The Connective Tissues: Bone
Jack L. Haar, Ph.D.
Learning objectives
Students should be able to:
1.
2.
3.
4.
5.
Describe the architecture of a long bone.
Describe the histology and function of bone components.
Describe the differences between woven and lamellar bone.
Describe how bone grows.
Explain how bone is deposited and to describe the configuration of the lamellae
deposited.
6. Explain how bone is resorbed.
7. Compare and contrast intramembranous and endochondral bone formation.
Explain how long bones grow.
I.
II.
Bone
-
Bone is alive, has great tensile strength and is light
-
Responds to hormonal and metabolic changes
Classification of Bone
A.
B.
Shape
1.
Flat
2.
Long
Mechanism of formation
1.
Intramembranous
-
2.
Endochondral
-
C.
mesenchymal C.T. changes into bone
cartilage is replaced by bone
Age - relates to microscopic structure
1.
Immature or woven
-
2.
very cellular, irregular matrix, low mineral content
Mature or lamellar
-
matrix is arranged in layers
-
bone may be spongy or compact
D.
Type - by texture of the bone, based on macroscopic structure
1.
compact
2.
spongy or cancellous or trabecular
-
E.
III.
A.
Other features
1.
Bone is vascular
2.
Growth is ONLY apposition
Architecture of a long bone
Regional divisions
1.
2.
Diaphysis (shaft)
-
Compact outside; marrow cavity inside
-
Red marrow is the site of blood cell formation or hematopoiesis
-
Yellow marrow is filled with adipose C.T.
Epiphysis
-
3.
Compact outside; spongy bone inside
Epiphyseal plate ("line" if growth has stopped)
-
B.
composed of spicules of bone with spaces between
Periosteum
Separates epiphysis from diaphysis
1.
Covers outer surface of bone except for articular surfaces.
2.
Layers - as in cartilage
a. Outer layer of dense FECT
b. Inner osteogenic layer
C.
D.
E.
Endosteum
1.
Lines marrow cavity, i.e., all internal surfaces of bone
2.
Composed of single layer of osteoblasts or osteogenic cells
Articular cartilage1.
Covers articular surfaces
2.
Hyaline cartilage
3.
Not covered by perichondrium
Blood Supply
1. Vessels of the periosteum become entrapped as bone is being laid down by periosteum; they become
vessels within canals of mature bone.
2.
3.
F.
IV.
Nutrient vessels
a.
Enter diaphysis of compact bone
b.
Supply marrow cavity
Epiphyseal vessels
a.
Enter near epiphyseal plate
b.
Supply epiphysis
Nerves
1.
Abundant sensory nerves in periosteum but not to bone cells.
2.
3.
Vasomotor nerves accompany blood vessels within the bone.
Architecture of a flat bone (many of skull bones)
A.
Inner and outer tables of relatively thick compact bone
B.
Diploe
C.
V.
1.
Spongy bone between the tables
2.
Thickness varies with the specific bone
Periosteum
1.
On outer surface is pericranium
2.
On inner layer is part of dura mater of brain
Histology of Bone
A.
Components
1.
Osteoblasts
a.
Osteoblasts or their precursors form a row of cuboidal cells on
surfaces of bone both periosteal and endosteal surfaces.
b.
Nucleus
c.
d.
(1)
Frequently located on side away from bone
(2)
Prominent nucleolus
Cytoplasm
(1)
Very basophilic due to extensive RER
(2)
Frequently, a negative Golgi image is seen
(3)
Numerous mitochondria
Mechanism of bone deposition
1)
Osteoid is synthesized first by osteoblasts
-
2)
Type I collagen fibers
Ground substance – primarily chondroitin sulfate
Ossification of osteoid then occurs
Results from osteoblasts secreting alkaline phosphatase
causing calcium deposition in osteoid along the collagen
fibers.
2.
Osteocytes
a.
2.
Osteoblast that have become completely surrounded by bone matrix; lie in lacunae as do
chondrocytes
b.
Shaped like a flattened ovoid with long processes extending from it through canaliculi in the
bone toward other osteocytes. Gap junctions join cells
c.
