Kharkov National Medical University
BONE
Department of histology, cytology and
embryology
BONE:
as a Tissue (single)
as an Organ (many tissues)
Types of Bone Tissue
(Classification)
According to its architecture:
•Spongy (cancellous) and Compact (dense)
According to its fine structure:
•Primary (woven) and Secondary (lamellar)
According to its histogenesis:
•Intramembranous and Endochondral
1.
• Compact bone is also called dense bone. 2.
Compact bone is composed of closely packed
osteons. An osteon is also called a Haversian
system.
• Spongy bone is also referred to as cancellous
(trabecular) bone. The mineralized tissue is
seen as plates and bars. Trabeculae are the
bars seen in the spongy bone. Hollow marrow
spaces are also present.
• Immature bone is woven bone.
It is non-lamellar bone or bundle bone.
Functions of Bone:
3.
• Supports soft tissues
• Protects vital organs (cranium, thoracic
cavity)
• Contains bone marrow
• Reservoir of Ca++, PO4 to maintain
constant concentrations in body fluids
• Allows body to move
General structure
4.
• Cells:
- Osteoprogenitors (stem cells)
– Osteoblasts
– Osteocytes
– Osteoclasts
• Bone matrix:
- Organic components- osteoid (50%): type I collagen fibers
and ground substance
- Inorganic components - Calcified material, lacunae,
canaliculi, Calcium hydroxyapatite
• Periosteum and Endosteum
• Bone marrow
Main Cells:
5.
1.Osteoblasts are bone-forming cells
• Synthesize organic components of matrix
(collagen fibers type I, proteoglycans,
glycoproteins.)
• Collagen forms osteoids: strands of spiral
fibers that form matrix
• Influence deposit of Ca++, PO4.
• Active vs inactive osteoblasts
• Estrogen, PTH stimulate activity
Osteoblasts
2.Osteocytes
6.
• Mature bone cells that sit in lacunae with
filopodia in the canaliculi
• Gap junctions between osteocytes provide
nutrition (15 cells in a row)
• Maintain bony matrix; long living cells
• Stimulated by calcitonin; inhibited by
PTH
Osteocyte with cytoplasmic extensions
(filopodia)
Osteocytes with filopodia inside of the
Canaliculae of Lacunae
3.Osteoclasts
are bone-resorptive cells
7.
• Derived from monocytes; engulf bony
material
• Active osteoblasts stimulate osteoclast
activity
• Large, branched, motile, multinucliated
cells lying in Howship’s lacunae
• Secrete enzymes that digest matrix
Osteoclasts
Histology of Compact Bone
Osteon (Haversian system)
8.
• The whole complex of concentric lamellae of
the bone surrounding a canal containing blood
vessels, nerves, and loose connective tissue is
called a Haversian system, or osteon .
• Lacunae containing osteocytes are found
between , and occasionally within, the
lamellae. In each lamella, collagen fibers are
parallel to each other.
• Haversian canals communicate with marrow
cavity, periosteum, other canals through
Volkmann’s canals
Haversian system or 9.
osteon
is
a Structural
and functional unit
of the Compact Bone
OSTEONS
10.
Periosteum. The outer surface of the bones
is covered by a double-layered coat of
connective tissue. The outer fibrous layer
of periosteum is dense connective tissue.
The inner or osteogenic layer is a loose CT
containing osteoprogenitor cells.
Endosteum lines the marrow cavity. It is
condensed reticular tissue that contain bone
and blood cell precursors.
Bone Formation
11.
• Ossification - the process by which
bone is formed
• Bone formation occurs in four
situations:
–Formation of bone in an embryo
–Growth of bones until adulthood
–Remodeling of bone
–Repair of fractures
Bone Development
12.
• Bone may develop directly from
mesenchyme or by the replacement
of cartilage (indirectly).
• The process of replacing other
tissues with bone is called
ossification
OSSIFICATION
13.
begins at the 6th week of embryo
development and continues up to age of 25
Types:
Intramembranous ossification
flat bones of skull, mandible and clavicle
Endochondral ossification
most of the bones in the body formed
in the place of hyaline cartilage
Intramembranous (direct)
Ossification
• Intramembranous ossification -bone develops
from mesenchyme.
– Mesenchymal cells start to secrete the
organic components (primary ossification
center). Differentiate into osteoblasts.
Further differentiation into osteocytes.
– Spicule formation
– Formation of spongy and compact bone.
