Bone tissue is the major structural and supportive connective tissue of the body. Osseous tissue forms the rigid part of the bones that make up the skeletal system.

1. HUMAN BONE
TISSUE

2. CONSTITUTION OF BONE TISSUE
- Extracellular Bone Matrix;
- Bone Cells.
Functions:
1. Support;
2. Protection;
3. Movement;
4. Storage.

3. BONE MATRIX
• Organic Part :
- Collagen;
- Proteoglycans;
- Glycoproteins.
• Inorganic Part:
Calcium
phosphate crystals
called
hydroxyapatite:

4. BONE MATRIX
• The collagen and mineral components: Responsible for the
major functional characteristics of bone.
Without
mineral
Without
collagen
Flexibility Fragile

5. BONE CELLS
• Produce the bone matrix, become entrapped within it, and break it down
so that new matrix can replace the old matrix.
Bone cells are categorized as:
 Osteoblasts; Osteocytes; Osteoclasts.

6. OSTEOBLASTS
• Extensive endoplasmic reticulum and numerous ribosomes;

7. OSTEOBLASTS
• Produce collagen and proteoglycans, which are packaged into vesicles
by the Golgi apparatus and released from the cell by exocytosis;
• Form vesicles that accumulate calcium ions (Ca2+), phosphate ions
(PO2
4), and various enzymes used to form hydroxyapatite crystals.

8. OSTEOCYTES
• They produce components needed to maintain the bone matrix;
• Lacunae: Spaces occupied
by the osteocyte cell
bodies;
• Canaliculi: Spaces occupied
by the osteocyte cell
processes;

9. OSTEOCYTES
• Bone differs from cartilage in that the processes of bone cells are in
contact with one another through the canaliculi

10. OSTEOCLASTS
• Large cells with several nuclei;
• Responsible for the resorption, or breakdown of bone;
• Ruffled border – Projections where the plasma membrane of
osteoclasts contacts bone matrix.

11. OSTEOCLASTS
• Hydrogen ions are pumped across the ruffled border and produce an
acid environment : Decalcification of the mineralized bone matrix;
By endocytosis
some of the
breakdown
products are
taken into the
osteoclast.
BONE REABSORPTION

12. PERIOSTEUM E ENDOSTEUM
• Layers of osteogenic cells and conjuntive tissue that covers the
internal and external surfaces of the bones.
• Outer layer: periosteum;
-Collagen fibers – Sharpey's
fibers penetrate the bone
and the periosteum hold
firmly to the bone;
- Fibroblasts.

13. PERIOSTEUM E ENDOSTEUM
• Inner layer:
Endosteum
• Osteogenic flattened
cells : Cover the
trabecular bone
cavity , the medullar
channel, and
Volkmann channel.
The aim of both layers is to promote the nutrition of bone tissue
and provide new osteoblasts for bone growth and fracture repair.

14. TYPES OF BONE TISSUE
• Compact bone
- No visible cavities
• Cancellous Bone
- Full of interconnecting channels

15. TYPES OF BONE TISSUE
•Short bones (tarsals in the foot,
e.g.):
-Cancellous bone in the center;
-Compact bone recoating all its
peripheral surface.
•Flat bones (cranium,e.g.):
-Two thin layers of compact bone;
-Enclosing between them: variable
quantity of cancellous bone.
•Interstices of the cancellous bone
and the medullary cavity in the
diaphysis of the long bones are
occupied by bone marrow.

16. TYPES OF BONE TISSUE
• Primary Bone
(Temporary):
- First bone tissue that
appears in
embryonic
development and in
fraction repair.
- Collagen fibers are
arranged in irregular
arrays.

17. TYPES OF BONE TISSUE
• Lamellar Bone:
-Mature bone with
collagen fibers that
are arranged in
lamellae.
-Fibers very well
organized around a
vascular canal:
Haversian canal.

18. OSTEOGENESIS
• Intramembranous ossification:
-In which osteoblasts differentiate directly from mesenchyme and begin
secreting osteoid;
-Most flat bones are produced and is so called because it takes place
within condensations of embryonic mesenchymal tissue.
Groups of
mesenchymal cells in
a "membrane" or
sheet of this
embryonic tissue,
round up and
differentiate as
osteoblasts producing
osteoid.

19. INTRAMEMBRANOUS OSSIFICATION:
Cells trapped in the
calcifying matrix
differentiate as
osteocytes.
Woven bone is produced in
this manner, with
vascularized internal
spaces that will form the
marrow cavity and
surrounded on both sides
by developing periosteum.

20. INTRAMEMBRANOUS OSSIFICATION:
Remodeling of the
woven bone produces
the
two layers of compact
lamellar bone with
cancellous bone in
between, which is
characteristic of flat
bones.
• The starting point for bone formation is
called a Primary ossification center.
• The ossification centers of a bone grow
radially and finally fuse together, replacing
the original connective tissue.

21. OSTEOGENESIS
• Endochondral ossification:
- Cartilage is present during the process;
- It is the process responsible for the formation of short and long bones.

22. ENDOCHONDRAL OSSIFICATION (LONG BONE FORMATION)
Primary Center of
ossification (in the
middle of the diaphysis) :
1. Pericondrium becomes
periosteum;
2. Formation of bone
collar by
intramembranous
ossification of the
pericondrium;
3. Calcification of the
matrix (Hypertrophy,
apoptosis of the
chondrocytes and
mineralization of the
matrix);

23. ENDOCHONDRAL OSSIFICATION (LONG BONE FORMATION)
4.
The blood vessels
from the
periosteum which
carry
osteoprogenitor
cells:
Invade the cavity
left by the
chondrocytes and
the cells proliferate
and differentiate
into osteoblasts;

24. ENDOCHONDRAL OSSIFICATION (LONG BONE FORMATION)
5. Formation of the
trabeculae (Primary
bone):
The osteoblasts secrete
osteoid which will
mineralize and form
trabecula.
Osteoclasts break down
spongy bone to form
the medullary cavity .

