Bones have several key functions: support, protection, movement, mineral storage, and blood cell formation. There are four main types of bones based on shape: long, short, flat, and irregular. Bone anatomy includes the diaphysis, epiphysis, growth plate, periosteum, marrow cavity, and endosteum. Bones are supplied by blood vessels that enter through the periosteum and Volkmann's canals. There are two types of bone tissue: compact bone made up of concentric osteons, and spongy/cancellous bone made of trabeculae within the marrow cavity. Bone is remodeled throughout life by osteoclasts that resorb bone and osteoblasts that

1. BONE
bone tissue (osseous tissue)

2. FUNCTIONS OF BONE
• Support
• Protection (protect internal organs)
• Movement (provide leverage system for skeletal muscles,
tendons, ligaments and joints)
• Mineral homeostasis (bones act as reserves of minerals
important for the body like calcium or phosphorus)
• Hematopoiesis: blood cell formation
• Storage of adipose tissue: yellow marrow

3. SHAPE OF BONES
• Long bones (e.g., humerus,
femur)
• Short bones (e.g., carpals,
tarsals)
• Flat bones (e.g., parietal bone,
scapula, sternum)
• Irregular bones (e.g.,
vertebrae, hip bones)
• Sesamoid bone (patella)
• Pneumatized bones (ethmoid)

5. BONE ANATOMY
Diaphysis: long shaft of bone
Epiphysis: ends of bone
Epiphyseal plate: growth plate
Metaphysis: b/w epiphysis and diaphysis
Articular cartilage: covers epiphysis
Periosteum: bone covering (pain sensitive)
Sharpey’s fibers: periosteum attaches to
underlying bone
Medullary cavity: Hollow chamber in bone
- red marrow produces blood cells
- yellow marrow is adipose
Endosteum: thin layer lining the medullary
cavity

6. BLOOD AND NERVE SUPPLY OF BONE
• Bone is supplied with blood by:
• Periosteal arteries accompanied by
nerves supply the periosteum and
compact bone
• Epiphyseal veins carry blood away
from long bones
• Nerves accompany the blood
vessels that supply bones
• The periosteum is rich in sensory
nerves sensitive to tearing or tension

7. Periosteum & Endosteum
• Bone covered by dense CT called periosteum.
• Cover whole bone except articular surface.
• Consist of 2 layers: fibrous (outer) & cellular (inner)
• Cavities within bone covered by endosteum (thin layer CT)
• E.g. medullary & trabecular cavities.

8. TYPES OF BONES
Compact Bone – dense
outer layer
Spongy Bone – (cancellous
bone) honeycomb of
trabeculae (needle-like or
flat pieces) filled with bone
marrow

9. Microscopic structures
2 types of bone:
1. Primary bone:
also called immature or woven bone.
• First bone tissue appear in embryonic stage & during bone repair.
• Contain collagen type I fibre, osteocyte & less mineral content.
• Replaced by secondary bone.

10. 2. Secondary bone:
also called mature or lamellar bone.
• lamellar bone (groups of elongated tubules called lamella)
• majority of all long bones
• protection and strength (wt. bearing)
• concentric ring structure.
• Adult skeleton consist of this tissue.
• Characterized by lamellar arrangement of collagen fibre., osteocytes
(in lacunae).
• Lacunae are connected by canaliculi.

11. COMPACT BONE: (OSTEON)
Bulk of compact bone
composed of cylindrical
subunits called haversian
system or osteon.
Lamellae constituting the
osteon called haversian
lamellae, surrounding haversian
canal.
Osteon surrounded by thin
layer of mineralized bone
matrix.

12. - blood vessels and nerves penetrate periosteum through horizontal
openings called perforating (Volkmann’s) canals.
• Central (Haversian) canals run longitudinally. Blood vessels and
nerves.
- around canals are concentric lamella
- osteocytes occupy lacunae which are between the lamella
- radiating from the lacunae are channels called canaliculi (finger like
processes of osteocytes)
- Lined by osteoprogenitor cells, contain one or two BV, nerve fibre, loose
CT.

