A basic course on bone formation, structure, function
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Basic Course in Bone Function, Structure and Health
Our bones provide support for other tissues and protect
underlying tissues. For example the skull bones protect
the brain, and the ribs protect the thoracic organs.
Skeletal muscles attach to bones, and when they contract
they pull on the bones - causing movement. Did you know
that your bones are also good for storage?
Fat is stored in the cavities within our long bones, and
minerals (especially calcium) form part of the bone
matrix. When your blood supply of calcium runs low,
calcium can be reabsorbed from bone tissue. Why is
calcium important? Calcium is needed for proper nervous
system function, and is also necessary for the
contraction of muscles.
Another important job of the bone tissue is to form
blood cells (hematopoiesis). Blood cells are formed in
the red bone marrow, which is found in the ends of our
long bones, and also in flat bones, like the sternum.
Two types of bone tissue:
·spongy bone - lightweight, but able to withstand
stress, marrow is found in the spaces between the bone
·compact bone - very dense and strong, found on the
outside of all our bones
We'll classify bones by their shape:
·long bones - longer than they are wide, The bones of
your arms, legs, fingers, and toes are long bones. There
is an outer collar of long bone, with spongy bone
inside.
·short bones - cubelike, about as tall as they are wide.
Your tarsal (ankle) and carpal (wrist) bones are short
bones. Short bones have a thin compact bone outer layer,
and are filled with spongy bone.
·sesamoid bones - a special class of short bones. These
bones are embedded in tendons, and are usually small,
except for the patella (kneecap). The number of sesamoid
bones varies from person to person.
·flat bones - as their name suggests, these bones are
flat! Your skull bones, sternum, and ribs are flat
bones. These bones have 2 plates of compact bone with a
layer of spongy bone inside.
·irregular bones - again, these bones are perfectly
described by their name! Examples are some of the skull
bones, the vertebrae, and hip bones. Again, there is a
layer of compact bone on the outside, with a spongy bone
"middle".
Long Bone Anatomy
Which bones are classified as long bones? If you said
the bones of the arms, legs, feet, and hands you
remember what you read above! All long bones have
similar anatomy, only the size of the bones makes the
femur (thigh bone) different from the metacarpals (bones
in the hand.)
diaphysis - the shaft of the long bone. Compact bone
"collar" with a cavity in the middle known as the...
medullary cavity - the cavity in the diaphysis of long
bones, filled with yellow marrow (fat)
epiphysis - the ends of the long bone, filled with
spongy bone and red bone marrow
epiphyseal plate - the area of longitudinal bone growth,
composed of cartilage which becomes ossified as growth
occurs. The epiphyseal plate remains throughout
childhood and into adolescence.
epiphyseal line - the remnant of the epiphyseal plate in
mature bones
periosteum - a two layered connective tissue membrane
which surrounds the diaphysis. The outer layer is made
of dense irregular CT, the inner layer contains
osteoblasts (bone-builder cells), and osteoclasts
(bone-destroyer cells.) The osteoclasts and osteoblasts
continually lay down and remove bone matrix under the
influence of two hormones - calcitonin, and parathyroid
hormone (more on these later.) The periosteum brings
blood and lymph vessels and nerves into the bone.
Sharpey's fibers - attach the periosteum to the bone
endosteum - a delicate connective tissue membrane which
lines the inner bone surfaces
articular cartilage - the top of the epiphyses are
covered with hyaline cartilage, which helps protect the
bone ends and allows for smoother joint movement
Anatomy of Other Bone Types
Flat, short, and irregular bones have a periosteum-covered
surface of compact bone, and endosteum-covered spongy
bone on the inside. The inner layer of spongy bone in
flat bones is known as diploe.
Bone Marrow
There are two types of bone marrow:
·yellow marrow - adipose tissue found in the diaphysis
and parts of the epiphysis in long bones
·red marrow - found in the epiphyses of some long bones
(especially the head of the femur and humerus) and the
diploe of flat bones. This tissue is known as
hematopoietic tissue (hemato = blood, poietic =
beginning, genesis, forming.) All blood cell types
originate here - not just the red blood cells.
Microscopic Structure of Compact Bone
Compact bone is organized into individual functional
units known as osteons. Each osteon is composed of
concentric layers of tissue known as lamellae. The
tissue is a combination of collagen fibers and deposited
minerals. The osteocytes (mature bone cells) maintain
the bone matrix, and wall themselves off in "caves" like
the cartilage cells do. So in bone we have osteocytes in
lacunae (instead of chondrocytes in lacunae in
cartilage.)
The osteocytes get nutrition and rid themselves of waste
products via a central canal in the osteon known as the
Haversian canal, or the central canal. Canals that run
perpendicular to the Haversian canals are known as
Volkmann's canals. This network of blood vessels
provides the nutrition to the bone, you may not have
realized that bone was as vascular as it is. So, how do
the individual cells get the nutrition they need if they
are in one of the outer lamellae in an osteon? The
answer lies in the canaliculi - tiny "canals" which
connect the osteocytes. The osteocytes extend their cell
membranes and cytoplasm into these channels, and there
the exchange is made. The picture below on the left is a
close-up of the osteocytes with the radiating canaliculi.
