Histology of bone

2. Introduction
 Bone tissue along with other
CT like cartilage, fibrous tissue,
fat, blood vessels, nerves, and
hematopoietic elements form
the individual bones.
 Bones are the organs of the
skeletal system; bone tissue
is the structural component
of bones.

3. Introduction
 Bone tissue is a specialized form of connective tissue
characterized by a mineralized extracellular matrix.
 Mineral = calcium phosphate in the form of hydroxyapatite
crystals [Ca10(PO4)6(OH)2]
- Calcium Carbonate: CaCO3
- Magnesium Hydroxide: Mg(OH)2
- Fluoride and Sulfate
 Matrix = mainly collagen (type I, VI) along with other
matrix proteins
 All collagen molecules ~ 90% of total weight of bone matrix

4. Function
1) storage for elements and minerals - homeostatic
regulation of blood calcium levels
2) mechanical structures for movement and protection
of viscera,
3) a home for hematopoietic tissue, and
4) Storage of adipose tissue: yellow marrow

5. BONE TISSUE
Bone tissue is classified
 compact bone
 dense layer forms the outside of the bone
 spongy bone (cancellous bone)
 spongelike meshwork consisting of
trabeculae
 The spaces within the meshwork are
continuous and occupied by marrow and
blood vessels.

6. Classified according to shape
 the location of spongy and compact
bone varies with bone shape.
 Long bones - longer in one dimension
 e.g., humerus, femur
 Short bones - nearly equal in length and
diameter
 Have compact, spongy bone and a
marrow space on the inside
 Articular surfaces are covered with
hyaline cartilage
 e.g., carpals, tarsals, patella

7.  Flat bones - thin and platelike
 e.g., parietal bone, scapula,
sternum
 Thick compact bone with an
intervening layer of spongy bone.
 Irregular bones
 e.g., vertebrae, hip bones,
ethmoid bone

8. GENERAL STRUCTURE OF BONES
Periosteum
 Is an outer fibrous sheath of
dense regular connective
tissue covering of the bone
except articular surface.
 Two layers
 outer fibrous layer
 an inner cellular
(osteoprogenic) layer
 is well defined If active bone
formation is in progress

9. Periosteum…
 The relatively few periosteal cells are
capable of undergoing division and
becoming osteoblasts under appropriate
stimulus.
 Sharpey’s fibers
 Collagen fibers from ligaments and
tendons extend directly into the bone
tissue, where they are continuous with the
collagen fibers of the extracellular matrix of
the bone tissue.

10. Bone Cavities
 The marrow cavity and the spaces
in spongy bone contain bone
marrow
 Red bone marrow
 normally restricted to the spaces
of spongy bone in the adult
 yellow marrow – consists mostly
of fat cells
 can revert to red marrow e.g.
extreme blood loss

11. Endosteum
 Is often only one cell thick a layer
of connective tissue cells lining
Bone cavities
 consists of osteoprogenitor cells
that can differentiate into
osteoblasts, and bone-lining cells –
endosteal cells layer

12. BLOOD AND NERVE SUPPLY OF BONE
 Bone is supplied with blood by
 Periosteal arteries
 Nutrient artery (diaphysis and
epiphysis)
 Metaphyseal arteries
 Nerves accompany the blood
vessels that supply bones
 The periosteum is rich in sensory
nerves sensitive to tearing or
tension
 Bone tissue lacks lymphatic vessels;
lymphatic drainage occurs only from
the periosteum.

13. Mature Bone/lamellar bone
 composed of cylindrical units called osteons or Haversian systems
 osteons consist of concentric lamellae of bone matrix surrounding
a central canal, the osteonal (Haversian) canal, which contains the
vascular and nerve supply of the osteon.

14.  Within the bone matrix are spaces called
lacunae, each containing osteocyte.
 The osteocyte extends numerous processes
into small tunnels called canaliculi.
 communicate by gap junctions with other
osteocytes
 canaliculis generally arranged in a radial
pattern with respect to the Haversian canal
 serves for the passage of
substances between the osteocytes and blood
vessels

16.  Between the osteons are remnants of
previous concentric lamellae called
interstitial lamellae
 Circumferential lamellae follow the
entire inner and outer circumferences
of the shaft of a long bone
 Perforating canals (Volkmann’s
canals)
 blood vessels and nerves travel from
the periosteal and endosteal surfaces
to reach the osteonal canal; they also
connect osteonal canals to one
another
 not surrounded by concentric
lamellae

19.  Mature spongy bone is structurally similar to
mature compact bone except that the tissue is
arranged as trabeculae.
 The matrix of the bone is lamellated
 osteocytes get nutrients directly from circulating blood.

