Bone or osseous tissue is a special type of highly vascular, mineralized & constantly changing rigid connective tissue that forms the framework of our body.
Composition of Bone
1. Bone cells
• Osteocytes
• Osteoblast
• Osteoclasts
2. Extracellular matrix
• Organic
- Type-I collagen fibers
- Protein
- Glycosaminoglycans (Mucopolysaccharides)
• Inorganic
- Abundant calcium & phosphorus

Classification of Bones
A/c to the development
Membranous bone
Ex: Most of the flat bone
Cartilaginous bone
Ex: All the long bones

A/c to Macroscopical view
Compact bone
Ex: Shaft of typical long bone
Spongy or cancellous bone
Ex: Each end of a typical long bone

A/c to position
Axial skeleton
Ex: Skull, ribs, sternum & vertebrae
Appendicular skeleton
Ex: Bones of limbs

A/c to the size & shape
(Morphological classification of bone)
1. Long bone
→Typical long bone
Ex: Humerus, radius, ulna, femur, tibia, fibula

→Miniature long bone
Ex: Metacarpals, metatarsals, Phalanges

→Modified long bone
Ex: Clavicle
2. Short bone
Eg. Carpal & tarsal
3. Flat bone
Eg. Scapula, sternum, ribs, parietal, frontal
4. Irregular bones
Eg. Vertebrae, hip bone
5. Pneumatic bones
Eg. Maxilla, sphenoid, ethmoid
6. Sesamoid bone
Eg. Patella, pisiform
Bone cells
• Osteoprogenitor cell
• Osteoblast
• Osteocyte
• Osteoclast

• Young, immature bone-forming cells
• Located at the lining of the growing surface of bone tissue side by side
• Form the organic components of bone matrix
• Increase surface area of bone
• Secrete enzyme alkaline phosphatase
Fate of osteoblast
• Some converted into osteocytes
• Some persist beneath the periosteum as undifferentiated mesenchyme

• Mature, inactive cell derived from osteoblast
• Lie within spaces called lacunae, situated between lamella of matrix
• Only one osteocyte found in each lacunae.
• Canaliculi radiating from lacunae
• Regulate phosphate metabolism
• Maintain homeostasis of blood calcium. Plays a role in the removal or deposition of calcium when required.
• Maintains bone tissue
• Responsible for hormones controlling bone growth & development

• Derived from macrophage/monocyte (which are derived from hematopoietic stem cell)
• Causes dissolution/breakdown of bone matrix
• Bone resorption, decalcification of the bone and helps to increase the Ca++ level in blood
• Secretes collagenase & other lytic enzymes to breakdown bone matrix

Periosteum is a dense tough white fibrous connective tissue membrane covers the external surface of bone expect its articular surfaces where the bone is covered by articular cartilage.
• Outer fibrous layer:
Consists of dense fibrous connective tissue with a network of blood vessels, lymphatics & nerves
• Middle vascular layer
• Inner osteogenic layer:
Consists of more loosely arranged connective tissue, collagenous fibers with connective
tissue cells (osteoprogenitor cells)

• It provides a media for the attachment of the muscles, tendons, ligaments, etc.
• Responsible for regeneration of bone (as osteoprogenitor cells present with enough blood supply)
• As it is more vascular, it forms a vascular bed from which it sends capillaries to supply the underlying bone
• Prevents overgrowth of bone
• Helps in bone formation
• Protection of bone
Nice to know:
Role of periosteum in the growth of long bone:
• Periosteum consists of a dense fibrous outer layer of collagen bundles & fibroblasts.
• The innermost cellular layer of the periosteum contains mesenchymal stem cells called osteoprogenitor cells, which divide by mitosis & differentiate into osteoblasts.
• Osteoprogenitor cells play a prominent role in bone growth & repair. So, periosteum plays a role in new bone formation.
Long Bone
3 types of long bones
• Typical long bone
• Miniature long bone
• Modified long bone
Typical long bone
Elongated shaft (called as diaphysis) with two expanded ends on either side of the shaft (called as epiphysis).
One shaft & Two epiphysis
The medullary cavity is present in the shaft which contains bone marrow.
Only one primary ossification center
Two or more than two secondary ossification centre.
Eg. Humerus, radius, ulna, femur, tibia, fibula
Miniature long bone:
Quite smaller in appearance than typical long bone
One shaft & only one epiphysis (only one expanded end/epiphysis) also called as head here.
Primary Ossification center lies on head/epiphysis/expanded end.
Only one secondary ossification center.
Eg. Metacarpals, Metatarsals, Phalanges
Modified long bone
Devoid of medullary cavity
Mostly ossify in membrane
Two primary ossification centers
Two or more than two secondary ossification center
Eg. Clavicle, body of vertebrae

