Bone tissue is a type of specialized connective tissue composed of cells and an extracellular matrix. The matrix is made up of collagen fibers and hydroxyapatite crystals that give bone its rigidity. There are three main cell types involved in bone tissue: osteoblasts which form new bone, osteocytes embedded in the matrix, and osteoclasts which resorb bone. Bone has two types of internal structures - compact bone which is dense and cancellous bone which is spongy. Bone tissue is continuously remodeled through the actions of osteoblasts and osteoclasts throughout life.

1. Bone tissue
"Mohmmadrjab" S. Seder
Faculty of Medicine
Histology

2. Table of contents
Introduction
Bone Cells
Bone Matrix
Periosteum and Endoesteum
Types of Bone
Osteogenesis
Bone Remodeling and Repair
Metabolic Role of Bone
Joints

3. Introduction
 Bone tissue also called
(osseous tissue) is a
type of specialized dense
connective tissue.
 Bone tissue is made up
of different types of bone
cells.

4.  Bone tissue has a honeycomb-like matrix internally,
which helps to give the bone rigidity.

5.  As the main constituent of the adult skeleton, Bone
tissues have many Functions:
 Movement.
 Provides solid support for the body.
 Protects vital organs such as those in the cranial and
thoracic cavities.
 Encloses internal (medullary) cavities containing bone
marrow where blood cells are formed.
 Serves as a reservoir of calcium, phosphate, and other
ions.
 Endocrine regulation.
 Storage of minerals and fat, and hematopoiesis.

7.  Normal bone is composed of 67% mineral, 33%
organic matrix, 5 to 10% water, and <3% lipids.
Calcium and phosphorus are chief minerals found in
the bone along with small amount of carbonate,
magnesium.

8. Classification of Bones by shapes

9.  Bone is a specialized connective tissue composed of
calcified extracellular material:
 The bone matrix
 Three major cell
types:
 Osteocytes (Oc)
 Osteoblasts (Ob)
 Osteoclasts (Ocl)
M: mesenchymal regions

10.  Osteocytes, which are found in cavities (lacunae)
between bone matrix layers (lamellae), with
cytoplasmic processes in small canaliculi (L. canalis,
canal) that extend into the matrix.
 Osteoblasts, growing cells which synthesize and
secrete the organic components of the matrix.
 Osteoclasts, which are giant, multinucleated cells
involved in removing calcified bone matrix and
remodeling bone tissue

11.  Osteogenic cells are undifferentiated stem cells.
They are the only bone cells that can divide. When
they do, they differentiate and develop into
osteoblasts.

12.  Bone lacuna is a small cavity within the bone
matrix, containing an osteocyte.

13.  Osteons are cylindrical structures that contain a
mineral matrix and living osteocytes connected by
canaliculi, which transport blood. They are aligned
parallel to the long axis of the bone. Each osteon
consists of lamellae, which are layers of compact
matrix that surround a central canal called the
Haversian canal.

16. Number 1 indicates a recently formed osteon.
Number 2 indicates a somewhat older osteon.
Number 3 indicates interstitial lamellae, representing the remnants of still older osteons.

17.  All bones are lined on their internal and external
surfaces by layers of connective tissue containing
osteogenic cells; endosteum on the internal surface
surrounding the marrow cavity and periosteum on
the external surface.

21.  Because of its hardness, bone cannot be sectioned
routinely. Bone matrix is usually softened by
immersion in a decalcifying solution before paraffin
embedding, or embedded in plastic after fixation
and sectioned with a specialized microtome.

22.  Bone marrow is a (spongy) soft connective tissue
that is found inside a cavity, called the marrow cavity.
 There are two types of marrow in adults, yellow
bone marrow, and red bone marrow.
 Yellow bone marrow consists mostly of fat. while red
bone marrow contains hematopoietic stem cells
that give rise to red blood cells, white blood cells, and
platelets in the process of hematopoiesis.

