1. BONE HISTOLOGY

2. OBJECTIVES Identify main functions of bone tissue
Identify the major parts of a long bones
Describe the cells found in bone tissue
Describe the components of an osteon
Compare and contrast intramembranous and endochondral ossification.

3. BONE FUNCTION 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

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

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)
Medullary cavity: Hollow chamber in bone
- red marrow produces blood cells
- yellow marrow is adipose
Endosteum: thin layer lining the medullary cavity

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

7. COMPACT BONE: (OSTEON) EXTERNAL LAYER
- called lamellar bone (groups of
elongated tubules called lamella)
- majority of all long bones
- protection and strength
(wt. bearing)
- concentric ring structure
- blood vessels and nerves penetrate
periosteum through horizontal
openings called perforating
(Volkmann’s) canals.

8. COMPACT BONE: (OSTEON) EXTERNAL LAYER
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 lacunea are channels called canaliculi (finger like
processes of osteocytes)

9. COMPACT BONE Lacunae are connected to one another by canaliculi
Osteon contains: - central canal
- surrounding lamellae
- lacunae
- osteocytes
- canaliculi

10. 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

11. HISTOLOGY OF BONE Histology of bone tissue
Cells are surrounded by matrix.
- 25% water
- 25% protein
- 50% mineral salts
4 cell types make up osseous tissue
Osteoprogenitor cells
Osteoblasts
Osteocytes
Osteoclasts

12. Osteoprogenitor cells:
- derived from mesenchyme
- all connective tissue is derived
- unspecialized stem cells
- undergo mitosis and develop
into osteoblasts
- found on inner surface of
periosteum and endosteum.

13. Osteoblasts:
- bone forming cells
- found on surface of bone (arrow)
- no ability to mitotically divide
- collagen secretors
Osteocytes:
- mature bone cells
- derived form osteoblasts
- do not secrete matrix material
- cellular duties include exchange of nutrients and waste with blood.

14. Osteoclasts
- bone resorbing cells
- bone surface
- growth, maintenance and bone repair
Abundant inorganic mineral salts:
Tricalcium phosphate in crystalline
form called hydroxyapatite
Ca3(PO4)2(OH)2
Calcium Carbonate: CaCO3
Magnesium Hydroxide: Mg(OH)2
Fluoride and Sulfate

15. 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

16. Bone Formation Formation of Bone in an Embryo
cartilage formation and ossification occurs during the sixth week of embryonic development
two patterns
Intramembranous ossification
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

17. Intramembranous Ossification
An ossification center appears in the fibrous connective tissue membrane
Osteoblasts secrete bone matrix within the fibrous membrane
Osteoblasts mature into osteocytes

18. Endochondral Ossification
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.
Articular
cartilage
Enlarging
chondrocytes within
calcifying matrix
Epiphysis
Epiphyseal
cartilage
Marrow
cavity
Diaphysis
Marrow
cavity
Primary
ossification
center
Secondary
ossification
center
Blood
vessel
Bone
formation
Blood
vessel
Cartilage
model
Replacement of hyaline cartilage with bone
Most bones are formed this way (i.e. long bones).

19. Longitudinal Bone Growth
Longitudinal Growth (interstitial) – cartilage continually grows and is replaced by bone
Bones lengthen entirely by growth of the epiphyseal plates
Cartilage is replaced with bone as quickly as it grows
Epiphyseal plate maintains constant thickness

20. GROWTH IN LENGTH
The growth in length of long bones involves two major events:
Growth of cartilage on the epiphyseal plate
Replacement of cartilage by bone tissue in the epiphyseal plate
- osteoblasts located beneath the
periosteum secrete bone matrix and
build bone on the surface
- osteoclasts located in the endosteum
resorbs (breakdown) bone.

21. GROWTH OF CARTILAGE ON THE EPIPHYSEAL PLATE
- epiphyseal plate (bone length)
- 4 zones of bone growth under hGH.
1- Zone of resting cartilage (quiescent):
- no bone growth
- located near the epiphyseal plate
- scattered chondrocytes
- anchors plate to bone
2- Zone of proliferating cartilage (prolferation zone)
- chondrocytes stacked like coins
- chondrocytes divide

22. - large chondrocytes arranged in columns
3- Zone of hypertrophic (maturing) cartilage
- large chondrocytes arranged in columns
- lengthwise expansion of epiphyseal plate
4- Zone of calcified cartilage
- few cell layers thick
- occupied by osteoblasts and osteoclasts
and capillaries from the diaphysis
- cells lay down bone
- dead chondrocytes surrounded by a calcified
matrix.
Matrix resembles long spicules of calcified
cartilage.
Spicules are partly eroded by osteoclasts and then
covered in bone matrix from osteoblasts: spongy
bone is formed.

23. APPOSITIONAL BONE GROWTH
Growing bones widen as they lengthen
Appositional growth – growth of a bone by addition of bone tissue to its surface
Bone is resorbed at endosteal surface and added at periosteal surface
Osteoblasts – add bone tissue to the external surface of the diaphysis
Osteoclasts – remove bone from the internal surface of the diaphysis
Figure 6-6

24. Thank you