Short Bone
• Cuboidal in shape & presents six surfaces.
• Out of these four surfaces are articular & the remaining two surfaces give attachment to muscles, ligaments & are pierced by blood vessels.
• Eg. Carpal & tarsal bones
Flat bone
• Consists of two outer plates of compact bone with middle sponge bone containing marrow.
• In term of skull called as
- Inner & outer tables of compact &
- Middle layer of diploe
• Intervening spongy tissue in the bones of the vault of the skull, is known as the diploe.
Eg: Most of the bones of the vault of the skull, sternum, ribs, sternum, scapula

Pneumatic bone:
• Bones containing air-filled spaces are called as pneumatic bone.
• Eg: Bone present around nasal cavity like sphenoid, ethmoid, maxilla, frontal bone of the skull
Sphenoid bone Sphenoid sinuses
Maxilla Maxillary air sinuses
Ethmoid Ethmoidal air sinuses
Frontal Frontal air sinuses
Function of pneumatic bone/ Air sinuses
• Makes the bone lighter
• Helps in the resonance of the vibration of sound.
• Acts as an air-conditioning chamber by adding humidity & temperature to the inspired air & making air suitable for the purpose of the body.
Clinical significance:
Infection from the nasal cavity extends into the air sinuses & produces ‘Colds in the head’.
Sesamoid bone
• Word sesamoid means a seed.
• These bones develop as seeds in the tendons of some muscles, when these tendons are subjected to friction during the movements of joints.
Sesamoid bones act as pulleys for muscular contraction.
Major examples:
• Patella, in Quadriceps femoris.
• Pisiform, in Flexor carpi ulnaris.
• Two bones beneath the heads of 1st metatarsal, in Flexor Hallucis Brevis
Peculiarities/Characters of sesamoid bone
• Develop in the tendon of muscles
• Ossify after birth
• Devoid of periosteum
• Absence of Haversian system
Parts of Growing long bone
1. Epiphysis:
• Expanded ends of long bone which ossify from secondary ossification center.
2. Epiphyseal plate:
• Plate of hyaline cartilage which separates the growing diaphysis from epiphysis.
3. Diaphysis:
• Elongated tubular portion between the two epiphysis which developed from the primary ossification center.
• After complete growth, it is called a shaft.
4. Metaphysis:
• Part of diaphysis immediately adjacent to the Epiphyseal plate which is the site of advancing bone growth.
Metaphysis is present between the epiphysis & diaphysis that contain the Epiphyseal plate (growth plate) which is of hyaline cartilage in a growing bone.
When the bone stops growing, this growth plate/Epiphyseal plate is replaced by osseous tissue & the Epiphyseal plate is demarcated by Epiphyseal line.
After the cessation of the growth plate, no further longitudinal growth of bone occurs.
Types of epiphysis
1. Pressure epiphysis:
• Transmits body weight
• Example
Head of femur & humerus, condyles of femur & tibia
2. Traction epiphysis:
• Provides an area for the insertion of the muscles, tendons
• Example
Tubercles of humerus, Trochanter of the femur
3. Atavistic epiphysis:
• Ossified from different ossification centers.
• Example
Coracoid process of scapula, posterior tubercle of the talus
4. Aberrant epiphysis
• Develops at the proximal end
• Eg. 1st metacarpal bone
Growing end of long bone:
• It is the cartilaginous end where the secondary center appears first & unites last with the diaphysis.
• Growing end of long bone is present in the area of epiphysis that appears 1st & fuses last.
• Relatable to law of ossification.
• It is situated against the direction of the nutrient foramen of that bone.