Nucleus
d.
Cytoplasm
e.
Transport materials between blood and bone to maintain
surrounding matrix
Osteoclasts
a.
b.
c.
d.
e.
Large cells with up to 15-20 nuclei
Located on surface of bone, frequently in depressions called
Howship's lacunae or resorption canal
Derived from monocytes in bone marrow
Involved with bone resorption, via proteolytic enzymes
Cytology
1)
Nuclei resemble those of osteoblasts and osteocytes;
prominent nucleolus
2)
Cytoplasm
a)
b)
c)
4.
frothy or vacuolated
Surface facing bone is deeply ruffled or infolded and
highly active
Contains lysosomal granules of acid phosphatase
Intercellular matrix of bone
a.
Organic portion - about 35% of dry weight
1)
Collagen type I fibers - 90% of organic matrix
a)
b)
2)
Glycosaminoglycanes (Protein-polysaccharides)
(primarily chondroitin sulfates)
a)
b)
3)
b.
Formed by osteoblasts
Demonstrate the 640 A banding
Cements fibers
Composed of less chondroitin sulfates than
cartilage; therefore, matrix appear more
acidophilic than cartilage
Function - provides bone with great toughness and resiliency
Inorganic portion - about 65% of dry weight
1)
Crystals resembling hydroxyapatite - Ca10
(PO4) 6 (OH)2
a)
b)
c)
2)
3)
Calcium ions
Phosphate ions
Some citrate, carbonate
Deposition - as slender needles along collagen fibers
Function - Provides bone with its hardness
B.
Architecture of Bone
1.
General Considerations
Histological preparation of bone; decalcification or ground bone
2. Compact Bone
a.
b.
Types of lamella
1)
Haversian system (osteons) - primary structure of compact bone
a)
Haversian canal at center
b)
Concentric lamellae
c)
Canaliculi interconnect lacunae
2)
Inner and outer circumferential lamellae
3)
Interstitial lamellae
Miscellaneous structures present
1)
2)
3)
4)
3.
Resorption canal
Sharpey's fiber
Volkmann's canals
Cement lines - refractile line separating adjacent lamellar groups which are collagen poor and
canaliculi do not cross them.
Spongy or cancellous bone
a.
Composed of thin trabeculae
b.
Lamellar bone but with no H. systems
c.
All surfaces covered by endosteum
BE SURE YOU UNDERSTAND THE FOLLOWING TERMINOLOGY
A.
Compact vs. spongy: refers to macroscopic structure of bone
B.
Woven vs. Lamellar: refers to microscopic structure of bone
C.
Intramembranous vs. endochondral: refers to the two mechanisms of bone formation
VI.
Mechanisms of bone formation (ossification)
A.
Intramembranous - formed directly from mesenchyme or a connective tissue layer (endosteum or
periosteum).
1.
Ossification with mesenchymal origin
a.
Mechanisms of ossification
1)
At primary center of ossification a clustering of mesenchymal
cells and blood vessels occurs
b.
c.
2)
Mesenchymal cells multiply and differentiate into osteoblasts
3)
These osteoblasts then secrete bony, organic matrix around
themselves forming a spicule of woven bone having low mineral content
Growth and appearance
1)
Irregular growth from original spicule forms:
2)
Anastomosing spicules or trabeculae = spongy
(cancellous) bone.
Conversion of spongy to compact bone
1)
All spaces within spongy bone contain c.t., blood vessels and
osteogenic cells.
2)
Osteoblasts lining such a space lay down matrix in circular
lamellae around periphery.
3)
Such lamellation continues, decreasing size of original space.
4)
Eventually, only a central canal with its blood vessel and
osteogenic cells remain, completely surrounded by lamellae
(= primitive Haversian system).
2.
Ossification with connective tissue origin, such as perichondrium, endosteum or
periosteum.
a.
Mechanism of ossification
1)
Osteogenic cells of endosteum or innermost cells of
of periosteum or perichondrium lay down lamella of bone.
b.
Growth and appearance
1)
Lamella conforms to shape of bone or cartilage model of
"bone" on which it is being deposited.
