Bone Tissue Objectives
1. List and describe the cell types, fibers and components of the extracellular
matrix of bone.
There are five types of fixed bone cells. Osteocytes are firmly encased. They are
mature cells embedded in calcified bone matrix. Osteocytes do not divide; they lie in lacunae
just like cartilage cells. They have the function of maintaining the extracellular matrix
surrounding the cell. Osteocytes are well separated by calcified bone matrix. They have
cytoplasmic processes, which extend to adjacent cells thorugh small tunnels of calcified
matrix called canaliculi. The cells are arranged in concentric circles around a central blood
vessel. The blood vessels run in the central canal (Haversion Canal) of the osteon
(Haversian system). Unlike cartilage, bone is vascular. Osteons are the basic structural and
functional unit of compact bone. The cells with their processes create a chain of cells which
extends from the blood vessel to the outermost ring of cells near the edge of the osteon. The
cement line is the outer most limit of the osteon. It is a region deficient in collagen fibers and
is lighter in appearance than the rest of the osteon. The osteoid is a layer of unmineralized
matrix adjacent to the cell membrane.
Osteoblasts are immature cells which are not capable of cell division. They are found
on the surface of existing bone. Cuboidal in shape, they are dark, or basophilic, and are
closely packed together. This gives the appearance on an epithelium forming a lining on the
outside surface of the bone tissue. This is the part that actively lays down bone matrix. All
bone growth is appositional.
Osteoclasts are the cells that reabsorb the bone. They are large and multinucleated
with 2-70 nuclei. Located on the surface of bone in a depression called Howship's lacunae
(resorption bays). These cells dissolve away the bone matrix to form the depression the cell
sits in and with continued action may form tunnels in the bone. Under a light microscope
these form a striated border in the ECM. This is the place of active bone reabsorption eroding
away ground substance and minerals; all that is left is the collagen fibers. This is different
than the ruffled border. At the EM level, the ruffled border is the site of active bone
resorption. This is a specialized region of the cell membrane where long finger-like processes
extend out into the matrix. The region of the ECM just outside the ruffled border is called the
subosteoclastic compartment. This has a very acid microenvironment and contains
carbonic anhydrase in vesicles. These produce H+ ions which are pumped outside the cell
to make the acidic microenvironment. The acidic nature dissolves the mineral and promotes
degredation of the organic matrix. Collagenase and hydrolases are proteins secreted by
osteoclasts to help degrade the collagen fibers. Vesicles in the adjacent cytoplasm suggest
resorption of these components into the cell. The sealing zone is at the edge of the ruffled
border. This is where the cell membrane uses integrins to bind to the extracellular matrix
and seal the membrane to the bone matrix to limit the extent of the subosteoclastic
compartment. The osteoclasts originate from a granulocyte-macrophage progenitor
(macrophage). In the bone multiple macrophage progenitor cells fuse to form osteoclasts.
This is due to parathyroid hormone and calcitonin.
Osteoprogenitor cells are not identifiable morphologically. They are either reserve
mesenchymal cells or pericytes. They are undifferentiated cells that differentiate into
osteoblasts. They are not identifiable until the begin to differentiate into osteoblasts.
Bone lining cells are the cells that line all bone surfaces. They look like simple
squamous epithelium but are not epithelium. There is no free surface and it is not always
continuous. They are found in three places: all vascular channels (aversion canals),
endosteum (lining of the bone tissue against the bone marrow), and the periosteum (bone
version of periochondium). These cells are meant to help maintain the microenvironment in
the bone tissue. The main functions of bone lining cells are to: separate the bone tissue from
other tissue (bone marrow), form interconnecting network with other bone lining cells,
osteocytes and osteoblasts by cellular processes, and are implicated in sensing stress within
the bone tissue and initiating internal bone remodeling.
The extracellular matrix is composed of two parts: an organic and an inorganic
portion. The organic portion of the ECM consists of collagen fibers and ground
substance. Most of the organic matrix is type I collagen fibers. These collagen fibers give
the bone its tensile strength and flexibility. The second component is the ground substance.
There is not much ground substance and of the scant amount around, there are small
amounts of proteoglycans. The function of the organic material gives flexibilty and strength to
the bone tissue. The few proteoglycans are in the form of aggrecan without the characteristic
amount of water binding. There are three major glycoproteins present: osteonectin is a
glycoprotein which binds plasma membrane to the collagen fibers (n&n to m&m), osteocalcin
binds Ca+ to the matrix, and bone sialoprotein binds cells to matrix via binding sites for
integrins and collagen receptors.