Intramembranous Ossification
• Condensation of mesenchyme into trabeculae
• Osteoblasts on trabeculae lay down osteoid
tissue (uncalcified bone)
• Calcium phosphate is deposited in the matrix
forming bony trabeculae of spongy bone
• Osteoclasts create marrow cavity
• Osteoblasts form compact bone at the surface
• Surface mesenchyme produces periosteum
Remodeling is secondary bone
formation
Osteoclasts erode the primary bone
matrix, blood vessels, nerves and
lymphatics invade the cavity and
osteogenic cells develop in osteoblasts and
osteocytes, which create concentric
lamellae and osteons.
Remodeling helps reshape growing bones
to adapt to changing loads.
INTRAMEMBRANOUS OSSIFICATION
14.
1.Mesenchyme cells differentiate into osteoblasts. Osteogenic
islands.
2.Osteoblasts secrete osteoid matrix. Osteoblasts surround
themselves with bone matrix, forming osteocytes. It is osteoid
stage.
3.Osteoid becomes mineralized through crystallization of
Ca++ salts using enzyme alkaline phosphatase and is called
primary ossification center(OC). It is ossification stage and
formation of spicules.
4.Blood vessels begin to grow spicules that meet and fuse
together.
Woven Bone (primary spongy bone).
5. Osteoclasts erode the primary bone matrix. It is remodeling
or Secondary bone formation.
Endochondral (indirect)
Ossification 1
• Bone develops from pre-existing model
– perichondrium and hyaline cartilage
• Formation of primary ossification center (OC)
and marrow cavity in shaft of model
–
–
–
–
bony collar developed by osteoblasts
chondrocytes swell and die
stem cells give rise to osteoblasts and clasts
bone laid down and marrow cavity created
Endochondral Ossification 2
• Secondary ossification centers and marrow
cavities are formed in the ends of the bone
– same process
• Cartilage remains as articular cartilage and
epiphyseal (growth) plates
– growth plates provide for increase in length of
bone during childhood and adolescence
– by early twenties, growth plates are gone and
primary and secondary marrow cavities united
Endochondral bone formation 15.
1.Cartilage model
2.The periosteal bone collar (perichondral
ossification)
3.Proliferation,hypertrophy,calcification of the
cartilage. Formation of primary marrow cavity
and Periosteal bud- small cluster of blood vessels
4. Primary ossification center
5.Secondary ossification center
7.Secondary bone formation and remodeling
8.Bone growth in length and girth
Structure of the epiphyseal plate:
16.
1. Zone of reserve cells (resting cartilage): A thin layer (3-6
cells wide) of small, randomly oriented chondrocytes adjacent
to the bony trabeculae on the articular side of the growth plate.
2. Zone of proliferation: Chrondrocytes are stacked in
prominent rows, mitotic figures and the cartilage matrix
becomes more basophilic 3. Zone of maturation: No mitoses;
gradual cellular enlargement. 4. Zone of hypertrophy:
Chrondrocytes and their lacunae increase in size.
5. Zone of calcification: Deposition of minerals in the matrix
surrounding the enlarged lacunae causing cell death.
6. Zone of ossification: Osteoblasts deposit bone matrix on
the exposed plates of calcified cartilage. 7. Zone of
resorption: Osteoclasts absorb the oldest bone spicules.
Bone Growth and Remodeling
• Bones increase in length
– interstitial growth of epiphyseal plate
– epiphyseal line is left behind when cartilage gone
• Bones increase in width = appositional growth
– osteoblasts lay down matrix in layers on outer surface
and osteoclasts dissolve bone on inner surface
• Bones remodeled throughout life
– Wolff’s law of bone = architecture of bone determined
by mechanical stresses
• action of osteoblasts and osteoclasts
– greater density and mass of bone in athletes or manual
worker is an adaptation to stress
BONE GROWTH in LENGTH
• on epiphyseal side chondrocytes continually
reproduce by epiphyseal plate
• on diaphyseal side chondrocytes swell and become
surrounded by calcified matrix, chondrocytes die,
bone replaces chondrocytes
bone is eroded away by osteoclasts, marrow cavity
forms
• epiphyseal plate remains constant until 18 years in
females,20 years in males
• epiphyseal plate becomes ossified after 25 years
• It is stop growth of the bone.
BONE GROWTH in girth
(DIAMETER)
Compact bone grows in thickness by
proliferation and differentiation of
osteoprogenitor cells in the inner layer of the
periosteum and due to deposition of new
ossified tissue on the outer surface of the bone.
Marrow cavity increases in diameter
Balance between osteoblasts and osteoclasts
depends from Hormonal regulation.