2)
May be a lamella added to a trabeculum of spongy bone or a
lamella added to compact bone (i.e., Haversian canal or
circumferential lamellae).
3)
Bone around shaft is laid down by newly converted
periosteum forming "periosteal band." This bone is formed
intramembranously, not endochondrally and is referred to as
perichondrial bone formation.
3.
Intramembranous bones
Bones which develop entirely intramembranously are the flat bones of the skull - frontal, parietal, occipital,
temporal, and parts of the mandible. However, intramembanous bone formation is partially responsible for
the development of most other bones.
B.
Endochondral - formed by replacement of cartilage model
1.
Occurs during the development of bones at base of skull, of extremities,
and of vertebral column, pelvis, ribs.
2.
Occurrences before ossification begins
a.
Hyaline cartilage model of the bone is formed in the fetus from
mesenchyme
b.
Chondrocytes at center of model (at what will become the primary
center of ossification) mature and greatly hypertrophy at expense of surrounding matrix.
(Recall “Regressive changes in cartilage”)
c.
Such chondrocytes produce alkaline phosphatase which provides a
calcifiable cartilage matrix
d.
Calcification of cartilage occurs, making matrix impermeable to
metabolites.
e.
Chondrocytes die from lack of nutrition
f.
3.
4.
Simultaneously, vascularization increasing in the perichondrium:
1)
Stimulates multipotential properties of perichondrium to stop laying down cartilage and
to begin laying down periosteal band.
2)
Provides vessels, the periosteal bud, which penetrate into area of calcified cartilage
Mechanism of ossification
a.
Bone around shaft is laid down by newly converted periosteum
forming "periosteal band" by intramembranous ossification.
b.
At primary center of ossification
1)
Periosteal bud supplies blood vessels and osteogenic cells to
area of calcified cartilage
2)
Bone is laid down by osteogenic cells on calcified cartilage
framework
Newly-formed endochondral bone at ossification centers looks like spongy bone because its
framework is the calcified cartilage spicules.
a.
b.
Center (core) is of calcified cartilage (blue with H and E) and
Periphery is composed of bone (pink to red with H and E).
5.
Endochondral ossification in diaphysis of long bones
a.
b.
Cartilage cells surrounding primary center become more organized forming the epiphyseal
plate
1)
Undergo a phase of interstitial growth (proliferation) resulting in growth in length
2)
Appear as longitudinal rows of chondrocytes
Resorption of spongy bone originally laid down at primary center occurs
1)
Periosteal band is increasing in size
2)
Spongy bone is not needed in center of bone now for support
3)
Therefore, a recognizable zonation is produced in the hyaline
cartilage of developing long bone
a)
Resting zone (zone of reserve cartilage)
b)
Zone of proliferation
c)
Zone of maturation - cells increase in size or hypertrophy
d)
Zone of calcification (of cartilage)
e)
Zone of degeneration
f)
Zone of ossification
g)
Area of bone resorption
c.
In epiphysis (secondary center for ossification)
1)
Ossification follows same pattern as at primary center producing a
network of spongy bone.
2)
Some areas of epiphyseal cartilage are not ossified.
a)
Articular cartilage on bone surface
Cartilage of epiphyseal plate
1)
2)
3)
Composed of the hyaline cartilage responsible for
all subsequent growth in length
Shows the zonation described above
Retains its same thickness (due to activity of zone
of proliferation) until growth ceases and epiphyses
"close"
VII. Bone Growth
A.
Long bones grow in length from epiphyseal plate
B.
Long bones grow in width by bone deposited by periosteum or
endosteum
1.
Circumferential lamellae - The outer circumferential lamellae may come
to occupy a substantial portion of the bone. It is nourished by vessels
running longitudinally through primary nutrient canals.
2.
C.
Flat bones of the skull
1.
2.
VIII.
Haversian systems
Deposition of bone in the suture
Deposition of bone on convex surface and resorption of bone on concave surface
Remodeling of bone - involves both deposition and resorption
A.
B.
C.
Definitions
1.