Simple concept:
Growing end means which keep on growing till last.
Thus, fused last with diaphysis.
Direction of nutrient foramen:
• Humerus - Down toward the elbow
• Radius & Ulna - Up toward the elbow

So, in upper extremity Growing end
• For humerus - Upper end (toward the shoulder end)
• For radius & ulna - Lower end (toward the wrist end)

Direction of nutrient foramen:
• Femur - Upward away from knee
• Tibia & Fibula - Downward away from the knee
So, in lower extremity Growing end
• Knee ends of femur, tibia & fibula i.e
(Lower end of femur) &
(Upper end of tibia & fibula)
Even though Tibia doesn't follow the law of ossification, 
but still, the upper end of the tibia is growing end.
For the direction of nutrient foramen
“To the elbow I go, from the knee I flee”
(Toward the elbow, away from knee)
Law of union of epiphysis/ Law of ossification
• The Epiphyseal center which appears first, unit last with the diaphysis & vice-versa.
• That means epiphysis that ossify first fuse with diaphysis last
& the epiphysis that ossifies last fuses first with diaphysis except fibula.
Fibula violet the law of ossification.
• Secondary ossification center appears first in the lower end,
that means epiphysis of lower end of fibula ossify first but also fused with diaphysis first.
• As the upper epiphysis of fibula fused last, the upper end of the fibula is the growing end of the fibula as usual.
• There is no violation in determining the growing end. It's same for all long bones (opposite to the direction of nutrient foramen)
In case of typical long bone
• Epiphysis develops from the secondary ossification center.
• Diaphysis (shaft) develop from the primary ossification center.
The primary ossification center is that from where, the bone starts ossified which appears before birth.
The primary ossification center is that from which, the main part of the bone is ossified & is appears before birth, usually during the 8th week of intrauterine life, which forms diaphysis (later called as the shaft of a long bone).
The secondary ossification center is that from which accessory part of a bone is ossified & is appears mostly after birth, which forms the epiphysis.
Compact & Spongy Bone:
• Compact bone
Dense area without cavities
• Spongy bone
Area with numerous interconnecting cavities
• In typical long bones, the shaft (diaphysis) is chiefly compact bone surrounding a medullary cavity.

Medullary cavity present within diaphysis (shaft) contains yellow bone marrow - (contain fat store cells).
• In typical long bones, the expanded end called epiphysis is composed of spongy bone.

Red Bone Marrow is present within the spaces of spongy bone.
Difference between compact bone & spongy bone
Compact bone Spongy bone
Compact bone has developed Haversian system. Spongy bone has no developed Haversian system.
Shows dense area generally without any cavities Shows areas with numerous interconnecting cavities.
Red Bone Marrow converted to Yellow Bone marrow present within the medullary cavity in adult age. Red Bone marrow within the numerous marrow spaces persist throughout the life.

Blood supply of long bone
• Epiphyseal vessels
Supply spongy part & red bone marrow
• Junxtra epiphyseal vessel
Supply epiphyseal end of long bone
• Periosteal vessels (vessels of periosteum)
Supply outer 1/3rd of compact bone of diaphysis
• Nutrient artery
It is tortuous before entering the bone, so that it is not damaged during muscular contraction.
Enter the medullary cavity where it divides into ascending & descending branches.
Each branch anastomoses with the Epiphyseal, metaphyseal & periosteal arteries.
- Medullary cavity
- Inner 2/3rd of compact bone of diaphysis
- Metaphysis

Haversian system / Osteon:
Histologically, the compact bone consists of numerous cylindrical units, each of which is known as Haversian system or osteon.
* Haversian system is absent in spongy bone.
Histological structure of Compact bone/ Haversian system
• Haversian canal
Long longitudinal central canal of Haversian system
Surrounded by 4-10 concentric lamellae
Contain blood vessels, nerves & loose connective tissue

• Volkmann’s canal (perforating canal)
Haversian canal is connected with the medullary cavity & with the outer surface of bone by numerous oblique channels known as Volkman's canal.
Contain small artery & nerves

• Lamellae:
Each Haversian canal is surrounded by 4-10 concentric lamellae.
Made up of bony matrix.
Composed of Type-I collagen fibers.