14. - Irregular interval b/w osteon are occupied by interstitial lamellae.
- Outer circumferential lamellae (beneath periosteum, parallel to outer surface
of bone).
- Inner circumferential lamellae (beneath endosteum, surrounding marrow
cavity).
- Lacunae are connected to one another by canaliculi
- Osteon contains: - central canal
- surrounding lamellae
- lacunae
- osteocytes
- canaliculi

16. SPONGY BONE (CANCELLOUS BONE):
INTERNAL LAYER
- Trabecular bone tissue (haphazard arrangement).
- Filled with red and yellow bone marrow
- Osteocytes get nutrients directly from circulating blood.
- Short, flat and irregular bone is made up of mostly spongy bone

17. HISTOLOGY OF BONE
• Histology of bone tissue
Cells are surrounded by matrix.
4 cell types make up osseous tissue
• Osteoprogenitor cells
• Osteoblasts
• Osteocytes
• Osteoclasts

18. Matrix
Organic component:
• 35% of dry weight.
• Consist of:
• More than 90% of Collagen type I fibres.
• Proteoglycans ( chondroitin sulfate & Keratan sulfate)
• Glycoproteins ( Osteonectin, osteocalcin & osteopontine)

19. Abundant inorganic mineral salts:
- 65% of dry weight.
- Tricalcium phosphate in crystalline form called hydroxyapatite
Ca3(PO4)2(OH)2
- Calcium Carbonate: CaCO3
- Magnesium Hydroxide: Mg(OH)2
- Fluoride and Sulfate

20. • Osteoprogenitor cells:
- Derived from mesenchyme
- Lines haversian & volkman canal
- Unspecialized stem cells
- Spindle shaped, ovoid nucleus,
Scanty cytoplasm.
- Undergo mitosis and develop
Into osteoblasts
- Found on inner surface of
periosteum and endosteum.
Cells of Bone Tissue

21. • Osteoblasts:
- Bone forming cells
- Found on surface of bone (arrow)
- No ability to mitotically divide
- Collagen secretors
- Secrete organic component
of matrix.
- Secrete unmineralized matrix called osteoid
- Secrete enzyme alkaline phosphatase
- Cuboidal or low columnar shaped
- Posses PTH receptors, at activation they secrete cytokine which
stimulate osteoclast activity

22. • Osteocytes:
- Mature bone cells
- Derived from osteoblasts
- Do not secrete matrix material
- Do not perform mitosis
- Flat almond shaped cells
- Cellular duties include exchange of nutrients and waste with blood.
-Maintain bone matrix & blood calcium levels.
- Death of cells result in resorption of bone matrix
- es PTH stimulate to resorb Ca+2 from matrix.

23. • Osteoclasts
- Bone resorbing cells.
- Present at bone surface.
- Growth, maintenance and bone repair.
- Imp role in remodelling & renewal of bone.
-On surface cells are located in shallow
Depression called resorption bays (Howship’s
Lacunae)
- large, multinucleated, motile cells
- lower part is called Ruffled border b/c finger like process due to folds of
plasmalemma.

24. • Resorption has 2 steps:
1. Dissolution of inorganic component.
2. Digestion of organic component.