The picture on the right is an electron micrograph which
illustrates the junction of two osteocytes in a
canaliculi.
Microscopic Structure of Spongy Bone
Spongy bone does not have osteons, instead the bone
tissue looks like a sponge - the bony pieces are called
trabeculae, and they are arranged along the lines of
greatest force to a bone, not haphazardly as you might
think. The trabeculae are only a few cell layers thick
and house the osteocytes, again connected by canaliculi.
Blood vessels run throughout the spongy bone, and marrow
fills the spaces between the bone tissue.
Chemical Composition of Bone
Remember our definition of connective tissue? By now,
you should easily say "living cells in a non-living
matrix." The matrix of bone is partially "organic", and
partially "inorganic". The organic portion is composed
of the collagen fibers secreted by the osteocytes and
osteoblasts, and some glycoproteins. The organic portion
of the matrix gives the bone strength and flexibility.
The inorganic portion of the matrix is the
hydroxyapatites, a fancy name for mineral salts. Calcium
is one of the most important mineral, but you'll also
find phosphorous, magnesium, and others. The mineral
salts are deposited by the osteocytes around the
collagen fibers. The inorganic matrix gives the bone
hardness.
Try soaking a chicken bone in a jar of vinegar for a few
days - the minerals will leach out, leaving only the
organic matrix. You'll definitely appreciate calcium
when you observe the vinegar-soaked bone - and may be
convinced to drink more milk!
Osteogenesis
Bone, like all other connective tissues, is formed from
mesenchyme - the embryonic connective tissue. Before you
are born this mesenchyme is replaced by the mature
connective tissues. Before ossification occurs your
skeleton is composed of fibrous connective tissue
membranes (most skull bones and the clavicles, or collar
bones) or cartilage (all other bones).
Ossification occurs in two different ways, depending on
the "forerunner" tissue. Intramembranous (within a
membrane) ossification occurs in the bones built from
fibrous connective tissue membranes, and endochondral
ossification occurs in the cartilage "bones."
Intramembranous Ossification
All bones formed via intramembranous ossification are
flat bones.
1. Mesenchymal cells in the center of the membrane
differentiate into osteoblasts, which then begin to
secrete the organic bone matrix.
2. The osteoblasts then secrete the mineral portion of
the matrix. The matrix being formed is called woven
bone, an immature bone tissue that will eventually
become spongy, or compact bone, depending on the
location.
3. As the matrix is secreted the osteoblasts become
trapped in their lacunae, and then become mature bone
cells (osteocytes).
4. The ossified area fuse, and blood vessels and red
bone marrow fill the spaces between the trabeculae
("little beams" of spongy bone).
5. Mesenchymal cells form around the ossified portion of
the membrane and form a periosteum. Remember that the
periosteum is a double-layered membrane - the outer
portion is fibrous connective tissue, and the inner
layer is osteogenic, that is, it is "bone forming."
6. The periosteal osteoblasts begin to form bone plates
around the spongy bone middle. Think of an oreo cookie -
the filling is the spongy bone, and the cookies are the
compact bone. In intramembranous ossification the middle
is formed first, then the osteoblasts in the periosteum
form the "cookies."
7. The woven bone plates are "refined" into compact bone
and are mineralized.
Endochondral Ossification
Remember that endo means within, and chon is our root
word for cartilage. Endochondral ossification occurs
within cartilage bone models, and occurs in most bones
of the body. This process is somewhat more complicated
than intramembranous ossification because the cartilage
must be replaced with the bone.
1. Ossification begins at a primary ossification center.
In long bones this is in the center of the diaphysis.
2 and 3. The perichondrium (the membrane surrounding all
cartilage in the body) differentiates into a periosteum,
and the chondroblasts differentiate into osteoblasts.
4. The osteoblasts secrete osteoid around the cartilage,
forming a bony collar around the diaphysis. This again
is not the mature bone, but is known a woven bone, which
will be eventually replaced by compact bone.
5. Chondrocytes within the cartilage bone will undergo
hypertrophy, or an increase in cell size. Minerals will
then be deposited in the area, which lessens nutrient
delivery to the cartilage cells, which then die.
6. A periosteal bud invades the newly formed cavity in
the cartilage. The periosteal bud contains blood and
lymph vessels, nerves, and osteoblasts and osteoclasts.
7. The osteoblasts (from the periosteal bud) secrete
matrix around the cartilage remnants - forming
trabeculae.
8. Ossification spreads in both directions from the
primary ossification center. Osteoclasts will then
remove the newly formed bone, and open up the medullary
cavity.
9. After birth, secondary ossification centers form in
the epiphyses, and the same steps will occur, except
that the spongy bone is not reabsorbed by the
osteoclasts and no medullary cavity is formed.
Some of the cartilage in a long bone will remain
unossified to allow for bone growth (the epiphyseal
plate), and the cartilage at the top of the epiphyses
will remain unossified (the articular cartilage).
