BONE

Definition

It is a highly specialized mineralized connective

tissue which makes up the body skeleton and is

one of the hardest structures of the animal body.

1. It provides shape and support for the body.

2. It also provides site of attachment for tendons and muscles, which are essential

for locomotion.

3. It protects the vital organs of the body.

4. Bone serves as a storage site for minerals especially ca and ph. Which mobilized

according to the body needs and provides the medium, the marrow for the

development and storage of blood cells.

5. Manufacture the blood elements (from bone marrow).

6. Bone remodeling responsible for development, growth movements, fracture and

repair.

7. Bone marrow is the main source of Stem cell until now.

Functions of bone

• Bone is in a continuous dynamic state and possesses a certain degree of

toughness and elasticity.

• Resorption with pressure& Deposition with tension

• Systemically, hormonal factors control the bone physiology; locally,

mechanical forces (including tooth movement), growth factors, and

cytokines also have regulatory functions.

• Bone resists compressive forces best and tensile forces least. Bone also

resists forces applied along the axis of its fibrous component. Fractures

of bone thus occur most readily because of tensile and slicing stresses.

• Human body contains more than 300 bone in children and 206 bone in

adults

Properties of bone

Bones may be classified according to their

Long bones

Short bones

Flat bones

Irregular bones

Sesamoid bones

Mature bone Lameller

Compact bone (cortical bone)

Cancellous bone (spongy bone)

Endochondral bones

Intramembranous bones Woven or immature Non Lameller

Bundle Bone

Classification of bone

Long bones These bones are long and slender. They are longer than they are wider. The long bones include the bones of

the arm—humerus, radius, ulna; leg—femur, tibia, fibula; fingers and toes—each phalanx (individual bone of finger or

toe, all together the phalanges); palms of hands and soles of feet—metacarpals and

metatarsals.

Short bones These bones are usually cube shaped of nearly equal length and width.

They consist of spongy bone which is covered by a thin layer of compact bone. They

contain bone marrow, but no marrow cavity. These bones include bones of wrist—

carpals and ankle—tarsals.

Flat bones These bones are thin, flat, curved, with no marrow cavity. Spongy bone is present between upper and lower layer

of compact bone. Flat bones include bones of sternum, ribs, scapula, clavicle, and bones that form roof of the skull, parietal,

frontal, temporal and occipital.

Irregular bones

These bones have complex shapes, notched or with ridges and

are not included in any of the above categories. These bones are

primarily made of spongy bone, which is covered with a thin

layer of compact bone, with bone marrow, but no marrow cavity.

The irregular bones include bones of vertebrae, facial bones

(ethmoid, sphenoid) pelvic bones (ischium and pubis), calcareous

(heel bone) and mandible.

Sesamoid bones These bones develop in tendons, where there is considerable

pressure, tension or friction. Patella (knee cap) is a good example.

Intramembranous bones These bones are formed by replacement of sheet like connective tissue membrane with bony tissue. There is no intermediate cartilage involved. This ossification occurs in the cranial and facial flat bones of the skull, mandible and clavicles.

Endochondral bones A cartilage is formed as precursor on which new bone is laid down These bones are formed by replacement of hyaline cartilage with bony tissue. This type of ossification occurs in bones of trunk and extremities as TMJ and long bone.

Endochondral bone Intramembranous bones

Classification based on development

Developmentally, bones are classified as

Classification based on microscopic structure Histologically, bones are classified as mature bone

Mature bone (Lamellar) is further classified as compact bone

and cancellous bone.

Compact bone (cortical bone) consists of tightly, packed

osteons or haversian systems, forming a solid mass. Since the

bone mass is arranged in layers, it is also called lamellar bone

Cancellous bone (spongy bone) has a honeycomb appearance,

with large marrow cavities and sheets of trabeculae of bone in

the form of bars and plates

Woven or (Non Lamellar) immature bone is the first formed

bone with irregularly oriented collagen fibers of varying diameters.

This type of bone is not usually seen after birth. However

it is seen in the alveolar bone and during healing of fractures.