The organic portion is the component resonsible for great structural strength of the
bone. It consists of all the mineral salts located in the extracellular matrix. Most of the total
bone weight is the inorganic component. The ions are found in the form of hydroxyapetite
crystals. These are primarily composed of CaPO4. They are aligned parallel to the collagen
fibers in the matrix. The hydration shell consists of water surrounded with each crystal.
These crystal shells strengthen the bone big time. Bone is a biphasic material where the
qualities of two materials are joined together into one stronger material. The inorganic matrix
has the great strength of the mineral but is brittle. The organic matrix has the great strength
and flexibility of the collagen fibers. The two components come together to form a structural
material that is strong but also flexible enough to not be brittle.
The extracellular fibers present in the bone tissue are an enormous part of the
classification of bone tissue. Classification by the arrangement of extracellular fibers is by
lamellar or woven bone. Collagen type I fibers are the primary fiber type found in bone.
They are densely packed. Collagen type I is laid down in layers in which the fibers all run in
parallel in a single direction. The fibers are arranged in alternating layers with fiber direction
at right angles to the preceding layer. They are arranged like the layering of plywood. Each
individual layer is called a lamellae. Lamellar bone is bone tissue with layered arrangement
of collagen fibers. Lamellar bone = mature bone = secondary bone. They are almost at
right angles to each other. Woven bone is bone with no layered collagen fibers, but random
orientation of bundles of collagen fibers running in all directions. Lamellar bone is stronger
than woven bone.
2. List and describe the components of the organic and inorganic bone matrix.
The extracellular matrix is composed of two parts: an organic and an inorganic
portion. The organic portion of the ECM consists of collagen fibers and ground
substance. Most of the organic matrix is type I collagen fibers. These collagen fibers give
the bone its tensile strength and flexibility. The second component is the ground substance.
There is not much ground substance and of the scant amount around, there are small
amounts of proteoglycans. The function of the organic material gives flexibilty and strength to
the bone tissue. The few proteoglycans are in the form of aggrecan without the characteristic
amount of water binding. There are three major glycoproteins present: osteonectin is a
glycoprotein which binds plasma membrane to the collagen fibers (n&n to m&m), osteocalcin
binds Ca+ to the matrix, and bone sialoprotein binds cells to matrix via binding sites for
integrins and collagen receptors.
The organic portion is the component resonsible for great structural strength of the
bone. It consists of all the mineral salts located in the extracellular matrix. Most of the total
bone weight is the inorganic component. The ions are found in the form of hydroxyapetite
crystals. These are primarily composed of CaPO4. They are aligned parallel to the collagen
fibers in the matrix. The hydration shell consists of water surrounded with each crystal.
These crystal shells strengthen the bone big time. Bone is a biphasic material where the
qualities of two materials are joined together into one stronger material. The inorganic matrix
has the great strength of the mineral but is brittle. The organic matrix has the great strength
and flexibility of the collagen fibers. The two components come together to form a structural
material that is strong but also flexible enough to not be brittle.
3. List and describe the individual components of bone and relate these to bone
function.
Cells are important because they maintain the extracellular matrix, they also participate
in the breakdown and buildup of bone. The extracellular fibers help to provide the bone with
its flexibility and strength. The ground substance helps to bind inorganic ions which provide
massive structural strength.
4. Identify and describe the structural and functional unit of the two types of bone
tissue.
The extracellular fibers present in the bone tissue are an enormous part of the
classification of bone tissue. Classification by the arrangement of extracellular fibers is by
lamellar or woven bone. Collagen type I fibers are the primary fiber type found in bone. They
are densely packed. Collagen type I is laid down in layers in which the fibers all run in parallel
in a single direction. The fibers are arranged in alternating layers with fiber direction at right
angles to the preceding layer. They are arranged like the layering of plywood. Each
individual layer is called a lamellae. Lamellar bone is bone tissue with layered arrangement
of collagen fibers. Lamellar bone = mature bone = secondary bone. They are almost at right
angles to each other. Woven bone is bone with no layered collagen fibers, but random
orientation of bundles of collagen fibers running in all directions. Lamellar bone is stronger
than woven bone.
