach them via the matrix.
. The lacunae of these cells become empty and form small cavities.
. The nutrient artery then penetrates the perichondrium and bone through the nutrient foramen. This stimulates the osteoprogenitor cells in perichondrium to form osteoblasts which then lay down a thin shell of compact bone called as periosteal bone collar. The perichondrium is now called the periosteum.
. Development of primary ossification center
. Periosteal capillaries grow into the disintegrating calcified cartilage. These vessels along with the osteoblasts, osteoclasts, and red bone marrow cells are together k/a the periosteal bud.
. The capillaries induce the growth of a primary ossification center, a region where bone tissue will replace most of the cartilaginous tissue.
. Osteoblasts then begin to deposit bone matrix (osteoid) over the remnants of the calcified cartilage forming spongy bone trabeculae.
. As the ossification center enlarges , the osteoclasts break the newly formed trabeculae forming the Medullary cavity.
. The Medullary cavity is then filled with bone marrow.
. Development of diaphysis and epiphysis
.
. The diaphysis is replaced by solid compact bone, the center of which contains the red bone marrow. When blood vessels enter the diaphysis, secondary ossification centers develop, usually around the time of birth.
. In the secondary ossification centers the bone formation is similar to that of the primary ossification centers except that the spongy bone remains in the interior of the epiphysis (no Medullary cavities are formed in the epiphysis).
. Full transition to lamellar bone
. Invasion of blood vessels
. Primary osteon
. Mineralization




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