This document discusses the histology of bone. It describes bone as a specialized connective tissue composed of cells, an organic matrix, and inorganic minerals. The main cell types are osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts. Bone has two types of tissue - compact bone which is dense, and cancellous or spongy bone found deeper within bones. Microscopically, bone is made up of concentric rings called osteons surrounding central canals, with osteocytes housed in lacunae between lamellae. This detailed structure allows for nutrients and waste exchange to support the living cells.

1. Histology of bone
Dr. P. Christabel Divya

2. Introduction
• Solid support for the body
• Protects vital organs such as those in
the cranial and thoracic cavities
• Harbors cavities containing bone
marrow where blood cells are
formed
• Reservoir of calcium, phosphorous
and other ions that can be released
or stored in a controlled fashion to
maintain constant concentration in
body fluids

3. Constituents
• Specialized connective tissue
• Cells
• Osteoprogenitor cells
• Osteoblasts
• Osteocytes
• Osteoclasts
• Calcified extracellular matrix
• Organic matter
• Fibres – Type 1 collagen
• Ground substances – Proteoglycans (aggregcan)
and Glycoproteins (Osteonectin) and water
• Inorganic material – 50% of dry weight
• Calcium hydroxyapatite
• Other ions - Bicarbonate, citrate, magnesium,
potassium, sodium ions

4. Bone membranes
• Bones are lined on both internal and
external surfaces by layer of connective
tissue containing osteoprogenitor cells
Endosteum – On the internal surface
surrounding the marrow cavity
Periosteum – On the external surface

5. Bone Cells
• Osteoprogenitor cells
• Osteoblasts
• Osteocytes
• Osteoclasts

6. Osteoprogenitor cells
• Stem cells of mesenchymal origin
• Resembles fibroblast in
appearance
• Located over the bone surfaces
(on both the periosteal and
endosteal aspects)
• Can proliferate and convert
themselves into osteoblasts

7. Osteoblast
• Bone forming cell
• Cuboidal to columnar shape with basophilic cytoplasm
• Located at the surfaces of bone matrix, usually side by
side in a layer
• Synthesize and secrete the organic components of
bone matrix, including type I collagen fibres,
proteoglycans and glycoproteins (osteonectin)

8. Osteoblast
contd
• Secrete matrix components at the cell surface in contact
with the existing bone matrix, producing a layer of new
material called osteoid (but not yet calcified) between
osteoblast layer and pre existing bone surface
• Completed by subsequent deposition of calcium salts into
the newly formed matrix

9. Osteocytes
• Cells of mature bone
• Lie in the lacunae of bone that lies in between the
successive lamellae and are regularly spaced through out the
mineralized matrix
• Osteoblasts that have become imprisoned in the matrix
during bone formation and differentiate further as osteocyte

10. Osteocytes contd
• In the transistion from osteoblast to
osteocyte, cells extend many long
dendritic process arising from osteocytes
• Establishes contacts with other
osteocytes and with bone lining cells
present on the surface of bone
• Maintain bone matrix and their death is
followed by matrix resorption
• Additional role in calcium homeostasis

11. Osteoclasts
• Bone removing cells
• Very large motile cells with multiple
nuclei, the surface of the cell is folded
into irregular projections (Ruffled
border)
• Found in relation to surface where bone
removal is taking place, lies within
enzymatically etched depression or
cavities in the matrix (Resorption bays or
Howship’s lacunae)
• Removal of bone by osteoclast involves
demineralization and removal of matrix

12. Periosteum
• External surface of any bone
• Consists of two layers
Outer
Inner
• Outer layer – Fibrous layer
• Inner layer – Cellular layer.
Contains osteoprogenitor cells
(Osteogenic layer)
• Richly supplied with blood.

13. Functions of
periosteum
Provides a medium through which
muscles, tendons and ligaments are
attached to bone. Fibers of the tendon
continue into the outer layers of bone
as perforating fibers of sharpey
Performs a nutritive function because
of the blood vessels supplying it

14. Functions of
periosteum contd

15. Types of bone
• Based on the gross appearance
Compact bone (cortical)
Cancellous bone (spongy)
• Compact – Dense area near the
surface (80% of mass)
• Cancellous – Deeper areas with
numerous interconnecting cavities
(20% of mass)

16. • In long bones
Epiphysis – spongy bone are
covered by a thin layer of compact
bone
Diaphysis – Composed of contact
bone with a thin region of spongy
bone on the inner surface around
the central marrow cavity
• Short bones
Cores of spongy bone surrounded
by compact bone
• Flat bone
Have two layers of compact bone
called plates, separated by a thick
layer of spongy bone called diploe

17. Types of
bone contd
• Based on the microscopic examination
Lamellar bone – Mature
Woven bone - Immature

18. Lamellar bone
• Made up of layers or lamellae
• Lamellus – thin plate of bone
consisting of collagen fibers and
mineral salts that are deposited
in a gelatinous ground
substance
• Mainly depends on the
arrangement of collagen fibers
• Fibers of one lamellus run
parallel to each other but those
of adjoining lamellae runs at
perpendicular direction

19. Woven bone
• Newly formed bone does not have a lamellar
structure
• Collagen fibers present in bundles that appear to
run randomly in different directions interlacing with
each other
• Later replaced by lamellar bone

20. Micro structure of bone
• Lamellae are organized around a central canal
(Haversian canal) and parallel to each other
• Osteon (Haversian system) –
Complex of concentric lamellae surrounding
a small central canal that contains nerves,
blood vessels and loose connective tissue
Between successive lamellae are lacunae,
each with one osteocyte, interconnected by
canaliculi containing the cells dendritic
process
Receives nutrients and oxygen from the
microvasculature in the central canal
Outer boundary of each osteon is cement
line, which is a more collagen rich layer

21. Micro structure of bone
• Osteons arranged as long cylinder parallel to the long axis
of the diaphysis
• Central canal surrounded by 4-10 concentric lamellae
• Central canal communicate with marrow cavity &
periosteum and with one another through perforating
channels (or Volkmann canals)
• Scattered among the intact osteon, numerous irregular
shaped groups of parallel lamellae called interstitial
lamellae
• Lamellae remaining from osteons partially destroyed by
osteoclast during growth and remodeling of bone

22. Micro structure
of bone
• External circumferential
lamellae – located
immediately beneath the
periosteum
• Inner circumferential
lamellae – Located around
the marrow cavity

23. Compact bone TS
• Ring like osteons (or haversian
systems)
• At the center of each osteon, there is
haversian canal
• Around the canal, there are concentric
lamellae of bone amongst which there
are small spaces called lacunae
(containing osteocytes)
• Delicate canaliculi radiate from the
lacunae; these contain cytoplasmic
processes of osteocytes

24. Compact bone TS
• Interstitial lamellae fill intervals
between haversian systems
• Near the surface of compact bone,
the lamellae are arranged in a
parallel manner. These are
circumferential lamellae
• Volkmann’s canal interconnecting
the adjacent Haversian canal may
be seen

25. Compact bone
LS
• Volkmann’s canal connecting Haversian
canal is seen
• Osteocytes within lacunae is seen

26. The end