5. Describe the morphological organization of spongy and compact bone tissue.
Cortical bone consists of the outer layer of the bone organ. It is also called dense or
compact bone. Cortical bone has more bone tissue than space per unit volume. Trabecular
bone is located inside the bone organ. Also known as cancellous or spongy bone, it is
defined as having more space than bone tissue per unit volume. The osteon is a major part
of cortical bone. Trabecular bone is composed of thing processes or fingers of bone tissue
called trabecula or spicules with large amounts of space between the trabeculae. Within
the space is the bone marrow cavity. The trabecula are made up fo trabecular packets,
angular shaped parcels fitted together to form the trabecula. They are lamellar bone due to
the ordered arrangement of collagen fibers. Only large trabecula contain osteons. The space
is called the bone marrow space and is filled with red or yellow bone marrow.
6. Describe the vascular supply to bone tissue and relate how nutrients and wastes
pass to cells from the vessels. Explain how the structure of bone affects this.
The osteon is the basic structural unit of compact bone. The central canal (Haversian
canal) contains the blood vessels. This lies in the center of the osteon. The bone is vascular.
The osteon is identifieable by its concentric layers of collagen fibers and osteocytes. Osteons
run parallel down the length of the bone. The cement line is the outer limit of the osteon. It
is a lighter staining layer at the periphery of the osteon deficient in collagen. The Volkmann
canals are canals that hold diagonally running blood vessels. They supply the blood to
vessels in the osteons. They look like osteon except they lack the concentric layers of cells.
They have to then diffuse nutrients across the matrix. This becomes more difficult as
mineralization occurs. When enough mineralization occurs, the cells might die and then
become interstitial lamellae.
7. Compare the blood supply of bone with that of cartilage.
Bone is vascularized, cartilage is not. In cartilage, the cells must wait for entry/repair to
reach the periochondium and then wait for more diffusion of progenitor cells. In the bone,
thanks to vascularization, nutrients and wastes are much more readily transferred.
8. List and describe how different terminology is used to describe structure and
function of bone tissue (compact bone, spongy bone, long bone…).
Bones can be classified by shape such as long bones and short bones. Irregular
bones include the hip and vertebrae. Flat bones are divided into two types: dipole (layer of
trabecular bone between two layers of compact bone), and inner & outer tableau (layer of
dense bone on the island outside the flat bones in the skull. Sesamoid bones are bones
formed within a tendon of a muscle. The patella is the largest sesamoid bone.
Bones can also be classified by gross structure. The epiphysis is the enlarged area at
each end of a long bone. The diaphysis is the shaft of a long bone. The metaphysis is the
cone shaped region connecting the epiphysis with the diaphysis. The epiphyseal plate is the
cartilage region found in the metaphysic. It is responsible for increasing the length of the long
bone.
Cortical bone is also known as dense bone or compact bone. Trabecular bone is
also known as cancellous bone or spongy bone.
Bone can also be classified by the arrangement of collagen fibers. Lamellar bone
is bone tissue with layered arrangement of collagen fibers. Lamellar bone = mature bone =
secondary bone. Woven bone is bone that has no layered collagen fibers, but random
orientation of bundles of collagen fibers running in all directions. Woven bone = immature
bone = primary bone.
9. List and describe the functions of bone tissue.
Bone is for structural support. This is based on inorganic salts impregnated in
organic matrix to give bone its rigidity and great strength. Bone maintains shape and form of
body and serves to protect soft tissues. Bone is also meant to act as a reservoir of mineral
ions for the rest of the body. The bone is continually being removed, rebuilt, or remodeled.
The mechanism for release ions form inorganic matrix is the process of internal remodling.
10. Relate the morphology of bone tissue with its functions.
The structural support is based on the structural bone. This consists of older mature
osteons and interstitial lamellae. It is the most heavily calcified bone tissue and is responsible
for supporting the weight of the body. As a metabolic function to store mineral ions for the rest
of the body, these ions can be released during osteoclast activity. Ca2+ and other ions are
released into the blood.
11. List and describe the theories of mineralization (calcification) of bone matrix.
Calcification is the deposition of Ca salt (mineral) into any tissue. In bone, the organic
matrix is laid down first. The organic bone is called osteoid (unmineralized bone matrix).
