Dr. Sami Zaqout JUST Dr. Sami Zaqout IUG

Dr. Sami Zaqout IUG Dr. Sami Zaqout IUG Dr. Sami Zaqout JUST

Dr. Sami Zaqout IUG Dr. Sami Zaqout IUG

Ossification

Classification of bones

Constituents of bone

Types of bone

Definition and functions of bones

Contents


Definition of bones

• A special form of connective tissue in

which calcium salts are deposited and

which provides a framework, or skeleton,

for the other tissues of the body.


Function of bones

• The rigid supporting framework of the body

• Levers for muscles

• Protection to certain viscera (e.g., brain, spinal

cord, heart, lung, liver, bladder)

• Contain marrow, which is factory for blood cells

• Storehouse of calcium and phosphate


Types of Bone

• 1. Compact or solid bone which is present in the shafts of long bone and in the outer thin layer of the spongy bone in old age.

• 2. Spongy or Cancellons bone which is present in the epiphyses of long bones, ribs, vertebrae, flat bones as skull, scapula, sternum, and sacrum.


Histological Slides

• Histological slides taken from compact bone can be prepared by two methods:

• 1. Decalcified Compact Bone: Bones are treated with nitric acid to remove their calcium.

• The sections are then cut and stained with Hx and Eosin to demonstrate: – Periosteum – Endosteum – Osteocytes – Bone marrow cells.


Histological Slides

• 2. Ground Compact Bone: The dry bone is ground to demonstrate: – Haversian canals – Volkman's canals – Lacunae and canaliculi


Constituents Of Bone

• 1. Bone Matrix which is: a calcified tissue.

• 2. Bone Cells which are: Osteogenic Cells, Osteoblasts, Osteocytes and Osleoclasts.

• 3. Periosteum which is the covering layer of bone from outside.

• 4. Endosteum which is the lining layer of bone from inside.


Bone Matrix

• The Chemical Structure Of Bone Matrix

• 1. Organic substances: They constitute about 30% and are mainly protein.

• 2. Inorganic Substances: They constitute about 45% and are mainly calcium.

• 3. Water: about 25%


Types Of Bone cells

• There are Four types of Bone Cells:- • 1. Osteogenic Cells: They can differentiate into

osteoblast cells. • 2. Osteoblast Cells: They are responsible for

calcification of bones and formation of their organic materials.

• 3. Osteocyte Cells: They are the actual mature bone cells.

• 4. Osteoclast Cells: They are responsible for bone resorption during ossification


1 - Osteogenic Cells or Osteoprogenitor Cells

• Osteogenic cells develop from embryonic

mesenchymal cells or from pericyte cells • They are present in periosteum, endosteum

and bone marrow cavities. • By cell modulation they change into

osteoblast cells. • They are very active during growth of bone

and healing of fractured bone.


2 - Osteoblast Cells

• They arise from osteoprogenitor cells. • Osteoblasts are rich in these enzymes:- • a) Alkaline phosphatase enzymes which facilitate

deposition of calcium. • b ) Pyrophosphatase Enzymes which inhibit the

action of pyrophosphate substances – (These pyrophosphate substances retard the process

of calcification). • Sites: Osteoblasts are found in: Periosteum, endosteum and in bone marrow

cavities.


2 - Osteoblast Cells

• Functions:

• 1. They synthesize the organic components of bone matrix.

• 2. The enzymes of osteoblasts are concerned with calcification of bone,

• 3. Osteoblasts change into osteocytes when they are surrounded by lacunae and by calcified matrix.


3 - The Osteocyte

• It is a mature flat bone cell present inside lacuna.

• It is surrounded by calcified matrix.

• Each cell is surrounded by a space or lacuna from which canaliculi arise.

• The canaliculi of the neighboring osteocytes form gap junctions at their sites of contact.


3 - The Osteocyte

• Osteocytes communicate with each other by filopodial processes.

• Tissue fluids usually pass through these canaliculi and lacunae in order to conduct nourishment to osteocytes and to remove the waste products from them.