Since this bone forms rapidly, it incorporates many osteocytes

Bundle bone is that bone in which the principal fibers of the periodontal ligament are anchored. The term ‘bundle’

was chosen, because, the bundles of the principal fibers continue into the bone as Sharpey’s fibers. The bundle bone is

characterized by the scarcity of the fibrils in the intercellular substance. These fibrils, more over, are all arranged at

right angles to Sharpey’s fibers. The bundle bone contains fewer fibrils than does lamellated bone, and therefore it

appears dark in routine Hematoxylin and eosin stained sections and much lighter in preparations stained with silver

than does lamellated bone. These fibers are mineralized at the periphery and have a larger diameter. These fibers are

less numerous than the corresponding fiber bundles in the cementum on the opposite side of the periodontal ligament.

The collagen adjacent to bone is always less mature than that adjacent to cementum. In some areas, the alveolar bone

proper consists mainly of bundle bone.

Bundle bone is formed in areas of recent bone apposition. Lines of rest are seen in bundle bone.

Radiographically, it is also referred to as the lamina dura, because, of increased radiopacity, which is due to the

presence of thick bone without trabeculations, that X-rays must penetrate and not to any increased mineral content.

33%

Organic matrix Osteoid tissue

67%

Inorganic

Hydroxyapatite crystals

(Ca10[PO4]6[OH]2),

Small plates, most of which lodge in the holes and pores of collagen

fibrils calcium phosphate, hydroxyl and carbonate. Citrate,

magnesium, sodium, potassium, fluoride, iron, zinc, copper,

aluminum, lead, strontium, silicon and boron are present in small

quantities.

28% collagen 90%TypeI 5% Type V

5%

noncollagenous

matrix proteins

Osteocalcin 15%, osteopontin, bone sialoprotein,,osteonectin

25%, proteases, protease inhibitors and a variety of cytokines,

bone morphogenetic proteins (BMP) superfamily, including

BMP-2, BMP-7, TGF-β, insulin like growth factors

(IGF-I and IGF-II), platelet derived growth factor (PDGF) and

fibroblast growth factor (FGF). IGF-I, PDGF and TGF-β

increase the rapidity of bone formation and bone repair. and

proteoglycans; growth factors and serum proteins

Cells Hard Intercellular ground substance

Osteoblasts

Osteocyte

Osteoclast

Osteoprogenitor

BONE CELLS

1 - OSTEOPROGENITOR ( osteogenic stem cell) CELLS.

2 - OSTEOBLAST

3 - OSTEOCYTE

4 - OSTEOCLAST

1 -OSTEOPROGENITOR CELLS (Osteogenic )

Location: in the deepest layer

of periosteum and lining the

vascular canals of compact

bone.

Origin: mesenchymal in origin.

Function: according to the

function needed they divide by

mitosis to give any type of

bone cell.

Morphology: resemble

mesenchymal cell with pale

stained nucleus and little

esinophilic cytoplasm.

2 - OSTEOBLASTS

Location: on the surface of bone. Origin: from osteoprogenitor cell. Function: bone forming cell (osteoid bone). Histology: they are round or slightly elongated cell with the long axis perpendicular to the bone surface . basophilic cytoplasm rich in alkaline phosphatase . .It is a protein forming cell. (1) Large open faced distally placed nucleus. (2) Mitochondria. (3) R.E.R. (4) Golgi apparatus. (5) Secretary granules. (6) Free ribosomes. (7) Cytoplasm. (8) Alkaline ph.

Secretory vesicles containing an amorphous material that is presumed to be matrix precursors(4).

Exocytosis of these secretory products occurs from all parts of the cell surface

* When trapped in bone called osteocyte.

3 - OSTEOCYTES

They are the principal cell of bone. Location: during bone development, the incorporated osteoblasts in bone matrix called osteocytes. Origin: incorporated osteoblasts. Function: 1. provide good supply of nutrient and oxygen to mentain bone tissue and prevent their own death. 2. Play important role in minerals storage and resorption (high metabolic activity). 3. resorption of bone by osteocytes called (osteocytic osteolysis). Histology: * they have flat cell body with several branching process that anastomose with neigbouring cells and with the nearst perivascular region. * Their body present in lacuna and their process canal cull. * have basophilic cytoplasm * R.E.R. * Golgi * Mitochondria * with fat droplet and glycogen. * The junctions between osteocytes are gab junction

Location: present on the surface of resorbed bone, in a bay like depression

called how ships lacuna.