Resorption (Absorption) cavity - irregular in shape, lined with osteoclasts,
not ringed by concentric lamellae
2.
Primitive Haversian system - osteon formed by conversion of spongy to
compact bone.
3.
Primary bone - original bone formed.
4.
Secondary bone - laid down after resorption of primary bone
Phases of internal remodeling
1.
Resorption of primary bone
2.
Osteocytes undergo some as yet poorly understood change
3.
Osteoclastic erosion
4.
Death of osteocytes
When resorption stops osteoblasts begin bone formation around edge of resorption cavity
1.
Secondary Haversian systems are formed
2.
Cement line separates primary and secondary bone
1)
2)
3.
D.
IX.
A zone very poor in collagen
Lamella not traversed by canaliculi
Remains of old Haversian systems form the interstitial lamellae
Continues throughout life with third, fourth, etc., orders of Haversian systems being formed.
Fracture repair
1.
Fibroblasts and capillaries form granulation material around break
(procallus).
2.
Procallus becomes dense FECT and finally cartilage which gaps the ends
of the bone fragments.
3.
New bone originates from periosteum and endosteum and replaces the
cartilage callus - bony callus
4.
Reorganization occurs with resorption of excess bone and internal
reconstruction.
X. Joints
Areas where bones are capped by connective tissue thus permitting movement to varying degrees.
A.
Synarthroses - limited or no movement
1.
Synostosis - bone to bone
2.
Synchondrosis - bone joined by hyaline cartilage
3.
Syndesmosis - bones joined by interosseous ligament
B.
Diarthrosis - joints having great mobility
1.
Ligaments and a capsule of c.t. maintains the relationship between two bones.
a.
Histology of the capsule
i. Synovial membrane - Formed by a layer of squamous or cuboidal cells arranged in folds which
line the interior of the articular cavity. Originate from mesenchyme. Lines the joints everywhere
except over articular cartilage. It is supplied with blood vessels, nerves, and lymphatics.
b.
2.
Fibrous capsule - the outer fibrous layer which is continuous with the fibrous layer of periosteum
of bones. It is relatively inelastic.
The capsule forms the articular cavity which contains synovial fluid.
a.
b.
Synovial fluid is a filtrate of blood and contains a high
concentration of hyaluronic acid. Synovial fluid facilitates sliding
of articular surfaces of the bones which are covered by hyaline
cartilage having no pericondrium.
Proteoglycan molecules of the articular cartilage help absorb
mechanical pressure experienced by joints. Movement of fluids out
of and into cartilage during this process is essential for good
nutrition of the cartilage.
Structures identified in this section
Bone Tissue
Blood vessels
Bone marrow
Canaliculi
Compact bone
Decalcified bone
Ground bone
Howship's lacunae
Intercellular matrix
Lacunae
Lamellae of bone
Lamellar bone
Loose CT
Organic matrix
Osteoblasts, active
Osteoblasts, inactive
Osteoclasts
Osteocytes
Osteoid
Periosteum
Spicules
Spongy bone
Woven bone
Organ structures
Articular cartilage
Diaphysis
Endosteum
Epiphyseal plate
Epiphysis
Flat bone, diploe
Flat bone, inner table
Flat bone, outer table
Hyaline cartilage
Metaphysis
Muscle
Periosteum, osteogenic layer
Spongy woven bone
Suture
Deposition and resorption
Cement lines
Deposition "rules"
First lamellae
Haversian canal
Haversian canal contents
Inner circumferential lamellae
Interstitial lamellae
Osteon (Haversian system)
Outer circumferential lamellae
Resorption canal
Intramembranous Formation
Canaliculi
Skeletal muscle
Skin
Spongy lamellar bone
Spongy woven bone
Endochondral formation
Bone deposition
Calcified cartilage
Cartilage spicules
Flat bones
Long bones
Periosteal band
Resting zone
Zone of degeneration
Zone of ossification
Zone of proliferation
Zone of resorption
Zones of maturation-hypertrophy-calcification
VIRTUAL SLIDE LINK
Ground bone:
Ground Bone