24.  All marrow is red in newborns, but by adulthood,
much of the red marrow has changed to yellow
marrow.
 In adults, red marrow is found mainly in the femur,
ribs, vertebrae, and pelvic bones.

27. Bone cells
 Osteoblast: a mononucleate cell from which
bone develops.
 Osteocyte: a mature bone cell involved with the
maintenance of bone.
 Osteoclast: a large multinuclear cell associated
with the resorption of bone.

30. Osteoblasts
 Osteoblast: a cell which secretes the substance of
bone.
 Originating from mesenchymal stem cells.
 Osteoblasts produce the organic components of
bone matrix, including type I collagen fibers,
proteoglycans, and matricellular glycoproteins such
as osteonectin.
 Active osteoblasts are located exclusively at the
surfaces of bone matrix.

32.  Matrix components are secreted at the cell surface
in contact with existing bone matrix, producing a
layer of unique collagen-rich material called osteoid
between the osteoblast layer and the preexisting
bone surface.

33.  Osteoid is the unmineralized, organic portion of the
bone matrix that forms prior to the maturation of
bone tissue. Osteoblasts begin the process of
forming bone tissue by secreting the osteoid as
several specific proteins.

34.  Bone mineralization is a well regulated process in
which crystals of calcium phosphate are produced
by bone-forming cells [osteoblasts] and are laid
down in precise amounts within the fibrous matrix.
 The process of matrix mineralization is not
completely understood, but basic aspects of the
process are shown in the Figure in the next slide.

36. Osteocytes
 Some osteoblasts become surrounded by the
material they secrete and then differentiate as
osteocytes enclosed singly within the lacunae
spaced throughout the mineralized matrix.
An osteocyte

37. C: Canaliculi
Osteocytes in lacunae

39. Osteoclasts
 Osteoclasts are very large, motile cells with
multiple nuclei that are essential for matrix
resorption during bone growth and remodeling.
 Osteoclasts are multinucleated due to their
origin from the fusion of bone marrow-derived
monocytes.

42. Osteoclast

45. Bone Matrix
 The bone matrix (also known as osteoid) is that
part of the bone tissue and forms most of the mass
of the bone.
 The bone matrix consists of about 33% organic
matter (mostly Type I collagen) and 67% inorganic
matter (calcium phosphate, mostly hydroxyapatite
crystals).

47.  The organic matter embedded in the calcified matrix
is 90% type I collagen, but also includes mostly
small proteoglycans and multiadhesive
glycoproteins such as osteonectin.

48. Periosteum and
Endoesteum
 External and internal
surfaces of all bones
are covered by CT of
the periosteum and
endosteum.

49. Periosteum
 Is a layer of dense CT on
the outer surface of bone,
bound to bone matrix by
bundles of type I collagen
called perforating (or
Sharpey) fibers.

50. Periosteum

51.  The periosteum’s inner layer is more cellular and
includes osteoblasts, bone lining cells, and
mesenchymal stem cells referred to as
osteoprogenitor cells. With the potential to
proliferate extensively and produce many new
osteoblasts, osteoprogenitor cells play a
prominent role in bone growth and repair.

52. Endosteum
 Endosteum is a thin layer of active and inactive
osteoblasts, which lines all the internal surfaces
within bone;
 Endosteum covers small trabeculae of bony
matrix that project into the marrow cavities.

54. Endosteum

58. Characteristic ENDOSTEUM PERIOSTEUM
Location Medullary canal, spongy bone,
Volkmann’s and Haversian canals of
all bones
Outer bone surfaces, except articular
surfaces; not in sesamoid bones
(patella)
Structure A single cellular layer, loose
connective tissue
Two layers: fibrous and cellular layer
(cambium), dense irregular connective
tissue
Thickness ~10 micrometers (0.01 mm) 13 0.1-0-5 mm
Function Bone growth, remodeling, repair Bone growth, remodeling, repair, bone
sensitivity, nourishment
Difference Between Endosteum and Periosteum

59. Types of Bone
 Gross observation of a bone shows 2 types of
bone:
 Compact (cortical / hard) bone: a dense area
near the surface, which represents 80% of the total
bone mass.
 Cancellous (spongy / trabecular) bone: deeper
areas with numerous interconnecting cavities,
constituting about 20% of total bone mass.