25. ENDOCHONDRAL OSSIFICATION (LONG BONE FORMATION)
• Secondary Center of
Ossification :
- Cartilage Tissue is
reduced to the Articular
Cartilage and the
Epiphyseal plate zone;
- Epiphyseal plate:
between the epiphysis and
the diaphysis;
- In adults who stop
growing the plate is
replaced by an epiphyseal
line.

26. ENDOCHONDRAL OSSIFICATION (LONG BONE FORMATION)
In the epiphyseal plate, five layers are distinguished:
Normal resting hyaline
cartilage
Chondrocytes undergo rapid
mitosis
Chondrocytes stop mitosis
and begin to hypertrophy
Chondrocytes are either
dying or dead
Osteoprogenitor cells invade the
area and differentiate into
osteoblasts

27. BONE REMODELING
When bone becomes old and it’s replaced by new bone
Osteoclasts: remove old bone
Osteoblasts: deposit new bone

28. BONE REMODELING
It is also
responsible
for the
formation of
new
osteons in
compact
bone

29. BONE REMODELING – NEW OSTEONS
Within already
existing osteons:
- Osteoclasts enter
a central canal
through the blood
vessels, remove
bone from the
center of the
osteon;
- New concentric
lamellae are formed
around the vessels.

30. BONE REMODELING – NEW OSTEONS
Osteoclasts in the periosteum:
- Remove bone: groove formation along the surface;
- Periosteal capillaries lie within these grooves and become surrounded to
form a tunnel as the osteoblasts of the periosteum form new bone;
- Additional lamellae then are added to the inside of the tunnel until an osteon
results.

31. BONE REMODELING
Bone is constantly being removed by osteoclasts, and new bone
is being formed by osteoblasts:
• The relative
thickness of
compact bone is
maintained;
• It leaves behind
portions of older
bone - interstitial
lamellae.

32. BONE REMODELING
It can modify the strength of the bone in response to the amount of stress
applied to it.
Mechanical stress
applied to bone
increases osteoblast
activity
• Applying weight to a
broken bone speeds
the healing process.
Removal of mechanical
stress decreases
osteoblast activity

33. BONE REPAIR
Bone can undergo repair following damage to it in four major steps
1. Hematoma formation:
- The blood vessels and surrounding periosteum are damaged: a
hematoma forms;
- The blood in a hematoma forms a clot .

34. BONE REPAIR
2. Callus formation:
- Callus : mass of tissue that forms at a fracture site and connects
the broken ends of the bone.

35. BONE REPAIR
2. Callus formation:
- Internal callus: fibers and cartilage;
- External callus: Bone–cartilage collar that stabilizes the ends of the
broken bone .

36. BONE REPAIR
3. Callus ossification:
- Cartilage in the external callus and fibers and cartilage of the
internal callus: replaced by woven, cancellous bone

37. BONE REPAIR
4. Remodeling of bone
- The woven bone of the internal callus and the dead bone adjacent to
the fracture site are replaced by compact bone;
- The internal callus is remodeled and the external callus is reduced in
size by osteoclast activity.

38. CALCIUM HOMEOSTASIS
Calcium moves into bone as osteoblasts build new bone
and out of bone as osteoclasts break down bone
Blood calcium
levels too low
Osteoclast
activity
increases
More calcium is
released by
osteoclasts from
bone into the
blood
Blood calcium
levels increase
Blood calcium
levels too high
Osteoclast
activity
decreases
Less calcium is
released by
osteoclasts from
bone into the
blood
Blood calcium
levels decrease

39. CALCIUM HOMEOSTASIS
Parathyroid hormone (PTH) from the parathyroid glands
is the major regulator of blood calcium levels.

40. CALCIUM HOMEOSTASIS
• PTH binds to its receptors on osteoblasts/stromal cells and these cells
produce a receptor for activation of nuclear factor kappa B ligand (RANKL)-
stimulating osteoclasts molecule

41. CALCIUM HOMEOSTASIS
• Increased PTH inhibits the release from osteoblasts/stromal cells of a
protein called osteoprotegerin (OPG) that inhibits the formation of
osteoclasts because it binds to RANKL
↑ PTH
↑ RANKL
↓ OPG
↑ Osteoclasts
↑ Calcium
in the blood

42. CALCIUM HOMEOSTASIS
• Calcitonin plays a minor role in calcium maintenance by inhibiting
osteoclast activity

43. 2. In the kidneys,
PTH increases
calcium
reabsorption from
the urine.
. And promotes the
formation of active
vitamin D, which
increases calcium
absorption from the
small intestine.

44. BONE DISORDERS
Osteopetrosis Defect in the formation of osteoclasts. The bones harden, becoming denser.
Rickets
Softening of bones in children due to deficiency or impaired metabolism of
Vitamin D.
Osteomalacia
Softening of the bones caused by defective bone mineralization secondary to
inadequate amounts of available phosphorus and calcium.
Osteoporosis
Bones become fragile and more likely to fracture. The bone loses density, which
measures the amount of calcium and minerals in the bone.
Cretinism
Stunted physical and mental growth due to untreated congenital deficiency of
thyroid hormones .
Giantism
Abnormally increased height because of excessive cartilage and bone formation
at the epiphyseal plates of long bones.
Acromegaly
Excess pituitary growth hormone secretion; it involves growth of connective
tissue after the epiphyseal plates have ossified.
Dwarfism A person is abnormally short, is the opposite of giantism

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