Then Ca++ and PO4- are deposited at the calcification front. All calcification is believed
under control of osteoblasts. There are two theories about how the mineral is deposited. The
cellular nucleation theory believes matrix vesicles act as initiators of calcification. Matrix
vesicles are small blebs of cytoplasm pinched off from cells containing the enzyme alkaline
phosphatase. This enzyme raises Ca++ and PO4- levels. This acts as the inhibitor of an
inhibitor. Pyrophosphate normally inhibits calcification. Alkaline phosphatase inhibits
pyrophosphate, thereby promoting calcification. The vesicles have Ca and PO4 receptors
that promote crystal formation. The macromolecular nucleation theory incorporates four
different molecules. Collagen fibers (heterogenous nucleation) contain gap regions between
tropocollagen molecules that initiates calcification (50% of crystals in bone are in gaps).
Osteocalcin is a glycoprotein which binds Ca+ and initiates mineralization in different areas
in the ground substance. Chondrocalcin is found in cartilage. It is a glycoprotein which
binds Ca+. They are diffusely distributed throughout the matrix. Both glycoproteins build up
in the matrix just prior to calcification. Sulfated proteoglycans normally hold Ca++. During
calcification they are modified to release the sequestered Ca++ for use in mineralization.
These proteoglycans are also associated with phosphate. There is a conformational change
in the sulfated proteoglycans so they can release the Ca++.
Mineralization of bone osteoid occurs as a two phase process. The primary
mineralization is only a couple of days but is 70% of total mineralization of the matrix. The
primary mineralization is under the control of the osteoblasts and new osteocytes. The
secondary mineralization is the second phase which lasts for several months. This is
responsible for the last 30% of mineralization.
12.
Define, describe, and differentiate between the processes of ossification,
calcification, and growth.
Ossification is synonymous with bone development and bone formation.
Intramembranous ossification occurs within a fibrous membrane or layer. The bone is formed
within the connective tissue proper. Endochondral ossification occurs within a hyaline
cartilage model of the bone. The hyaline cartilage model in the shape of the bone is formed
prior to any bone tissue being laid down. The cartilage is removed and replaced with bone
tissue. This process occurs in most of the bones of the body. Calcification is the deposition
of Ca salt in tissue. This involves depositing Ca and PO4 in organic matrix, called osteoid.
The two theories of bone mineral deposition are the cellular nucleation theory and the
macromolecular nucleation theory.
Bone growth occurs through Internal
remodeling/Haversian remodeling/cortical remodeling/secondary bone formation.
13. Describe the process of internal (Haversian) remodeling of bone and its structural
basis.
Bone growth occurs through Internal remodeling/Haversian remodeling/cortical
remodeling/secondary bone formation. This occurs in response to new stress on the bone.
New lamellar bone is laid down within the dense bone. The mechanism begins with the
bone lining cell network sensing new stress in the bone tissue and initiating internal
remodeling. It is thought that the bone lining cell network is responsible for the control and
access of osteoclasts to bone matrix. Osteoclasts begin to form from granulocytemacrophage progenitor cells. They fuse together to become osteoclasts. The Resorption
Cone (Resorption cavity, tunnel) is formed by numerous osteoclasts. These cavities
become tapered as they enlarge. This is the region where osteoclasts are actively removing
bone matrix. This results in the release of Ca++ and other ions into the blood.
Diagnostically, a resorption cavity is characterized by a large lumen in the bone matrix with
osteoclasts lining it. It is usually not in the center of an osteon. There are no circumferential
layers of osteocytes that run entirely around the large central canal.
The reversal zone is where there is no longer any bone resorption and bone
deposition has not yet begun. Macrophages here scavenge the debris and smooth out the
surface. The closing cone or forming osteon is where a blood vessel follows down the
forming resorption tunnel. It carries osteoprogenitor cells that migrate across the gap
between the blood vessel and the outer limit of the resorption cavity. The osteoprogenitor
cells attach to the surface of resorption tunnel and differentiate into osteoblasts. The
osteoblasts lay down a layer of bone osteoid (unmineralized bone matrix). The collagen
fibers are laid down in lamellae so this forms lamellar bone. The calcification front sweeps
through. The forming osteon is characterized by a large central canal with only a few
circumferential layers of osteocytes present. Because this is an ongoing process, the closing
cone is tapered along the length as the layers are built up in the older portion of the forming
osteon. The new osteon is lamellar bone due to the orderly arrangement of the collagen
fibers in each layer.