61.  In long bones, the bulbous
ends—called epiphyses are
composed of cancellous
bone covered by a thin layer
of compact cortical bone.
 In long bones,the cylindrical
part—the diaphysis is
almost totally dense
compact bone, with a thin
region of cancellous bone on
the inner surface around the
central marrow cavity.

62.  Short bones such as those of the wrist and ankle
usually have cores of cancellous bone surrounded
completely by compact bone.

63.  The flat bones that form the calvaria (skullcap)
have two layers of compact bone called plates,
separated by a thicker layer of cancellous bone
called the diploë.

65.  At the microscopic level both compact and
cancellous bones typically show two types of
organization:
 Mature lamellar bone, with matrix existing as
discrete sheets.
 Woven bone, newly formed with randomly
arranged components.

67. Summary of bone types and their organization

68. Lamellar Bone
 Lamellar bone represents the main type of bone
in a mature skeleton.
 Lamellar bone is characterized by the organized
arrangement of collagen fibers into layers or
lamellae.
 This arrangement gives lamellar bone greater
stiffness when compared to the disorganized
nature of woven bone.

69.  An osteon (or Haversian system) refers to the
complex of concentric lamellae, typically 100-250
μm in diameter, surrounding a central canal that
contains small blood vessels, nerves, and
endosteum.

70.  Each osteon is a long, sometimes bifurcated,
cylinder generally parallel to the long axis of the
diaphysis.
 Each osteon has 5-20 concentric lamellae around
the central canal that communicates with the
marrow cavity and the periosteum.
 Canals also communicate with one another
through transverse perforating canals (or
Volkmann canals) that have few, if any,
concentric lamellae.

71. L: Lamellae
C: Canaliculi
CC: Central canal
I: Interstitial lamellae
O: Osteocytes
An osteon

73. P: Perforating
(Volkmann) canals

74. Two photographs of the same area of an unstained section of compact bone, showing osteons
with concentric lamellae around central canals. Lamellae are seen only faintly by brightfield
microscopy (a), but they appear as alternating bright and dark bands under the polarizing light
microscope (b). Bright bands are due to birefringence from the highly ordered collagen fibers
in a lamella. Alternating bright and dark bands indicate that fibers in successive lamellae have
different orientations, an organization that makes lamellar bone very strong.

75.  Scattered among the intact osteons are numerous
irregularly shaped groups of parallel lamellae called
interstitial lamellae. These structures are lamellae
remaining from osteons partially destroyed by
osteoclasts during growth and remodeling of bone.

76.  Compact bone (e.g, in the diaphysis of long bones)
also includes parallel lamellae organized as multiple
external circumferential lamellae immediately
beneath the periosteum and fewer inner
circumferential lamellae around the marrow cavity.
 The lamellae of these outer and innermost areas of
compact bone enclose and strengthen the middle
region containing vascularized osteons.

80. Woven Bone
 Woven bone is nonlamellar and characterized by
bone tissue with a disorganized collagen I fibers
arrangement.
 It primarily develops embryonically and is
gradually replaced between three and four years
of age by lamellar bone.
 Woven bone is not frequently found in the adult
skeleton, except in pathological conditions (such
as Paget’s disease and osteosarcoma, ...)

81.  Woven bone typically has a lower mineral content
(it is more easily penetrated by x-rays) and a higher
proportion of osteocytes than mature lamellar bone.
 Immature woven bone forms more quickly but has
less strength than lamellar bone.

