CHAPTER 6 SKELETAL SYSTEM:
BONE TISSUE
Osteology--study
of bone & treatment of bone disorders
Anatomy--structure
of a long bone
Parts
of a typical long bone are listed and discussed on p. 172 – 173
Figure 6.1 P. 173
Bone
(osseous tissue) is a form of CT. It contains a large amount of matrix with
widely separated cells. Bone matrix is:
25% protein (collagen) fibers (organic)
25% water (inorganic)
50% crystallized mineral salts (inorganic)
bone matrix contains abundant
inorganic mineral salts. The most abundant one is calcium phosphate. It
combines with smaller amounts of calcium hydroxide and forms crystals of
hydroxyapatite. These crystals combine with calcium carbonate and ions of
magnesium, potassium, and sulfate. These salts are deposited in a network of
collagen fibers and, initiated by osteoblasts, the
whole mass hardens (calcification). Calcification cannot occur without the
collagen fibers. First the minerals must get in tiny spaces between fibers and
then crystallize (harden)
Hardness of bone—due to mineral salts
(inorganic)
Tensile strength of bone—means it holds
together and is not brittle—due to collagen fibers (organic)
1. Osteogenic cells—stem cells derived
from mesenchyme--can undergo mitosis. Some remain and continue to divide; some
differentiate into osteoblasts. Found in the periosteum, the endosteum and
canals in bone that contain blood vessels.
2. Osteoblasts--cells that form bone--do
not undergo mitosis. Secrete collagen and other organic components and actually
build all new bone. If we need more osteoblasts, they must develop from
osteogenic cells.
3. Osteocytes--mature bone cells that are
derived from osteoblasts. Principle cells of bone tissue. No mitosis. As
osteoblasts secrete bone matrix they surround themselves, leaving only a tiny
space they occupy, and mature into osteocytes, which do not secrete matrix.
Osteocytes carry on daily cellular activities of bone--exchange of nutrients
& wastes, etc.
4. Osteoclasts--develop from fusion of
many monocytes (phagocytic white blood cells) and settle in the endosteum.
Function is bone resorption (breakdown of matrix).
.
Despite appearances, bone is not completely solid. All types of bone have tiny spaces to provide channels for the blood vessels that supply it. Depending on size and distribution of spaces, bone tissue may be classified as compact or spongy.
1.
Compact (dense) bone—80% of skeleton--contains fewest spaces and therefore is
the strongest type of bone. Forms the external layer of all
bones and almost all of the diaphyses of long bones, which gives great strength
to allow the long bones to resist the stress of weight and movement.
Compact
bone is highly organized into a concentric ring structure, which allows
nutrients and O2 to reach all osteocytes with an absolute minimum of
spaces.
Central canal--open space containing
blood vessels—these run longitudinally
Concentric lamellae--rings of calcified
matrix
Lacunae--tiny spaces in matrix located
between rings--each completely filled with an osteocyte
Canaliculi ("little
canals")--radiate out from central canals to lacunae and also connect some
lacunae with each other. Contain ECF and processes (extensions) of osteocytes.
Nutrients diffuse through these canals.
Intersitital
lamellae (fragments of older osteons) fill spaces between concentric lamellae.
Circumferential lamellae encircle the bone just beneath the periosteum or
encircle the medullary cavity.
Blood
vessels from the periosteum reach the central canals through perforating
(Volkmann’s) canals, which run transversely.
2.
Spongy bone tissue—20% of skeleton--contains more spaces and has less strength.
Does not contain osteons--consists of lamellae arranged in an irregular
latticework made of thin plates of bone called trabeculae. Spaces between
trabeculae are small but macroscopic, and are filled with red marrow, which produces
blood cells. Osteocytes are found on superficial surfaces of lacunae. Blood
vessels from the periosteum penetrate spongy bone and provide blood that
circulates through the marrow cavities.
Spongy
bone is lighter in weight than compact. Its numerous spaces between trabeculae
provide the safest possible location for the red bone marrow, which must
continue to produce large numbers of blood cell throughout life.
Spongy
bone makes up most of short, flat and irregular bones (under a thin layer of
compact bone) and almost all of the epiphyses of long bones. Blood cells are
formed mostly in red marrow of spongy bone of the:
Hipbones
Ribs
Sternum
Vertebrae
Skull
Epiphyses of some long bones
Bone
tissue has a rich blood supply. Blood vessels pass into bones from the
periosteum.