3 - The Osteocyte

• Origin:

• Osteocytes are considered as mature osteoblasts surrounded by calcified matrix.

• Functions:

• 1. They are involved in the maintenance of the bone matrix.

• 2. They produce components needed to maintain the surrounding matrix.


4 - Osteoclast Cells

• Origin: • They are formed as a result of multiple blood monocytes

or macrophages or osteoprogenitor cells.

• It is a large cell with irregular cell membrane. • Each cell contains from 6 to 12 nuclei. • These multiple nuclei are the nuclei of the fused

monocytes.

• With the E/M:

Osteoclast shows a Ruffled border formed of many finger like processes projecting from the cell membrane.



• The osteoclasts are rich in lysosomes and in acid phosphatase enzymes.

• Osteoclasts secrete osteolytic enzymes which destroy bone matrix, therefore, they are surrounded by spaces called “Howship's lacunae"

• Sites: They are present in bone marrow cavities and endosteum of bone.



• Functions Of Osteoclast

• 1. They are concerned in bone resorption during ossification.

• 2. They secrete acid hydrolases that dissolve bone matrix during ossification.

• 3. They liberate hydrogen ions which play a role in decalcification of bone matrix

• 4. They remove bone debris during ossification.


Microscopic Structure Of Compact Bone

• Transverse section in the shaft of an adult long bone is composed of the following structures:

• 1. Haversian Systems or osteons.

• 2. Interstitial systems or lamellae between the Haversian systems.


Microscopic Structure Of Compact Bone

• 3. Outer and Inner circumferential lamellae which are present under the periosteum and near to the endosteum of the bone.

• 4. Periosteum and Endosteum

The external surface of bone is covered by the periosteum and its internal surface is lined by the endosteum.


The Periosteum

• It is a vascular CT. membrane which covers the bone from outside.

• It does not cover the bone at the articular surfaces of joints.


The Periosteum

• The periosteum is formed of two layers:-

• a ) Outer fibrous layer which is formed of collagenous fibers, fibroblast cells, nerve fibres and B.V.

• b) Inner osteogenic layer which is formed of osteogenic spindle-shaped cells.


The Periosteum

• Perforating fibers of sharper These calcified collagenous bundles which arise from the deep surface of the periosteum to be embedded like nails into the bone.

• They are present at the sites of attachments of tendons and ligaments of muscles to fix them more into the bone.


The Periosteum

• Functions of periosteum:

• It provides an attachment for muscles, tendons and ligaments.

• It provides the bone with blood supply and nourishment.

• It is important for formation of bone during its growth and after its fracture.


The Endosteum

• It lines the internal surface of bone and the bone marrow cavities.

• It is formed of a vascular C.T, membrane rich in osteogenic cells.

• Functions Of Endosteum: • 1. It supplies bone with blood

supply and nutrition. • 2. Its osteogenic cells

continuously supply bone with new osteoblast cells which are concerned with growth and repair of bone.


Bone Marrow

• Bone marrow occupies the marrow cavity in long and short bones and the interstices of the cancellous bone in flat and irregular bones.

• At birth, the marrow of all the bones of the body is red and hematopoietic.

• This blood-forming activity gradually lessens with age, and the red marrow is replaced by yellow marrow.


Bone Marrow

• At 7 years of age, yellow marrow begins to appear in the distal bones of the limbs.

• This replacement of marrow gradually moves proximally, so that by the time the person becomes an adult, red marrow is restricted to the bones of the skull, the vertebral column, the thoracic cage, the girdle bones, and the head of the humerus and femur.


The Haversian System Or Osteon

• It is the structural unit of compact bone.

• It is formed of :-

• a) Haversian Canal which runs parallel to the long axis of the bone. – It contains loose connective

tissue rich in blood vessels.

– It is supply the surrounding bone matrix and osteocytes with blood and nutrition.



• b) Concentric Bone Lamellae: – These are the calcified

osteoid tissue arranged in concentric layers one outside the other.

– These layers are from 4 to 20 in number.

– The bone cells (osteocytes) are arranged between these lamellae.