Origin: from osteoprogenitor cells.

Or blood cells (monocytes).

Theories of formation: (1) Division of nucleus without division of cytoplasm.

(2) Fusion of several mononuclear cells.

Function: bone resorption.

Histology: * large multinucleated cells (30 or more N).

With foamy esinophilic cytoplasm

* R.E.R. * Mitochondria.

* Golgi apparatus * lysosomal vacuoles.

* Free ribosomes * rich in acid phosphate.

* The plasma membrane facing the resorbed bone have ruffled

border.

4 – OSTEOCLASTS (origin)

BONE RESORPTION summary

1 – DECALCIFICATION. 2 – EXTRACELLULAR

DIGESTION.

3 – TRANSPORT OF

SOLUBLE PRODUCTS

TO EXTRA

CELLULAR FLUID OR

THE BLOOD

VESCULAR SYSTEM.

Citric & lactic acids. Acid protease & Collagenase enzymes,

Metabolic by- products Secreted by osteoclast at its

ruffled border.

.osteoclastactivity of Lysosomal

Fibroblasts, monocytes and macrophages also produce collagenase.

Mechanism not yet known.

Only calcium phosphate can be identified inside osteoclasts.

Osteoclasts

Osteocytes

Osteoblasts

Osteogenic cells

BONE CELLS summary

Osteoid is an unmineralized bone matrix on the surface, where

active bone formation is taking place. It is approximately

5–10 μm before the commencement of mineralization. The

mineralizing front is linear. Osteoid contains type I collagen

fibers, parallel to the bone surface, embedded in ground substance of proteoglycans,

glycoproteins and other proteins.

Just beneath the periostium All mature bones have a dense outer sheet of compact bone and

a central medullary cavity. The cavity is filled with red or yellow bone marrow in living

bone. This cavity shows a network of bone trabeculae. Trabecular, spongy or cancellous

bone are the terms used to describe this network.

The outer aspect of mature bone compact bone is surrounded by a condensed fibrocollagen

layer, the periosteum which is fibrouse sheath has two layers:

an outer layer which is a dense, irregular connective tissue termed

fibrous layer; and an inner osteogenic layer, next to the bone surface

consisting of bone cells, their precursors and a rich vascular

supply. The periosteum is active during fetal development. It

is also important in the repair of fractures.

The inner surface of compact and cancellous bone are covered

by a thin cellular layer called endosteum. In resting adult

bone, quiescent osteoblasts and osteoprogenitor cells are present

on the endosteal surfaces. These cells act as reservoir of new

bone forming cells for remodeling or repair.

BONE HISTOLOGY The periosteum (Cambium)

Osteoid

COMPACT BONE

SITES

EXTERNAL COVERING of

Mandible, Maxilla, RIBS, VERTEBRAE, FLAT BONES OF THE SKULL

SHAFT OF LONG BONES L.S. SHAFT OF LONG BONE T.S.

LAMELLAR PATTERNS (in the shaft of the long bone)

THREE PATTERNS OF LAMELLAR ORGANIZATION.

1 – Circumferential or basic

lamellae.

2 – Haversian lamellae.

3 – Interstitial lamellae.

1 - Outer circumferential lamellae Concentric Lamellae

Artery Vein

Osteons

Volkmann’s canal

COMPACT SPONGY

Bone Trabeculae

1 - Inner circumferential

lamellae

Haversian canal

3 - Interstitial lamellae

2 - Haversian lamellae

HAVERSIAN SYSTEM or Osteon (primary unit)

GROUND SECTION DECALCIFIED SECTION

Interstitial lamellae

Haversian lamellae

H. canal

Note: H. canal + H. lamellae = H. system or Osteon

B - SPONGY BONE

SITES

EPIPHYSIAL PLATE

RED BONE MARROW

SPONGY BONE

Cancellous bone, also known as spongy or trabecular bone, is one of the two types of bone tissue found in the human body. Cancellous bone is found at the ends of long bones, as well as in the pelvic bones, ribs, skull, and the vertebrae in the spinal column. It is very porous and contains red bone marrow, where blood cells are made. It is weaker and easier to fracture than cortical bone, which makes up the shafts of long bones.

HISTOLOGY

1. Resting lines:

- appear dark blue line in decalcified H and E section.