85. Osteogenesis
 Bone ossification, or osteogenesis, is the process
of bone formation.
 This process begins between the 6th and 7thweeks of
embryonic development and continues until about
age 25.
 Osteogenesis occurs by one of two processes:
 Intramembranous ossification
 Endochondral ossification

86.  Each of these processes(Intramembranous,
Endochondral ossification) begins with a
mesenchymal tissue precursor, but how it
transforms into bone differs.
 The names refer to the mechanisms by which the
bone forms initially; in both processes woven bone
is produced first and is soon replaced by stronger
lamellar bone.
 During growth of all bones, areas of woven bone,
areas of bone resorption, and areas of lamellar
bone all exist contiguous to one another.

87. Intramembranous vs. Endochondral ossification
 Intramembranous ossification directly converts the
mesenchymal tissue to bone and forms the flat
bones of the skull, clavicle, and most of the cranial
bones.
 Endochondral ossification begins with
mesenchymal tissue transforming into a cartilage
intermediate, which is later replaced by bone and
forms the remainder of the axial skeleton and the
long bones.

88. Intramembranous ossification
 Intramembranous ossification is the process of
bone development from fibrous membranes.
 It is involved in the formation of the flat bones of
the skull, the mandible, and the clavicles.
 Ossification begins as mesenchymal cells form a
template of the future bone.

89. (a) Mesenchymal cells group into clusters, and
ossification centers form.
(b) Secreted osteoid traps osteoblasts, which then
become osteocytes.
(c) Trabecular matrix and periosteum form.
(d) Compact bone develops superficial to the
trabecular bone, and crowded blood vessels
condense into red marrow.
Intramembranous ossification steps

92. Intramembranous ossification

93.  Five steps can summarize intramembranous
ossification:
 Mesenchymal cells differentiate into osteoblasts and
group into ossification centers.
 Osteoblasts become entrapped by the osteoid they
secrete, transforming them to osteocytes.
 Trabecular bone and periosteum form.
 Cortical bone forms superficially to the trabecular
bone.
 Blood vessels form the red marrow.

94. Endochondral ossification
 Endochondral ossification takes place within
hyaline cartilage shaped as a small version, or
model, of the bone to be formed.
 This process involves the replacement of hyaline
cartilage with bone.
 Forms most bones of the body.

95.  Five steps can summarize endochondral
ossification:
 Mesenchymal cells differentiate into chondrocytes and form
the cartilage model for bone.
 Chondrocytes near the center of the cartilage model undergo
hypertrophy and alter the contents of the matrix they secrete,
enabling mineralization.
 Chondrocytes undergo apoptosis due to decreased nutrient
availability; blood vessels invade and bring osteogenic cells.
 Primary ossification center forms in the diaphyseal region of
the periosteum called the periosteal collar.
 Secondary ossification centers develop in the epiphyseal region
after birth.

97. The epiphyseal
(growth) plate

98. The epiphyseal
(growth) plate

99.  The physeal growth plate is separated into various
sections based on pathologic characteristics:
 Reserve Zone
 Proliferative Zone
 Hypertrophic Zone
 Primary Spongiosa (zone of calcified cartilage)
 Secondary Spongiosa (zone of ossification)

100.  Reserve Zone
 Storage site for lipids, glycogen, proteoglycan
 Proliferative Zone
 Proliferating chondrocytes leading to longitudinal growth
 Hypertrophic Zone
 Site of chondrocyte maturation
 Within the hypertrophic zone, the chondrocytes go through a
transformation process. The chondrocyte mature and prepare
a matrix for calcification; then they degenerate which allows
calcium release for calcification of the matrix

101.  Primary Spongiosa (zone of calcified cartilage)
 Site for mineralization to form woven bone
 Vascular invasion occurs
 Secondary Spongiosa (zone of ossification)
 Internal modeling with the replacement of fiber bone with
lamellar bone
 External modeling with funnelization

102. Epiphyseal growth plate: Locations and zones of activity.

103. Growth disorders

104. Appositional growth
 Appositional growth is the increase in the
diameter of bones by the addition of bony tissue
at the surface of bones. Osteoblasts at the bone
surface secrete bone matrix, and osteoclasts on
the inner surface break down bone.