• c) Osteocytes: – These are the mature bone cells

which are present inside their lacunae and are surrounded by bone matrix.

– From these lacunae canaliculi project to anastomose with the canaliculi of other osteocytes.

– The tissue fluid pass through these lacunae and canaliculi from one cell to another, therefore the bone is considered as a well nourished tissue and is very rich in blood supply.



• The External Circumferential Lamellae:- – These lamellae are formed of

calcified osteoid tissue in which osteocytes are embedded.

– They are present under the periosteum and are arranged parallel to it.

• The Internal Circumferential

Lamellae:- – These lamellae are formed of

calcified osteoid tissue present adjacent to the endosteum.

– Osteocytes are embedded in these lamellae and are arranged parallel to the endosteum



• The Interstitial Or The Inter Haversian Lamellae:- – These are formed of calcified

osteoid tissue present between the Haversian systems.

– Osteocytes in these interstitial lamellae are irregularly arranged.


Volkmann's Canals

• These are transverse or oblique canals.

• They connect the Haversian canals together.

• They may also connect the Haversian canals with the periosteum and occasionally with the endosteum.

• Through these Volkmann's canals, the blood vessels of the Haversian systems anastomose with the periosteal blood vessels.


Spongy Or Cancellous Bone

• The long and short hones are formed externally of compact bone, but their endosteums are irregular due to presence of spongy bone.

• Cancellous bone looks spongy, with many vascular channels.



• It is formed of irregular bars or plates of bone separating between them multiple bone marrow cavities which are rich in blood vessels.

• The multiple bone marrow cavities are filled with active red bone marrow.



• Sites of Spongy Bone: – It is present in the

centre of ribs, vertebrae and the centre of flat bones as: skull, scapula, sternum and ilium.

– It is present also in the centre of the epiphyses of long bones.


Classification of

Bones

Regionally General shape


Bone Regionally

Number of bones Region of skeleton

Axial skeleton Skull

8 Cranium

14 Face

6 Auditory ossicles

1 Hyoid

26 Vertebrae

1 Sternum

24 Ribs

Appendicular skeleton Shoulder girdles

2 Clavicle

2 Scapula


Number of bones Region of skeleton Upper extremities

2 Humerus

2 Radius

2 Ulna

16 Carpals

10 Metacarpals

28 Phalanges

Pelvic girdle

2 Hip bone

Lower extremities

2 Femur

2 Patella

2 Fibula

2 Tibia

14 Tarsals

10 Metatarsals

28 Phalanges

206


Bone

General Shape

Long bones

Short bones

Flat bones

Irregular bones

Sesamoid bones


Bone General Shape

Long bones

Their length is greater than their breadth

Found in the limbs (e.g., the humerus, femur, metacarpals,

metatarsal, and phalanges) Long bones


Bone General Shape

Short bones

They are roughly cuboidal shape

Found in hand and foot (e.g., the scaphoid, launate, talus, and

calcaneum). Short bones


Bone General Shape

Flat bones

They are composed of thin inner and outer layers of compact

bones, the tables, separated by a layer of cancellous bone ,the

diploe

Found in the vault of the skull (e.g., the frontal and parietal bones)

Flat bones


Bone General Shape Irregular bones

Thin shell of compact bone with an interior made up of

cancellous bone

E.g., bones of skull, the vertebra, and the pelvic bones) Irregular bones


Bone General Shape Sesamoid bones

Small nodules of bone that are found in certain tendons

where they rub over bony surfaces

E.g., patella.. Sesamoid bones


Bone markings and formations

• Markings appear on bones wherever tendons, ligaments, and fascia attach.

• Other formations relate to joints, the passage of tendons, and the provision of increased leverage .