• Lacunae
Small spaces between the lamellae
Contain only one osteocyte.
* But lacunae of cartilage contain one or more than one chondrocytes.
• Canaliculi
Fine radiating channels
Connect the lacunae with each others & Haversian canal
Contain branches of blood vessels & cytoplasmic process of osteocyte.

Difference between periosteum & endosteum
Periosteum Endosteum
Cover the external surface of the bone except articular surface which is covered by hyaline cartilage Cover the internal surface around the medullary cavity
Consists of outer fibrous & inner osteogenic layers Consists of a single layer of flattened osteoprogenitor cells
Thicker than endosteum Thinner than periosteum

Difference between bone & cartilage
Bone Cartilage
The intercellular matrix is calcified, so it is hard. The intercellular matrix is uncalcified, so it is soft.
Highly vascular Avascular
Nerve supply present No nerve supply
Bones are arranged in a definite pattern termed as the Haversian system. No definite pattern of arrangements.
Outer covering called the periosteum. Outer covering called Perichondrium.

Bone marrow
It is a soft connective tissue that occupies the medullary cavity of long bones & all of the spaces between the trabeculae of spongy bone.
  • Red Bone marrow
  • Yellow marrow
Red Bone marrow:
• Found in fetal & young bones.
• Active in the production of blood cells.
• With increasing age, it converts into yellow marrow.
• In newborns, the entire skeleton is occupied by the red bone marrow.
• By the age of 20, red marrow is found only in diploe (middle spongy layer) of skull bones, in the ribs, sternum, vertebrae, & in the spongy part at the expanded end of long bones.

Yellow marrow:
• At the age of 20, all the marrow of long bones of the body becomes yellow except the proximal epiphysis of the femur & humerus.
• It consists mainly of adipose cells with an admixture of macrophages, undifferentiated mesenchymal cells & reticular cells.
Function of Bone Marrow
• Production of blood cells
• Destruction of RBC (act as part of the reticuloendothelial system)
• Store iron in the form of ferritin & haemosiderin
• Play a role in the inactivation of toxins or other toxic substances of the body.
• Performs immunological functions.
Bone marrow & Thymus regarded as the primary lymphoid organ. Why?
Why bone is regarded as connective tissue?
Develop from mesoderm - undifferentiated mesenchymal cells
It is composed of three elements of connective tissue
ie. Cells, Fibers & ground substances.
Intercellular matrix (fibers & ground substances) are maximum & cellular substances are minimum.

Bone is called a specialized rigid form of connective tissue.
• Other usual connective tissue has a semi-solid intercellular matrix.
• But the bone has a solid intercellular matrix.
• The intercellular matrix is calcified.
Just To Know:
Process of repair of fracture
Fracture of a bone - destroy of bone matrix & bone cells adjoining the fractures
Damaged blood vessels cause hemorrhage & form a blood clot.
Blood clot, cells & damaged bone matrix are removed by macrophages.
Proliferation of periosteum & endosteum, forming tissue around the fracture.
Primary soft bone is formed by endochondral & membranous ossification.
Further repair produces irregularly formed trabeculae of primary soft bone that temporarily unite the edges of the fractured bone, forming a hard bone callus.
The process of loss or removal of calcium from bone is called decalcification.
Pathogenesis of decalcification
• Decalcification of bone may be caused by excessive production of parathyroid hormone (hyperparathyroidism), which can cause increased osteoclastic activity,
• intense resorption of bone, elevation of blood calcium & phosphate levels
• & abnormal deposition of calcium in the arterial walls & kidneys (leading to kidney stone)