Bone is one the hardest tissues of the human

body, second only to cartilage in its ability to with

stand stress

Functions :

1. Support fleshly structures

2. Protect such vital organs

3. Harbors the bone marrow

4. Reservoir of calcium phosphat and others ions

Composed :

I. Bone Matrix

II. Cells :

• Osteocytes

• Osteoblasts

• Osteoclasts

I. BONE MATRIX

1. INORGANIC MATTER• Calcium Phosphat• Bicarbonaat• Citrate• Magnesium• Pottasium• Sodium

2. ORGANIC MATTER• Collagen Fibers• Amorphous Ground substance :

a. Chondroitin 4-sulfate

b. Chondroitin 6-sulfate

c. Keratan Sulfate

II. BONE CELLS

1. OSTEOBLASTS

• Synthesis of bone matrix

a. Type I Collagen

b. Proteoglycans

c. Glycoprotein

• Located at the surfaces of bone tissue, side by side, in away that resembles simple epithelium

• When actively : Cuboidal to columnar shape

• When actively declines : Flatten

OsteoblastOsteoclast

Osteocyte

MesenchymeBone Matrix

Newly formed matrix

2. OSTEOCYTES

• Lie in the lacunae

• One osteocytes in each

lacuna

• Canaliculi house cytoplasmic

process

• Processes of adjucent cell

make contact via gap

junction

This figure is section of bone tissue showing an osteocyte with its cytoplasmic processes surrounded by matrix. Ultrastructure compatible with a low level of synthetic activity is apparent in both nucleus and cytoplasm.

3. OSTEOCLASTS• Very large

• 5 to 50 contain nuclei

• Lie in howship’s lacunae

• Derived from the fusion of monocytes

• Secrete :

a. Acid

b. Collagenase

c. Other proteolytic enzymes

PERIOSTEUM AND ENDOSTEUM

External and intrnal surfaces of bone are covered by layers of bone forming cells and connective tissue called periosteum and endosteum.

1. Periosteum Outer Layer

- Collagen fibers

Bundle : Sharpey’s fibers- Fibroblasts

Inner Layer

More celllular (oteoprocenitor cells) is composed of

flattened cells to divided into osteoblasts

2. Endosteum

Lines all internal surfaces of cavities with in the bone

Composed :

a. Osteoprogenitor cells (single layer)

b. Small amount of connective tissue

Therefore, the endosteum is consider ably thinner than the

periosteum

Functions of periosteum and endosteum :

1. Nutrition of osseus tissue

2. Supply of new osteoblasts for repair or growth of bone

Interstitial lamellae

Inner Circumferential

lamellae

Haversian system (osteon)

Outer circumferential lamellae

Volkmann’s canal

PeriosteumEndosteum Haversian canal

Schematic drawing of the wall of a long bone diaphsis.

A. MICROSCOPIC EXAMINATION

1. Primary, immature or woven bone

2. Secondary, mature or lamellar bone

B. GROSS OBSERVATION (CROSS SECTION)

1. Dense area without cavities : Compact Bone

2. Areas with numerous interconecting cavities :

Cancellous (spongy) bone

Thick ground section of tibia illustrating the cortical compact bone and the lattice of trabeculae of cancellous bone

In long Bones :

1. Bulbous ends : Epiphyses

Spongy bone covered by a thin layer of compact bone

2. Cylindrical Part : Diaphysis

Almost totally composed of compact bone, with a small component of spongy bone on its inner surface around the bone marrow cavity.

Bone collar

Osteogenic bud

Primary ossification

center

Epiphysis

Epiphyseal plate

Epiphysis

Diaphysis

Compact

bone

Spongy bone

Secondary ossification

center

Secondary ossification center

Formation of a long bone on a model made of cartilage

In Short Bones :

Usually have a core of spongy bone completely surrounded by compact bone.

In Flat Bones (Calvaria)

Have two layers of compact bone called plated (tables), separated called the diploe.

PRIMARY BONE TISSUE

First bone tissue

Temporary, replace by secondary bone tissue, except :

- Near the sutures of the flat bones of the skull

- In tooth sockets

- In the insertion some tendons

Characteristics :

- Irregular array of collagen fibers

- Smaller mineral content

- Higher proportion of osteocytes than in secondary bone tissue

SECONDARY BONE TISSUE Usually found in adults Characteristics :

- Collagen fibers : Lamellae- Haversian canals- Haversian system or osteon- Lacunae- The lamellae exhibit a typical organization consisting of :

1. Haversian system2. Outer circumferential lamellae3. Inner circumferential lamellae4. Intertitial lamellae

- The haversian canal communicate with :1. The narrow cavity2. The periosteum3. The volkman’s canals (do not have concentric

lamellae)

Interstitial lamellae

Inner Circumferential

lamellae

Haversian system (osteon)

Outer circumferential

lamellae

Volkmann’s canal

PeriosteumEndosteum Haversian canal

Schematic drawing of the wall of a long bone diaphsis.