1.
Usually a large artery called the nutrient artery penetrates the diaphysis of a
long bone near the center, entering through an opening called the nutrient
foramen. The nutrient artery reaches the medullary cavity and divides into
proximal and distal branches. These supply the red bone marrow and the inner
part of the compact bone of the diaphysis. Eventually, tiny branches reach the
haversian canals.
2.
Smaller arteries from the periosteum also enter the diaphysis at many points
through perforating (Volkmann’s) canals. These supply the outer part of the
compact bone of the diaphysis.
3.
Epiphyses and metaphyses are supplied by arteries in those areas, which also
supply associated joints.
Corresponding
veins drain the blood away.
1.
Sometimes a nutrient vein accompanies the nutrient artery.
2.
Many small veins are usually found in the epiphyses & metaphyses
3.
Many very small periosteal veins complete the job
Nerves
accompany blood vessels deep into the bone and nerves are also found in the
periosteum.
Figure 6.4 P. 177
PHYSIOLOGY OF OSSIFICATION
FORMATION OF BONES IN THE EMBRYO, FETUS, INFANT AND CHILD (BUILDING BONE FOR THE FIRST TIME)
The
embryo first forms a skeleton made of mesenchymal cells, which are shaped
somewhat like bones. The mesenchyme is
replaced mostly by hyaline cartilage with some areas formed by fibrous CT
membranes. These are also shaped more or less like the finished bones will be.
Both of these tissues are derived from mesenchyme. This is complete by the 6th
week and the formation of bone (ossification or osteogenesis) begins at that
point. Two kinds of bone formation take place:
1.
Intramembranous ossification--formation of bone directly on or within the
fibrous membranes. Forms most of the flat bones of the skull,
the mandible and the clavicles. This type has no cartilage model
involved.
a. Mesenchyme cells cluster in the fibrous
membrane, differentiate into osteogenic cells and then into osteoblasts. This
cluster is known as a center of ossification and a large number form, scattered
over the membrane.
b. The osteoblasts secrete a collagenous
fiber network in which Ca salts are deposited.
c. As a cluster of osteoblasts becomes
completely surrounded by matrix, the plate of bone thus formed is called a
trabecula and trapped osteoblasts become osteocytes.
d. Trabeculae fuse into the latticework of
spongy bone and spaces fill with red bone marrow. Blood vessels grow into the
space between trabeculae.
e. The original fibrous membrane condenses
and becomes the periosteum
f. Some of the surface layers of spongy
bone are reorganized into compact bone.
2.
Endochondral ossification--replacement of hyaline cartilage by bone. This forms
most of the bones of the body. It is best observed in long bones and the tibia
is most often used as the typical example.
a. Mesenchyme cells become chondroblasts,
which produce a hyaline cartilage model of the bone. A membrane called the
perichondrium develops around the cartilage model. The cartilage model grows
with the baby. Cartilage "bones" are solid.
b. Always at a point in the middle of the
diaphysis, steps begin which will lead to ossification. Some cartilage cells
enlarge and burst, raising the pH of the cartilage matrix and causing it to
calcify.
c. Remaining cartilage cells can no longer
receive nutrients (by diffusion) so they also die.
d. In this same area, a nutrient artery
penetrates into the cartilage model. This stimulates osteogenic cells in the
perichondrium to differentiate into osteoblasts.
e. The osteoblasts lay down a thin shell
of bone called the periosteal bone collar under the perichondrium, still in the
center of the diaphysis.
f. The part of the perichondrium that now
surrounds bone instead of cartilage is called the periosteum.
g. Capillaries of the periosteum begin to
grow into the calcified cartilage matrix. This leads to the development of the
primary ossification center (still in the center of the diaphysis). The
cartilage is replaced by spongy bone.
h. Spongy bone is formed through the
entire thickness of the diaphysis, but then osteoclasts break down the bone in
the center, forming the marrow cavity.
i. Ossification spreads both directions
from the center, up and down the shaft.
j. At a later time (about the time of
birth) separate secondary ossification centers appear at each epiphysis and begin
to change more cartilage to bone by the same process, except that spongy bone
fills the interior of the epiphyses, which have no central medullary cavity.
k. The hyaline cartilage of the model
remains in 2 areas:
1) Ends of the epiphyses (articular
cartilages)
2) Between the diaphysis and
epiphysis, where it forms the epiphyseal plate and is responsible for
lengthwise growth
Epiphyseal plate--area of hyaline cartilage that remains
between the diaphysis and epiphyses (Figure.