25. BONE FORMATION
• The process of bone formation is called ossification
• Bone formation occurs in four situations:
1) Formation of bone in an embryo
2) Growth of bones until adulthood
3) Remodeling of bone
4) Repair of fractures

26. • Formation of Bone in an Embryo
• cartilage formation and ossification occurs during
the eighth week of embryonic development
• two patterns
• Intramembranous ossification
• The replacement of membranous sheet of
mesenchyme.
• Flat bones of the skull and mandible are
formed in this way
• “Soft spots” that help the fetal skull pass
through the birth canal later become
ossified forming the skull
• Endochondral ossification
• The replacement of cartilage by bone
• Most bones of the body are formed in this
way including long bones

27. Stages of Intramembranous Ossification
• Results in the formation of cranial bones of the skull
(frontal, perietal, occipital, and temporal bones) and the clavicles.
• All bones formed this way are flat bones
• An ossification center appears in the fibrous CT membrane
• Bone matrix is secreted within the fibrous membrane
• Woven bone and periosteum form
• Bone collar of compact bone forms, and red marrow appears

28. Intramembranous ossification
(osteoid is the organic part)

29. Mesenchymal
cell
Collagen
fiber
Ossification
center
Osteoid
Osteoblast
Ossification centers appear in the fibrous
connective tissue membrane.
• Selected centrally located mesenchymal cells cluster
and differentiate into osteoblasts, forming an
ossification center.
1

30. Osteoid
Osteocyte
Newly calcified
bone matrix
Osteoblast
B one matrix (osteoid) is secreted within the
fibrous membrane and calcifies.
• Osteoblasts begin to secrete osteoid, which is calcified
within a few days.
• Trapped osteoblasts become osteocytes.
2

31. Mesenchyme
condensing
to form the
periosteum
Blood vessel
Trabeculae of
woven bone
Woven bone and periosteum form.
• Accumulating osteoid is laid down between embryonic
blood vessels in a random manner. The result is a network
(instead of lamellae) of trabeculae called woven bone.
• Vascularized mesenchyme condenses on the external face
of the woven bone and becomes the periosteum.
3

32. Fibrous
periosteum
Osteoblast
Plate of
compact bone
Diploë (spongy
bone) cavities
contain red
marrow
Llamellar bone replaces woven bone, just deep to
the periosteum. Red marrow appears.
• Trabeculae just deep to the periosteum thicken, and are later
replaced with mature lamellar bone, forming compact bone
plates.
• Spongy bone (diploë), consisting of distinct trabeculae, per-
sists internally and its vascular tissue becomes red marrow.
4

33. Endochondral ossification
• Modeled in hyaline cartilage, called cartilage model
• Also called intracartilagenous ossification.
• Gradually replaced by bone: begins late in second month of development
• Perichondrium is invaded by vessels and becomes periosteum
• Osteoblasts in periosteum lay down collar of bone around diaphysis
• Calcification in center of diaphysis
• Primary ossification centers
• Secondary ossification in epiphyses
• Epiphyseal growth plates close at end of adolescence
• Diaphysis and epiphysis fuse
• No more bone lengthening

34. Enlarging
chondrocytes within
calcifying matrix
Chondrocytes at the
center of the
growing cartilage
model enlarge and
then die as the
matrix calicifies.
Newly derived
osteoblasts cover the
shaft of the cartilage
in a thin layer of
bone.
Blood vessels penetrate
the cartilage. New
osteoblasts form a
primary ossification
center.
The bone of the
shaft thickens, and
the cartilage near
each epiphysis is
replaced by shafts of
bone.
Blood vessels invade the
epiphyses and osteo-blasts
form secondary centers of
ossification.
Cartilage
model
Bone
formation
Epiphysis
Diaphysis Marrow
cavity
Primary
ossification
center
Blood
vessel
Marrow
cavity
Blood
vessel
Secondary
ossification
center
Epiphyseal
cartilage
Articular
cartilage
Endochondral Ossification

35. BONE REMODELING
- Bone continually renews itself
- Never metabolically at rest
- Enables ca+2 to be pulled from bone when blood levels are low
- Osteoclasts are responsible for matrix destruction
- Produce lysosomal enzymes and acids
• Lysosomal enzymes (digest organic matrix)
• Acids (convert calcium salts into soluble forms)
- Spongy bone replaced every 3-4 years
- Compact bone every 10 years

36. Thank you