Bone Growth
A child's long bones grow in length via activity in the
epiphyseal plates, but bones also need to become thicker
as a child grows. This thickening process is known as
appositional growth.
Longitudinal Growth
This process is very similar to endochondral
ossification. The cartilage cells at the "top" of the
epiphyseal plate undergo mitosis, lengthening the bone.
Meanwhile, cartilage cells at the "bottom" of the EP
undergo growth not in the number of cells, but in the
individual size of the cells - hypertrophy. The
hypertrophied cartilage cells die as their matrix is
calcified, leaving spicules of cartilage. Osteoblasts
below the EP will then lay bone matrix around the
cartilage spicules, forming spongy bone, which, as
before, will be eventually be reabsorbed by the
osteoclasts.
This same process will occur in the articular cartilage
at the ends of the long bones.
Appositional Growth
Osteoblasts under the periosteum secrete matrix on the
outside surfaces of the bone, thus increasing the
thickness of the bone. Meanwhile osteoclasts on the
inside of the bone reabsorb some of the matrix to
prevent the bone from becoming too thick and heavy.
Bone Remodeling
Our bone tissue is continually being replaced and
remodeled to fit the demands we place on our skeleton.
Bone remodeling is done by osteoblasts and osteoclasts.
Osteoblasts will secrete new matrix wherever an injury
has occurred, or when added strength is needed.
Osteoclasts have a unique modification on their cell
surface - they have what is known as a ruffled border,
formed by protein filaments within the cells extending
the cell membrane out in raised projections. This border
acts like Velcro, helping to adhere the osteoclast to
the bone being reabsorbed. The osteoclasts secrete
enzymes and acids onto the surface of the bone breaking
down the matrix.
Two hormones are involved in regulating calcium levels
in the blood. Calcium is an important mineral - it is
necessary for muscle contraction and the transmission of
nervous impulses, among other things. When blood calcium
levels are low parathyroid hormone is secreted.
Parathyroid hormone is made in the parathyroid glands,
which lie underneath the thyroid gland in the neck. PTH
causes the osteoclasts to increase their activity -
releasing calcium from the bone matrix. When blood
calcium levels rise as a result of osteoclast activity
PTH will no longer be secreted, and the osteoclasts will
"shut off."
When there is excess calcium in the blood, calcitonin
will be secreted by the thyroid gland. Calcitonin will
cause calcium to be deposited in the bone matrix, thus
reducing calcium levels.
Fracture Repair
Your bones have an amazing ability to heal themselves -
in fact a doctor treating a broken bone is merely acting
like a catalyst, helping to speed up the healing
process. There are 4 major steps in fracture repair:
1. A hematoma forms at the site of injury - remember
that a hematoma is caused by ruptured blood vessels
leaking blood into the tissue space. Cells that are
normally served by these blood vessels begin to die.
2. A fibrocartilaginous callus is formed. New blood
vessels grow into the area, macrophages appear to help
clean up the debris, and fibroblasts and osteoblasts
from the periosteum and endosteum migrate to the injured
area. The fibroblasts secrete collagen fibers which
helps connect the bone ends. Some fibroblasts
differentiate into chondroblasts and begin secreting
cartilage in the area, thus forming fibrocartilage. An
excess amount of tissue is added around the outside of
the bone to help splint it - this tissue is known as the
external callus. (The internal callus, is of course, the
fibrocartilage directly between the broken bone ends.)
3. Formation of the bony callus. Once the
fibrocartilaginous callus is in place osteoblasts and
osteoclasts which migrated in from the periosteum and
endosteum begin to ossify the callus, thus converting
the fibrocartilage to a bony callus.
4. Bone remodeling occurs. The woven bone of the callus
is modified into spongy and compact bone depending on
location. A new medullary cavity is formed inside, and
the excess bone of the external callus is removed.
Bone Diseases
Osteoporosis
Osteoporosis is a very common bone disorder where bone
reabsorption occurs faster than bone deposition. This
leaves bones that are of normal composition, but are
thin and light - or porous. The picture below on the
left is normal spongy bone. The picture on the right is
osteoporotic bone.
Osteoporotic bone is easily fractured. One of the more
common sites of bone fracture is directly underneath the
ball of the femur - the so called broken hip. The
vertebrae can also be damaged by compression fractures.
What are the risk factors for developing osteoporosis?
·post-menopausal female - when estrogen production stops
the bones become less dense
·lack of exercise - remember that bone tissue adapts to
stresses put upon it. Weight-bearing exercise throughout
your life helps to strengthen bones
·dietary factors - inadequate calcium, protein, and
vitamin D
To learn more about osteoporosis, visit the web site of
the National Osteoporosis Foundation.
Osteomalacia
If you have the bone disease osteomalacia your bones are
poorly mineralized and soft. Bones which must bear a lot
of weight will deform. Rickets is the analogous disease
in children, but is more severe since children's bones
are growing rapidly.
The cause of osteomalacia is vitamin D deficiency.
Paget's Disease
Paget's disease occurs when bone is excessively
reabsorbed and formed. The bone formed is not of normal
composition - there is an excess of woven bone and a
lack of mineralization. |
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