6.7 P. 182) 1.
Zone of resting cartilage-- anchors
epiphyseal plate to epiphysis
2.
Zone of proliferating cartilage--stacks of
young chonddrocytes that continuously
divide
3. Zone of hypertrophic
cartilage--new
cartilage cells mature
4. Zone of calcified cartilage--cartilage cells
die and matrix becomes calcified.
Osteoclasts
remove calcified cartilage matrix and osteoblasts spread into the area and lay
down bone. New chondrocytes are formed on the epiphyseal side of the plate and
old chondrocytes are replaced by bone on the diaphyseal side. The bone of the
diaphysis steadily increases in length. Spongy bone is formed first and later
replaced by compact bone.
All
lengthwise growth of bone must occur at the epiphyseal plate. This occurs
throughout childhood until early adulthood. A fracture in the area of the
epiphyseal plate before growth is complete often causes that bone not to reach
normal length.
As
growth ends, cartilage cells of the epiphyseal plate die and are no longer
replaced by new ones. The epiphyseal plate becomes completely ossified (fusion
of epiphyses) and is now called the epiphyseal line. All epiphyses are
generally fused by age 25 or before.
Growth
in diameter occurs as osteoblasts from the periosteum add new bone on the
surface. This is known as appositional growth. As the bone thickens,
osteoclasts enlarge the marrow cavity.
Bone
must continually renew itself throughout life. Remodeling is the replacement of
old bone tissue with new bone tissue. Bone is never at rest. Matrix is
redistributed according to stress. Compact bone is formed from spongy bone.
Worn and injured bone is removed and replaced with new tissue. We hope to keep
a fairly steady balance between breakdown and building.
Remodeling
occurs at different rates in various body regions.
Distal femur--completely replaced every 4
months
Some bones may not be replaced or only
over years
Osteoclasts
are the cells of bone resorption. They remove both minerals and collagen fibers
in balance with replacement by osteoblasts. An osteoclast attaches tightly to
the bone surface, forming a leak-proof seal. The part of its plasma membrane
next to the bone is arranged in deep folds, forming what is called the ruffled
border. The osteoclast then secretes:
Enzymes which break down collagen
Acids which dissolve mineral salts
Materials
released from bone are absorbed into the bloodstream
Adequate
supplies of minerals and amino acids are necessary for the building of new
bone. Hormones continue to play a role. Sex hormones and hGH encourage the
maintenance and rebuilding of bone. Calcitonin and parathyroid hormone also
influence the process. Exercise (stress on bone) plays a vital role in
maintaining or increasing the strength of bone.
For
normal growth and remodeling we need:
1.
Minerals
Calcium
Phosphorus
Fluoride
Magnesium
Iron
Manganese
2.
Vitamins
Vit D--calcitriol allows us to absorb Ca
from the GI tract
Vit A
Vit C
Vit B12
Vit K
3.
Hormones
a. Insulin-like growth factors (IGFs)
produced in response to hGH (human growth hormone)--pituitary gland--general
growth of all tissues--great effect on height. This hormone causes cells to
produce insulinlike growth factors, which promote osteoblast production and
protein synthesis. Even after we have achieved full growth, this hormone plays
a major role in bone maintenance.
b. Sex hormones
(estrogens/testosterone)--appear in large amounts at puberty and promote
activity in the epiphyseal plate--but after a period of rapid growth the
cartilage cells "burn out" and all die--epiphyses then fuse. In
mature individuals the sex hormones promote bone maintenance and replacement.
In females, estrogen production tapers off beginning in the thirties, and this
is one reason for the problems with osteoporosis.
c. Calcitonin/Parathyroid
hormone—discussed in their own section
d.
Insulin
e. Thyroid hormones
f.
Anabolic steroids—tend to cause premature fusion of epiphyses
FRACTURE REPAIR
Fracture--any
break in a bone
See
P. 186 for list of types of fractures & pictures of some in Figure 6.9 P.
185
Reduction--aligning
the broken ends of the bone properly
Closed
reduction--done by means of only a cast, splint, etc.--ends are placed in
proper alignment and immobilized while healing takes place
Open
reduction--surgery--using pins, plates, screws, and/or wires to align
fragments--then also may have a cast or splint for support
Bone
healing is slow--bone cells grow slowly and deposition of matrix is slow.