Bone Surface markings

Example Bone marking

Linear elevation

Superior nuchal line of the occipital line Line

The medial and lateral supracondylar

ridges of the humerus

Ridge

The iliac crest the hip bone Crest

Round elevation

Pubic tubercle Tubercle

External occipital protuberance Protuberance

Great and lesser tuberosities of the

humerus

Tuberosity

Medial,lateral malleolus of the tibia

Greater & lesser trochanters of femur

Malleolus

Trochanter




Sharp elevation

Ischial spine, spine of vertebra Spine or spinous process

Styloid process of temporal bone Styloid process

Expanded ends

for articulation

Head of humerus, head of femur Head

Medial and lateral condyles of

femur

Condyle (knucklelike process)

medial and lateral epicondyles of

femur

Epicondyle (a prominence

situated just above condyle)




Depressions

Great sciatic notch of hip bone Notch

Bicipital groove of humerus Groove or sulcus

Olecranon fossa of humerus, acetabular

fossa of hip bone

Fossa

Small flat area

for articulation

On head of ribs for articulation with

vertebral body

Facet


Bone Surface markings Example Bone marking

Openings

Superior orbital fissure Fissure

Infraorbital foramen of the maxilla Foramen

Carotid canal of temporal bone Canal

External acoustic meatus of temporal bone Meatus


Ossification • Ossification is the process of formation of bone

which leads to its growth. • There are two methods for bone development or

bone ossification: – 1. Intramembranous. – 2. Endochondral.


Mechanism of ossification

• Bone development occurs as a result of the following two processes:

• 1. Bone Formation by osteoblasts: – The osteogenic cells in the primitive embryonic tissue change

into osteoblasts. – These osteoblasts can form osteoid tissue in the form of

irregular trabeculae of bone. – When the osteoblasts are completely surrounded by the bone

matrix, they are changed, into osteocytes.

• 2. Bone Resorption by osteoclasts:

– This process occurs during growth and remodeling of the formed bones.


(1) Intramembranous Ossification

• It occurs in: Flat bones of the face, skull and also in the clavicle.

• The site of the future bone is occupied by a mesenchymal membrane which is formed of matrix, blood capillaries and mesenchymal cells.



• 1. A centre of ossification appears in the middle of the mesenchymal membrane.

• At this centre, the blood supply increases and the mesenchymal cells are transformed into osteogenic cells which are then transformed into osteoblasts.



• 2. The newly formed osteoblasts synthesize the organic components of the matrix.

• They are rich in phosphatase enzymes which can deposit calcium on the newly formed matrix forming trabeculae of calcified osteoid tissue.

• The osteoblasts which are now surrounded by a solid matrix are then transformed into osteocytes.



• 3. The trabeculae of the newly-formed spongy bone extend from the center of ossification outwards in a radial manner.

• 4. The osteogenic cells on the outer and inner surfaces will form the periosteum and the endosteum.

• 5. Growth and remodeling of bone occurs by deposition of new bone by the osteoblasts and resorption of irregular bone from the opposite side by the osteoclasts.


(2) Endochondral

• This type of ossification occurs in the long bones which were originally formed of hyaline cartilage in the fetus.

• These cartilage models will be replaced by bone. • Ossification starts as primary centre of ossification in

the middle part of the long bone (Diaphysis). • Then secondary centres of ossification appear at

both ends of long bone (Epiphyses). • The epiphyseal disc and the part of the diaphysis

near to it are called the growing zone.


Stages Of Endochondral Ossification

1. Resting stage of hyaline cartilage

2. Proliferative stage of cartilage

3. Maturation or hypertrophy stage of cartilage

4. Calcification stage of cartilage

5. Stage of invasion

6. Stage of spongy bone formation

7. Stage of internal reconstruction or stage of remodeling

8. Stage of complete ossification


Growth of Bone

• Growth of Bone in Length: – Bones grow in length by the

proliferation of more cartilage cells at both epiphyseal discs.

– No further increase in the

length of the bone is possible after the replacement of the epiphyseal disc by bone.


Growth of Bone

• Growth of Bone In Width

– Bones grow in width (diameter) by deposition of subperiosteal bone under the periosteum and absorption of irregular bone at the endosteum in order to increase the bone marrow cavity with the increase in the thickness of the shaft.

– Growth of bone is affected by Hormones, minerals, nutrition and vitamins (especially vitamin D and C).

Dr. Sami Zaqout JUST Dr. Sami Zaqout IUG

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