20. Immature Bone
 the skeleton of a developing fetus
 Aka bundle bone or woven bone because of the interlacing
arrangement of the collagen fibers.
 Comparison with mature bone
 does not display an organized lamellated appearance.
 contains randomly arranged, relatively more cells per unit area.
 more ground substance, not heavily mineralized
 stains more intensely with hematoxylin whereas mature bone
stains more intensely with eosin
 Areas of immature bone are present in adults, especially
where bone is being remodeled. E.g. in the alveolar sockets
and where tendons insert into bones.

21. CELLS OF BONE TISSUE
cell types
1. osteoprogenitor cells,
2. osteoblasts,
3. osteocytes,
4. bone-lining cells
5. Osteoclasts
 bone tissue Cells are surrounded by matrix.
- 25% water
- 25% protein
- 50% mineral salts
Differentiated form of
the same basic cell type

22. cell types

23. Osteoprogenitor Cells
 derived from mesenchymal stem cells.
 is a resting cell that can differentiate into an osteoblast
and secrete bone matrix.
 found on the external and internal surfaces of bones
 Morphologically, they comprise the periosteal cells
that form the innermost layer of the periosteum and
the endosteal cells that line the marrow cavities, the
osteonal (Haversian) canals, and the perforating
(Volkmann’s) canals.

24. Osteoblast
 is the differentiated bone-forming cell that secretes bone
matrix /type I collagen and bone matrix proteins (BMPs)/
 active osteoblasts are cuboidal or polygonal in shape and
aggregate into a single layer of cells lying in apposition to the
forming bone.
 inactive osteoblasts are flat or attenuated cells that cover the
bone surface.
 Osteoblast processes communicate with other osteoblasts
and with osteocytes by gap junctions.

25. Osteocytes
 When completely surrounded by osteoid or bone matrix, the
osteoblast is referred to as an osteocyte and the space occupied is
lacuna
 Osteocytes processes communicate through the canaliculi with other
Osteocytes and bone-lining cells by gap junctions.
 They are responsible for maintaining the bone matrix.
 synthesize new matrix, as well as participate in matrix degradation →
maintain calcium homeostasis
 arranged with their long axes in the same direction as the lamellae.
 a reduced load on bone initiates expression of matrix metalloproteinases
(MMP)

26. Bone-Lining Cells
 Bone-lining cells are derived from osteoblasts and cover
bone that is not remodeling.
 layer of flat cells with attenuated cytoplasm
 on external bone surfaces = periosteal cells
 on internal bone surfaces = endosteal cells
 Cell processes contact one another and with osteocytic
processes → Gap junctions
 Function
 maintenance and nutritional support of the osteocytes and
regulate the movement of calcium and phosphate into and out
of the bone.

27. Osteoclasts
 are phagocytotic cells derived from fusion of hemopoietic
progenitor cells of neutrophilic granulocyte and
monocyte lineages. → multinucleated cells
 are bone-resorbing cells present on bone surfaces where
bone is being removed or remodeled
 a shallow bay called a resorption bay (Howship’s lacuna) can
be observed in the bone directly under the osteoclast.

29. Bone Formation
Formation of Bone in an Embryo
 two patterns
 Intramembranous ossification
 Flat bones of the skull and mandible are formed in this way
 “Soft spots” that help the fetal skull pass through the birth canal
later become ossified forming the skull
 Endochondral ossification
 The replacement of cartilage by bone
 Most bones of the body are formed in this way including long bones

30. Intramembranous Ossification
 An ossification center appears in
the fibrous connective tissue
membrane
 Osteoblasts secrete bone matrix
within the fibrous membrane
 Osteoblasts mature into osteocytes

31. Enlarging
chondrocytes within
calcifying matrix
Chondrocytes at
the center of the
growing
cartilage model
enlarge and then
die as the matrix
calicifies.
Newly derived
osteoblasts
cover the shaft
of the cartilage
in a thin layer of
bone.
Blood vessels
penetrate the
cartilage. New
osteoblasts form a
primary ossification
center.
The bone of the
shaft thickens,
and the cartilage
near each
epiphysis is
replaced by
shafts of bone.
Blood vessels invade
the epiphyses and
osteo-blasts form
secondary centers of
ossification.
Cartilage
model
Bone
formation
Epiphysis
Diaphysis Marrow
cavity
Primary
ossification
center
Blood
vessel
Marrow
cavity
Blood
vessel
Secondary
ossification
center
Epiphyseal
cartilage
Articular
cartilage
Replacement of hyaline cartilage with bone Most bones are formed
this way (i.e. long bones).
Endochondral Ossification

32. Reference
 Histology A text and Atlas with correlated cell and
molecular Biology, Micheal H. Ross, Lippincott
Williams & Wilkins, 2011 G.C.; 6th Ed
 Color Atlas and Text book of Histology, Leslie P.
Gartner, 2014 G.C.; 6th Ed