- This line is straight or slightly undulated.

- represent the incremental lines of bone formation (rest phase).

2. Reversal line:

- appear dark blue line in decalcified H and E section.

- Scalloped in shape.

- demonstrate osteoclast activity where the convexities.

Towards the old resorbed bone

3. Faint lines: - appear faint black line in section stained with silver

impregnation.

- Collagen fibers in every layer arranged to each other.

- Fibers or next layer change their direction with 450.

- The change of direction of collagen fibers from one layer to

other will lead to these faint lines between layers.

INCREMENTAL LINES

ENDOSTUEM

MEDULLARY SURFACE

MEDULLARY SURFACE

The inner surface of compact and cancellous bone are covered by a thin cellular layer called endosteum. In resting adult bone,

quiescent osteoblasts and osteoprogenitor cells are present on the endosteal surfaces. These cells act as reservoir of new bone forming

cells for remodeling or repair.

2-WOVEN BONE

Bone of the foetus = Embryonic bone.

- Callus of fracture =Bone of emergency.

- Healing sockets after tooth extraction.

- Other names * coarse fibered woven bone * embryonic bone

* temporary bone * bone of emergency.

- Found in areas where bone is laid down for the first time, as

in bones of foetus, callus of fracture and healing socket.

-Characterized by:

-1. irregular arrangement of collagen fibers.

2. Great no, great size, irregular arrangement of osteocytes.

3. Organic substance and inorganic so appear radiolucent in x-

ray (in spite of its presence histological).

- This type of bone is resorped and replaced by lamellar bone

(i.e. never change to lamellar bone but replaced with it).

Differences between immature bone and mature bone (woven bone and lamellar bone)

Woven bone Lamellar bone

Woven bone is an immature bone characterized by intertwined collagen fibers oriented in many directions, hence the name.

In lamellar bone, a distinctive, orderly arrangement is seen,

which is the result of repeated addition of uniform lamellae to

bony sur- faces during appositional growth. The direction of

collagen fibrils in any given lamella lies at right angles to that of the fibrils in the adjacent lamella.

In woven bone, there is a great amount of interfibrillar space that is occupied by mineral crystals

and acidic proteins.

In mature bone, the interfibrillar space is less.

The matrix of woven bone in hematoxylin and eosin section is tinged with blue indicating

that it has higher proteoglycan content.

Lamellar bone shows comparatively uniform acido- philic staining

of the matrix.

Rates of deposition and mineralization are faster for woven bone Hence, woven

bone shows higher proportion of osteocytes.

than lamellar bone

than lamellar bone.

Woven bone is enriched in BAG-75 (bone acidic glycoprotein- 75) and BSP (bone

sialoprotein). Osteopontin and type 1 collagen are seen in both woven and lamellar

bone matrices.

Lamellar bone is enriched in osteocalcin.

Mineral density is lower and water content higher in woven bone matrix.

Woven bone can be entirely removed by osteoclasts, whereas only a portion of lamellar matrix of a

given bone is resorbed at one time.

Matrix vesicles participate in initiation of mineralization of woven bone, whereas collagen mediated

mechanism is opera- tive in calcification of lamellar bone.

3 – BUNDLE BONE

BUNDLE BONE

- Found adjacent to periosteum and P.L. (areas of tension).

- It's matrix pattern sharing between woven and lamellar

bone.

- Characterized by presence of Sharpey's fibers.

- It has less no of fibers, less no of cells than woven bone,

but more ca. salts than woven bone so it appears more

radio-opague in x-ray and called lamina durra.

- Their fibers arranged parallel to the socket wall.

periosteum

medulla

bundle bone

Bundle bone

SITES

1 – DARKER IN H&E STAINED SECTIONS.

Dentin

Cementum

Periodontal l.

Bundle bone

Lamellar bone Bondle bone

BUNDLE BONE

PERIODONTAL L. F. B. SHARPEY’S FIBERS

PERIODONTAL L. F. B.

BANDLE BONE LAMELLAR BONE

SHARPEY’S FIBERS

2 – LIGHTER IN SILVER STAINED SECTIONS.

3 – MORE RADIOPAQUE THEN LAMELLATED BONE

BUNDEL BONE

LAMELLAR BONE

LAMELLAR BONE

BUNDEL BONE