Bone can be formaed in two ways :

I. Direct mineralization of matrix secreted by osteoblasts (intra membranous ossification)

II. Deposition of bone matrix preexiting cartilage matrix (endochondral ossification)

HISTOGENESIS

I. Intramembranous Ossification formed by intramembranous ossification :

• The frontal and parietal bones of the skull• The occipital and temporal bones of the skull• The mandible and maxilla

Mesenchymal Condensation Layer

Primary ossification Centre

Cells differentiate into osteoblast

New Bone Matrix is formed

Calcification

Encapsulation of some Osteoblast

Become Osteocytes

The beginning of intramembranous ossification

Mesenchyme Bone Blastema Primary bone Tissue

II. Endochondral Ossification

Takes place within a piece of Hyalin Cartilage whose shape resembles a small version or model of the bone to be formed.

Bone collar

Osteogenic bud

Primary ossification

center

EpiphysisEpiphyseal

plate

Epiphysis

DiaphysisCompact

bone

Spongy bone

Secondary ossification

center

Secondary ossification

center

Endochondral ossification consists of two phases :1. The first phase

Hypertrophy and destruction of the chondrocytes of the model of the bone, leaving expanded lacunae separated by septa of calcified cartilage matrix.

2. The second phase

Osteogenic bud consisting of

osteoprogenitor cells

Blood capillaries penetrates the spaces left

by the degenerating chondrocytes

The osteoprogenitor cells give rise to osteoblasts,

with cover the cartilaginous septa with

bone matrix

The septa of calcified cartilage tissue thus serve

as support for the beginning of ossification

THE OSSIFICATION CENTRE1. Primary ossification centre appears in the diaphysis

2. Secondary ossification centre arises at the centre of each epiphysis

Bone collar

Osteogenic bud

Primary ossification

center

EpiphysisEpiphyseal

plate

Epiphysis

DiaphysisCompact

bone

Spongy bone

Secondary ossification

center

Secondary ossification

center

When the bone tissue that originated at the secondary centres occupies the epiphysis, cartilage remains restricted to 2 places :1. Articular cartilage

- Persist throughout adult life

- Does not contribute to bone formation

2. Epiphyseal cartilage or the epiphyseal plate

- Connect epiphysis to diaphysis

- As the cartilage grows, it is replace continously by newly formed bone matrix mainly from the diaphyseal centre

Epiphysis

Epiphyseal plate

cartilage

Diaphysis

Articular cartilage

Secondary ossification

center (marrow space)

Primary ossification

center (marrow space)

Schematic drawings showing the 3-dimensional shape of bone spicules in the epiphyseal plate area. Hyaline cartilages is stipple, calcifed cartilage is black, and bone tissue is shown in color. The upper drawing shown the region represented 3-dimensionally in the lower drawing.

Epiphyseal cartilage, divided into five zones :

1. Resting zone

• With out morphologic changes in the cells

2. Proliferative zone

• Chondrocytes devided rapidly

• Form columns of stacked cells parallel to the long axis of the bone

3. Hypertrophic cartilage zone

• Large chondrocytes who cytoplasm has accumulated glycogen

• The resorbed matrix is reduced to thin septa

4. Calcified cartilage zone

• Simultaneous with the death of chondrocytes

• The thin septa become calcified

5. Ossification zone

• Endochondral bone tissue appears

• Blood capillaries and osteoprogenitor cells formed by mitosis of cells originating from the periosteum invade the cavities

• The osteoprogenitor cells form osteoblasts

• Osteoblasts turn form a discontinuous layer over the septa

• Over these septa, the osteoblasts deposit matrix

Ossification zone

Proliferative Zone

Hypertrophic cartilage zone

Calcified cartilage zone

Resting Zone

Photomicrograph of the epiphyseal plate, showing the change that take place in the cartilage and the formation of bone spicules

MECHANISMS OF CLASSIFICATION

Bone calcium is mobilized by two mechanisms :

1. Rapid Mechanism

The simple transference of ions from hydroxyapatite crystal

to interstitial fluid into the blood

2. Slow Mechanism

Depends on the action hormones

a. Parathyroid hormone

Activates and increases the number of cells (osteoclasts)

promoting resorption of the bone matrix with the

consequent liberation of calcium

b. Calcitonin

Inhibits matrix resorption (its effect, is the opposite of

parathyroid hormone)

FRACTURE REPAIR

When fractures :

• The damaged blood vessels product a blood clot

• Destruction of bone cells

• Death of Bone cells

During repair :

• The blood clot, cells and damaged bone matrix are removed by macrophages

Repair of a fractured bone by formation of new bone tissue through periosteal and endosteal cell proliferation

Periosteum Periosteal proliferation

Bone Newly formed primary bone

Callus

Newly formed secondary bone

Hyaline Cartilage

Healed fracture

The Periosteum and endosteum proliferation of osteoprogenitor cells

Formed a cellular tissue

Bone is formed by endochondral and intramembranous ossifications

Formed Trabeculae of primary bone (A Bone Callus)

The callus is gradually resorbed and replaced by secondary bone