Two principal hormones are involved in the process of internal remodeling: parathyroid
hormone and calcitonin. Parathyroid hormone is secreted due to low peripheral blood Ca++
levels. Parathyroid hormone stimulates the osteoclasts to increase their resorption activity.
This is done by: removing bone matrix and releasing Ca++ and by decreasing osteoblastic
activity. The net effect is to increase blood calcium ion levels. The receptors for parathyroid
hormone are on the osteoblasts. The osteoblasts secrete osteoclastic stimulating factor,
which goes to the osteoclast. Calcitonin is a hormone secreted when the Ca++ level in the
blood is too high. The receptors for calcitonin are on the osteoclasts. Calcitonin acts to
decrease osteoclastic activity and increase osteoblastic activity and deposit Ca++. The net
effect is to decrease blood Ca++ levels.
14. Know when remodeling first starts and when it stops in the body. Understand how
the hormones, parathyroid and calcitonin, help control the internal remodeling
process.
Remodeling first starts in response to new stress on the bone. Remodeling is
completed when the lamellar bone is laid down. Remodeling, as a complete process, is
always occurring and reoccurring in the body. Internal remodeling begins from the time bone
is first formed in the fetus until death.
Two principal hormones are involved in the process of internal remodeling: parathyroid
hormone and calcitonin. Parathyroid hormone is secreted due to low peripheral blood Ca++
levels. Parathyroid hormone stimulates the osteoclasts to increase their resorption activity.
This is done by: removing bone matrix and releasing Ca++ and by decreasing osteoblastic
activity. The net effect is to increase blood calcium ion levels. The receptors for parathyroid
hormone are on the osteoblasts. The osteoblasts secrete osteoclastic stimulating factor,
which goes to the osteoclast. Calcitonin is a hormone secreted when the Ca++ level in the
blood is too high. The receptors for calcitonin are on the osteoclasts. Calcitonin acts to
decrease osteoclastic activity and increase osteoblastic activity and deposit Ca++. The net
effect is to decrease blood Ca++ levels.
15. Describe the function of structural and metabolic bone. Be able to relate it to
structures seen in bone.
Structural bone is based on inorganic salts providing bone its rigidity and great
strength. Structural bone maintains the shape and form of the body. Structural bone consists
of older mature osteons and interstitial lamellae. It is the most heavily calcified bone tissue
and is responsible for supporting the weight of the body.
Metabolic bone acts as a reservoir of mineral ions for the rest of the body. The bone
here is continually being removed and rebuilt or remodeled. The mechanism for releasing
ions from the inorganic matrix is the process of internal remodeling. This is where osteoclast
activity takes place.
16. Know the relative rates of tissue turnover in the body.
One remodeling cycle takes about four months to complete. Compact bone internal
remodeling is about 3% per year. This is a much slower process than trabecular remodeling,
which is ~26% per year. Each individual bone and various parts of each bone have different
turnover rates. About 7.6% of all the bone in yoru body turns over per year. Primary
mineralization takes place for only 3-4 days. Secondary mineralization lasts for several
months.
17. Describe the morphology of trabeculae and their remodeling.
Trabecular bone is synonymous with cancellous or spongy bone. It is made up of thin
processes or fingers of bone tissue called trabecula or spicules. The trabecula are made up
of trabecular packets. These are angular shaped parcels fitted together to form the
trabecula. They are lamellar bone due to the ordered arrangement of collagen fibers. Only
large trabecula contain osteons. Trabecula are composed of a mosaic of trabecular packets.
Each packet is lamellar bone. Each trabeculae undergoes periodic replacement. Trabecular
remodeling is like internal remodeling in using osteoclasts to scoop out the cavity where the
new packet will form. A pit is gouged out of the trabecular surface by osteoclasts. It is then
filled in by osteoblasts one layer at a time. This process is much faster than the internal
remodeling in cortical bone.
Coumadin is a drug used for anticoagulate therapy. It negatively affects bone healing.
Coumadin prevents clotting by interfering with carboxylation of glutamic acid. This has the
side effect of interfering with the synthesis of osteocalcin. That prevents Ca+ deposition. In
long term warfarin therapy, the patient can get severe bone disorders.