Written by :
RAYAN S. ALBALLAA
Histology team, Group A
Bone & Bone Formation
Edited by:
Albara Marwa
Histology team, Group A
Components of Bones :
1. Bone Cells.
2. Calcified Matrix.
3. Periosteum (the outer covering).
4. Endosteum (the inner layer facing
the Marrow Cavity).
Types of Bone Cells :
1. Osteoprogenitor cells (Forming)
2. Osteoblasts cells (Immature cells)
3. Osteocytes (Mature cells)
4. Osteoclasts
Osteoprogenitor Cells
 Arise from MESENCHYMAL STEM
CELL.
 Differentiate into OSTEOBLASTS.
 Found in PERIOSTEUM &
ENDOSTEUM.
Osteoblasts :
 Derived from OSTEOPROGENITOR cells
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and can divide.
Have CYTOPLASMIC PROCESSES which
are extensions of the cytoplasm.
Basophilic cells on the surface of the bone
(in PERIOSTEUM & ENDOSTEUM).
Protein secreting cells.
Secrete the ORGANIC PART OF THE
BONE MATRIX.
Osteocytes :
 After the osteoblasts secrete the bone matrix, it
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becomes osteocyte in a small space called the
LACUNA.
Osteocytes are mature bone cells with flattened
nucleus and cytoplasmic processes.
Can not divide.
Maintain Matrix.
The lacunae (the plural of lacuna) are connected
together by small canals called CANALICULI.
Canaliculi contain the cytoplasmic processes of
the osteocytes.
GAP JUNCTIONS connect the processes of the
osteocytes in the canaliculi.
Osteoclasts :
 Multinucleated, motile, and acidophilic
cells in the ENDOSTEUM.
 Originate by FUSION OF CELLS in the
bone marrow.
 Secrete Enzymes that digest and remove
bone matrix forming cavities and canals
(to maintain the Ca++ level in blood).
 Have cytoplasmic processes called
RUFFLED BORDER.
Bone Matrix :

Bone matrix consists of two
components:
1. organic components :
 Type 1 collagen.
 Chondroitin sulfate.
2. inorganic components :
 Calcium & phosphorus forming
HYDROXYAPATITE CRYSTALS.
Bone Matrix :
 In H&E section, the decalcified bone
matrix is acidophilic.
 It shows the collagen type 1 and the bone
cells.
 The bundles of collagen in the matrix form
parallel layers called bone LAMELLAE.
Periosteum :
 It's the outer covering of a bone.
 It consists of two layers :
 1/ outer fibrous layer of dense connective
tissue attached to a bone by collagen fibers.
 2/ inner cellular layer (Osteogenic layer) of
osteoblasts and osteoprogenitor cells.
 Function : bone formation and repair.
Endosteum :
 it's a layer of cells on the the internal
surface of bone facing the marrow
cavity.
 The cells are the osteoprogenitor
cells, osteoblasts, and osteoclasts.
 Function : Bone formation and repair.
Types of Bones :
Bones exist in two forms :
 Compact bone : forms the outer part
of all bones in the body.
 Cancellous (spongy) bone: forms the
inner part of all bones and is more in
the epiphysis (the ends of a long bone)
than in the diaphysis (the shaft of a
long bone).
Compact Bone
 The matrix of a compact bone consists
of REGULAR lamellae (layers) of
calcified type 1 collagen.
 The lamellae form parallel cylinders
called OSTEONS or HAVERSION
SYSTEMS.
 Osteons are found deeply in the
compact bone.
Compact Bone
 The CONCENTRIC lamellae forming the
osteons are called OSTEONAL
LAMELLAE.
 Under the periosteum and endosteum,
the lamellae do not form osteons and
are called CIRCUMFERENTIAL
LAMELLAE.
Canals in a Compact Bone
 HARVERSIAN canal in the centre of each
osteon contains osteoblasts, osteoclasts,
and blood vessels.
 VOLKMAN'S canals contain blood vessels
and connect the harversion canals of
adjacent osteons.
 CANALICULI connect the lacunae with
Haversion canals for nutrition of the
osteocytes.
Cancellous (Spongy) Bones
 The lamellae of spongy bone do not
form osteons.
 The lamellae form
INTERCONNECTED TRABECULAE.
 (Small pieces of bone).
Cancellous (Spongy) Bones
 The lamellae in each Trabecula are
parallel to each other.
 The Trabeculae are separated by
bone marrow spaces lined by
endosteum.
 In Trabeculae, the canaliculi connect
lacunae to bone marrow for nutrition
of osteocytes.
OSSIFICATION =
OSTEOGENESIS =
BONE FORMATION
Bone Is Formed by Two
Methods :
1. INTRAMEMBRANOUS
OSSIFICATION.
2. ENDOCHONDRAL OSSIFICATION
(INTRACARTILAGENOUS
OSSIFICATION).
INTERMEMBRANOUS
OSSIFICATION :
 In this type of ossification, bone is formed
directly in a membrane of MESENCHYMAL cells
without the formation of cartilage. (Ex: Flat bones
of the skull)
 MESENCHYMAL cells differentiate into
OSTEOPROGENITOR cells and OSTEOBLASTS
which secrete bone matrix and form the
PERIOSTEUM.
 Calcium salts are deposited in the matrix to form
bone.
 OSTEOCLASTS remove part of the bone to form
MARROW SPACES (Ex: Frontal bone, Maxilla)
ENDOCHONDRAL
OSSIFICATION :
In this type of ossification, Hyaline
Cartilage is formed first and then
replaced by bone (Ex: Long bones)
Steps of Endochondral
Ossification
1.
2.
3.
4.
5.
6.
MESENCHYMAL cells first form Hyaline
cartilage.
Blood vessels enter perichondrium which
becomes periosteum and secretes BONE
COLLAR on surface of cartilage.
Cartilage in centre degenerates leaving Cavities.
Blood vessels and Osteoblasts from periosteum
enter the cavities to form THE PRIMARY
OSSIFICATION CENTRE IN THE DIAPHYSIS.
OSTEOBLASTS secrete bone matrix.
OSTEOCLASTS in the ossification centre
remove part of the new bone to form the bone
marrow cavity.
Epiphyseal Growth Plate of
Cartilage
 After ossification, a piece of cartilage
called EPIPHYSEAL GROWTH PLATE
remains between the epiphysis and
diaphysis.
 Ossification of the growth plate continues
up to the age of 20 years.
 The growth plate increases the length of
bone because its cartilage continues to
grow.
Ossification of Epiphyseal
Plate
Zones of ossification of epiphyseal plate :
1. Zone of cartilage reserve (resting).
2. Zone of proliferation of chondrocytes.
3. Zone of hypertrophy of chondrocytes.
4. Zone of calcification of cartilage.
5. Zone of ossification (formation of bone on
the calcified cartilage matrix).
6. Zone of resorption by osteoclasts.