Complete healing of a fracture takes about 1 year, although the bone is usually
functional in 6-8 weeks.
Steps
in fracture repair:
1.
FORMATION OF THE FRACTURE HEMATOMA: Blood vessels crossing the fracture line
are broken and blood forms a clot called the fracture hematoma (within 6-8
hours of the injury). Bone cells and periosteal cells in the immediate area of
the fracture die. Swelling and inflammation occur and bring phagocytic white
blood cells, which aid osteoclasts in removing dead tissue and debris. This
continues for several weeks after the injury. No real healing occurs here---this
is mainly a clean-up job.
2.
a. FORMATION OF THE PROCALLUS: Increased numbers of capillaries grow into the
area and the fracture hematoma becomes a procallus made up of granulation
tissue.
b. FORMATION OF THE FIBROCARTILAGE (SOFT)
CALLUS: Fibroblasts and osteogenic cells from the periosteum, endosteum and
marrow invade the procallus. The fibroblasts produce collagen fibers and
osteogenic cells from the area right at the break develop into chondroblasts
and form fibrocartilage, transforming the procallus into a
fibrocartilaginous (soft) callus. The
callus is a mass of repair tissue that bridges the broken ends. The two parts
of this stage require about 3 weeks.
3.
FORMATION OF THE BONY (HARD) CALLUS: Osteogenic cells in healthy bone tissue
(further away from the break) become osteoblasts and begin to produce spongy
bone, which joins the original fragments. The fibrocartilage of the soft callus
is converted to spongy bone and is then called a bony or hard callus. This
stage lasts 3-4 months.
4.
REMODELING OF THE CALLUS: Dead bone is reabsorbed by osteoclasts. Compact bone
replaces spongy bone. The thickened callus becomes more normal in appearance,
although a thickened area may remain. This stage lasts up to 6 months or more.
Bone
is the major calcium reservoir in the body. Calcium level in blood must be
carefully regulated. In extreme cases disturbances can result in death:
Too high--cardiac arrest
Too low--respiratory arrest
Nerve cells require precise levels to function
Calcium is involved in muscle contractions
Many enzymes require Ca as the cofactor
Blood clotting requires Ca
Regulated
by hormones, bone releases or takes up calcium to keep the blood level within
normal limits. These 2 hormones causing this are released with no regard to
bone strength, although they have a great influence over it.
1. Parathyroid hormone (PTH)--receptors in
the parathyroid glands detect a drop in blood Ca and this causes a release of
PTH. The effect is to increase the number and activity of osteoclasts, which
break down bone matrix and release Ca into the blood. In addition, PTH
decreases Ca loss in urine and enhances activation of Vitamin D. Osteoclasts
are just as likely to break down healthy bone as not. The only concern is
raising blood Ca.
2. Calcitonin--secreted by special cells
in the thyroid gland when blood Ca level rises above normal. It inhibits the
activity of osteoclasts and speeds deposition of Ca from the blood into the
bones by osteoblasts. Of course, this would strengthen bone, but that is not
the main reason we produce calcitonin. We are putting the calcium into bone to
givve a safe place to store in until it is needed.
Bone
can alter its strength in response to mechanical stress (pull of skeletal
muscles, pull of gravity, as well as lifting weights). Bone becomes stronger
with increased stress, so exercise is essential for bone health. It strengthens
bone by:
1. Increasing deposition of mineral salts
2. Increasing production of collagen fibers
3. Increasing production of calcitonin
With
no stress, bone remodeling gets out of balance, with more resorption than
deposition of new bone. Bone loses both collagen and minerals--loss of up to 1%
of bone mass per week.
Bedridden
Limb in cast
Astronauts
2
main effects:
1.
Loss of Ca and other minerals--begins around age 30 in women and accelerates at
40-45. Average loss of Ca 30% by age 70. Read about osteoporosis p. 189-190.
Also occurs in males but begins later (around 60) so has less years to progress
& male bones are often stronger to begin with.
2.
Decrease in rate of protein synthesis--decrease in collagen fibers which also
weakens bone--bones become brittle.