105. Bone Remodeling and Repair
 Is a lifelong process where mature
bone tissue is removed from the
skeleton (bone resorption) and new
bone tissue is formed (ossification
or new bone formation).
 These processes also control the
reshaping or replacement of bone
following injuries like fractures.

106. Main features of bone fracture repair

107. Metabolic Role of Bone
 Calcium ions are required for the activity of
many enzymes and many proteins mediating cell
adhesion, cytoskeletal movements, exocytosis,
membrane permeability, and other cellular
functions.

108.  The skeleton serves 99% of the
body’s total calcium.

109.  The conc. of calcium in the blood (9-10 mg/dL)
 The principal mechanism for raising blood calcium
levels is the mobilization of ions from hydroxyapatite
to interstitial fluid, primarily in cancellous bone.
 Ca+2 mobilization is regulated mainly by paracrine
interactions among bone cells.
 Two polypeptide hormones target bone cells to
influence calcium homeostasis: parathyroid
hormone (PTH), calcitonin.

110. Hypocalcemia
 Hypocalcemia, commonly known as calcium
deficiency disease, occurs when calcium levels in
the blood are low. A long-term deficiency can lead
to dental changes, cataracts, alterations in the
brain, and osteoporosis, which causes the bones
to become brittle.

111. Osteoporosis
 A medical condition in
which the bones become
brittle and fragile from
loss of tissue, typically as
a result of hormonal
changes, or deficiency of
calcium or vitamin D.

114. Calcium-Rich Foods
 Oranges
 Milk
 Cheese
 Fish
 Kiwifruit
 Yoghurt
 Kale
....

115. Joints
 Joints are regions where
bones meet, capped and held
together firmly by other
connective tissues, allowing at
least the potential for bending
or movement in that portion of
the skeleton.
 The type of joint determines
the degree of movement
between the bones.

116. Anatomy of Joints
 A joint is an articulation between two bones in the
body and are broadly classified by the tissue which
connects the bones.
 The three main types of joints are:
 Synovial
 Cartilaginous
 Fibrous
 Fibrous joints have fibrous tissue joining the bone
and these joints are typically very strong.

117. Classifications of Joints
2 Methods of Classification:
Functional Classification
*Focuses on the amount of movement allowed.
Structural Classification
*Focuses on the material that binds joints together

118. Classifications of Joints
Fibrous
Fixed
(Synarthrosis)
A. Synostosis /
Sutures
B. Gomphosis
C. Syndesmoses
D. Symphyses
Cartilaginous
Slightly movable
(Amphiarthrosis)
A. Pri. Cart. Joints
Synchondrosis
B. Sec. Cart. Joints
Symphysis
Synovial
freely movable
(Diarthrosis)
1. Plane
2. Hinge
3. Pivot
4. Bicondylar
5. Ellipsoid
6. Saddle
7. Ball and socket

119. Types of Joints

120. Synarthrosis
(Sutures, Gomphosis, Symphyses and Syndesmoses)
Synarthrosis (Fibrous joints) formed by Dense
connective tissue

122. Amphiarthrosis (Cartilaginous)
Are united by Hyaline cartilage or fibrocartilage
 Synchondrosis (Primary Cartilaginous Joints)
 Temporary synchondrosis
 Permanent synchondrosis
 Symphysis (Secondary Cartilaginous Joints)
 Intervartebral symphysis
 Manubriosternal symphysis
 Pubic symphysis

125. Diarthroses (synovial joints)

126. Synovial membrane

127. Articular cartilage

128. THANK YOU
THE END