Bone Histology

1. Bone Histology

2. Types of Bone Cells
1) Osteocyte: Mature bone cell
2) Osteoblast: Bone-forming cells
3) Osteocast: Bone-destroying cells
Remember, bone is a living tissue!

3. Microscopic Structure of a Compact Bone
Osteon System: A central (Haversian)
canal with concentric rings (lamellae) of
bone matrix running lengthwise. Very
strong!

4. • Lacunae: tiny cavities inside the lamellae
rings. This is where the osteocytes are
found.

5. • The central canal carries blood vessels
and nerves to all areas of the bone.

6. So how do all bone cells get
nourishment and contact the body
outside the bone?
Canaliculi (kan” ah-lik’-u-li): tiny canals that
radiate outward from the central canals to
each lacunae space.
Volkmann’s Canals: canals that run at right
angles to the central canals and perforate
the shaft of the bone.

8. Spongy bone (cancellous bone)
Trabeculae: Loosely organized lamellae
rings with osteocytes (no central canal).
Canaliculi connect the osteocytes.
filled with red and yellow bone marrow
- osteocytes get nutrients directly from
circulating blood.
Works like struts along lines of stress in
bone to offer strength, yet lightweight.
Grrrr!

9. Long 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

10. Diaphysis

11. Epiphysis

12. Epiphyseal (growth) plate

13. Medullary cavity

14. • 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

15. • 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.

16. • Osteoblasts:
- bone forming cells
- found on surface of bone
- 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.

17. • 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

18. • Bone formation (ossification) occurs in two ways
1- Intramembranous ossification
2- Endochondral ossification
Both methods above lead to the same bone
formation but are different methods of getting
there.
Ossification (osteogenesis) begins around the 6th -
7th week of embryonic life. At this time the
embryonic skeleton is made of fibrous
membranes and hyaline cartilage.

19. Hyaline Cartilage Review
a. Most abundant
b. Provides support, flexibility and
resilience
c. Located:
a. forming nearly all the fetal skeleton
b. articular cartilage: ends of moving
bones
c. costal cartilage: ribs to sternum
d. tip of nose
e. respiratory cartilage

21. • Skeletal Cartilage:
1. Chondrocytes: cartilage producing
cells.
2. Lacunae: small cavities where the
chondrocytes are encased.
3. Extracellular matrix: jellylike ground
substance.
4. Perichondrium: layer of dense
irregular connective tissue that
surrounds the cartilage.
5. No blood vessels or nerves

23. • Intramembranous (within the membrane)
ossification: Bone develops from a fibrous
membrane.
- flat bones of skull
- mandible
- clavicles
-mesenchymal cells become vascularized and
become osteoprogenitor cells and then
osteoblasts.
- organic matrix of bone is secreted
- osteocytes are formed
- calcium and mineral salts are deposited and
bone tissue hardens.
- trabeculae develop and spongy bone is formed
- red marrow fills spaces

24. Replacement of hyaline cartilage with bone is called
. Endochondral (intracartilaginous) ossification
Most bones are formed this way (i.e. long bones).
Where bone is going to form:
1- mesenchymal cells differentiate into
chondroblasts (immature cartilage cells) which
produces hyaline cartilage.
Perichondrium develop around new cartilage
2- Chondrocytes (mature) mitotically divide
increasing in length
This pattern of growth: interstitial growth.
- growth from within

25. Growth of cartilage in thickness occurs from
the deposition of new matrix to the
periphery formed by chondroblasts within
the perichondrium. Appositional growth.
Chondrocytes undergo hypertrophy, swell
and burst. pH of the matrix changes and
calcification is triggered. Ultimately,
cartilage cells die. Lacunae are now
empty.

26. Nutrients are supplied by way of the nutrient
artery passing through the perichondrium
through the nutrient foramen.
Osteoprogenitor cells are stimulated in the
perichondrium to produce osteoblasts.
A thin layer of compact bone is laid down
under the perichondrium called the
periosteal bone collar.
Perichondrium becomes periosteum

27. Osteoblasts begin to deposit bone matrix
forming spongy bone trabeculae.
In the middle of the bone, osteoclasts break
down spongy bone trabeculae and form a
hollowed out cavity called the medullary
cavity. This cavity will be filled with red
bone marrow for hemopoiesis.
The shaft of the bone is replaced (was
hyaline cartilage) with compact bone.

31. • Physiology of bone growth:
- epiphyseal plate (bone length)
- 4 zones of bone growth under hGH.
1- Zone of resting cartilage:
- no bone growth
- located near the epiphyseal plate
- scattered chondrocytes
- anchors plate to bone
2- Zone of proliferating cartilage
- chondrocytes stacked like coins
- chondrocytes divide

32. 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.

33. 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.

35. Age 18-21: Longitudinal bone growth ends when
epiphysis fuses with the diaphysis.
- epiphyseal plate closure
- epiphyseal line is remnant of this
- last bone to stop growing: clavicle

36. • Bone width: increase in diameter of bone
occurs through appositional growth .
- Osteoblasts located beneath the
periosteum secrete bone matrix and build
bone on the surface
- Osteoclasts located in the endosteum
resorbs (breakdown) bone.

37. • Bone Remodeling:
- bone continually renews itself
- never metabolically at rest
- enables Ca to be pulled from bone when
blood levels are low
- osteoclasts are responsible for matrix
destruction
- produce lysosomal enzymes and acids
- spongy bone replaced every 3-4 years
- compact bone every 10 years

38. - Blood calcium levels signal release of either
parathyroid hormone (PTH, secreted by
parathyroid gland) and calcitonin (secreted
by thyroid).
PTH causes calcium release from bone matrix
by stimulating osteoclast activity and bone
resorption.
Calcitonin inhibits bone resorption and causes
calcium salts to be deposited in bone matrix.
Vitamins A, C, D and B12 help in bone remodeling

39. • Fractures: Any bone break.
- blood clot will form around break
- fracture hematoma
- inflammatory process begins
- blood capillaries grow into clot
- phagocytes and osteoclasts remove
damaged tissue
- procallus forms and is invaded by
osteoprogenitor cells and fibroblasts
- collagen and fibrocartilage turns
procallus to fibrocartilagenous (soft) callus

40. - broken ends of bone are bridged by callus
- Osteoprogenitor cells are replaced by
osteoblasts and spongy bone is formed
- bony (hard) callus is formed
- callus is resorbed by osteoclasts and compact
bone replaces spongy bone.
Remodeling : the shaft is reconstructed to
resemble original unbroken bone.
Closed reduction - bone ends coaxed back into
place by manipulation
Open reduction - surgery, bone ends secured
together with pins or wires