OSSEOUS TISSUE AND SKELETAL STRUCTURE
The skeletal system consists of 206 named bones, cartilage, ligaments, and
connective tissues that connect or stabilize bones. Functions of the skeletal system
include: support for the body by providing a rigid framework, storage site for
minerals such as calcium and phosphorus and lipids in yellow marrow. The
skeletal system is an important site for production of blood cells. Red blood cells,
white blood cells and other blood elements are formed in red marrow. Another
function of the skeletal system is to protect delicate tissues and organs by
surrounding them with skeletal elements. For example the brain is encased in the
skull and the heart and lungs are surrounded by the boney sternum and rib cage.
One of the most important functions of the skeletal system is that of leverage
which allows for movement due to an interaction of the muscular and skeletal
systems. Bone tissue aids in acid-base balance in the body by absorbing or
releasing alkaline salts.
The skeletal system is divided into two major parts: the axial and the appendicular
skeleton. The axial skeleton consists of bones forming the axis of the body. These
support and protect organs of the head, neck and trunk and include the skull,
sternum, ribs and vertebral column and include the hyoid bone, the sacrum and the
coccyx. The appendicular skeleton consists of bones that anchor appendages to the
axial skeleton. These include both upper and lower extremities, shoulder and pelvic
girdles. It should be noted that the auditory ossicles are not a part of either skeleton
but for convenience are grouped with the axial skeleton.
Types of Bones
Bones are classified into several categories. Long bones are longer than they are
wide. They function as levers that act on skeletal muscles to produce movements.
Long bones can be found in the appendages, even fingers and toes. Short bones are
boxy and small; nearly cube-shaped and can be found in the wrist-carpals and
ankle-tarsals. They have limited movements. Thin bones that have roughly parallel
surfaces are called flat bones. These are found in the roof of the skull, sternum,
ribs, and scapula. They serve to enclose and protect soft organs and provide broad
surfaces for muscle attachment. Irregular bones include all that do not fall into any
of the previous categories. They have varied, complex shapes, sizes, and surface
features. Examples include the vertebrae, sacrum, coccyx, temporal, sphenoid,
ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid.
Sesamoid bones are shaped like sesame seeds. They develop in areas where there
is a great deal of friction. Most are only a few millimeters and the number in each
person is different however the patella are typically present in everyone. Finally
there are sutual bones. These are small bones located in sutures and are classified
by location and not by shape.
Bone Composition
Bone tissue is also called osseous tissue. It is a type of supporting connective
tissue composed of an extracellular matrix which contains protein fibers and
specialized cells. The matrix is hardened by calcium phosphate deposits. Calcium
phosphate combines with calcium hydroxide to make hydroxyapatite
Ca10(PO4)6(OH)2which is deposited around collagen fibers. Calcium phosphate
makes up 2/3rd of bone weight and 1/3 is provided by collagen. This hardening and
deposition of the calcium salts around the collagen fivers is called calcification.
Bone is hard because of the calcium salts and flexible due to the collagen fibers.
The minerals are deposited in such a fashion as to give bone a very high tensile
strength which makes it resistance to stretching and to be torn apart.
Bone Cell Types
There are four principle types of bone cells: osteogenic cells, osteoblasts,
osteocytes and osteoclasts. Osteogenic cells are unspecialized bone stem cells.
They are found in the outer (periosteum) and the inner (endosteum) coverings of
bone and in the central canals. They are the only type of bone cell that can divide.
They continually divide some go on to become osteoblast cells.
Osteoblasts are bone-building cells. The make and secrete the organic matter of the
bone matrix. New bone matrix is made through osteogenesis; a process which
makes and releases proteins and organic components of the matrix. This substance
is called osteoid and represents bone matrix before calcium salts have been added.
Osteoblasts lay down matrix until they are surrounded by it at which time they
become trapped nd are called osteocytes.
Osteocytes are mature bone cells and represent most of the bone cell population.
These cells reside in lacunae (little lakes); one cell/lacuna. Osteocytes cannot
divide. They function to maintain and monitor protein and mineral content of the
matrix. They participate in bone repair by converting back into osteoblasts or
osteogeneic cells at the site of injury. They are strain sensors and regulate bone
remodeling. Neighboring osteocytes are linked by gap junctions which permit
exchange of ions and small molecules
Osteoclasts are bone dissolving cells. They are formed by fusion of several stem
cells and therefore are quite large and contain several nuclei. They function to
remove bone by osteolysis or resorption. Osteoclasts secrete acids and proteolytic
enzymes which degrade minerals and fibers and dissolve boney matrix. This
releases matrix components into the blood and restores calcium and phosphorus
concentrations in body fluids.
The activities of osteoblasts and osteoclasts are continuous. Their actions must be
balanced. When osteoclasts remove calcium faster than osteoblasts can deposit
itbone weakens. When osteoblast activity predominatesbones get stronger and
more massive.
Types of Bone Tissue
There are two types of bone tissue: compact or dense bone and spongy or
cancellous bone. Compact bone is dense, hard, and relatively solid. It forms the
protective exterior of all bones. Spongy bone is found inside most compact bone. It
is very porous or full of tiny holes which form open networks of struts and platescalled trabeculae. Most bones contain both types of bone tissue. Spongy bone can
withstand stresses from many directions. It is lighter than compact bone and
reduces skeletal weight which makes it easier for muscles to move bones.
Compact Bone Structure
Compact bone has few spaces. It is the strongest type of bone. It provides
protection and support. It resists the stress produced by weight and movement. It is
made up of repeating, structural units called osteons or Haversian system. Each
osteon consisist of concentric circles or layers called lamellae around a central or
Haversian canal. The central canal runs parallel to the surface and contains blood
vessels. The rings are composed of mineralized, extracellular matrix. Between the
concentric rings are lacunae which each contain one osteocyte. Radiating from
each lacunae are canaliculi. Canaliculi run through layers connecting osteocytes to
each other. Interstitial lamellae fill the spaces between osteons. Perforating the
central canal are Volkmann’s canals or perforating canals which run perpendicular
to the surface. These canals carry blood vessels to deeper osteons and to marrow
cavity tissues.
Spongy Bone Tissue
The matrix composition of compact and spongy bone is the same. Osteocytes,
canalicui and lamellae have different arrangements. Spongy, trabecular or
cancellous bone has no osteons. This type of bone is found in the interior and is
protected by compact bone. The matrix is arranged in irregular patterns of thin
columns or struts called trabeculae (little beams) which form a thin, branching
open network filled with red marrow. This type of bone is light and reduces the
weight of the skeleton making it easier to move. This type of bone is located
where stresses come from several directions such as the proximal epiphysis of the
femur or where bone is not heavily stressed such as the flat bones like the sternum.
Bone Type and Bone Tissue Type Location
The relationship between compact and spongy bone and the relative proportions of
each varies with bone shape and with the function of the bone. In long bones, the
body, diaphysis or shaft is long and cylindrical and curved in 2 planes which give
strength. The central cavity, medullary canal or marrow cavity is filled with
marrow. This cavity is continuous with the expanded extremities at either end of
the bone-the epiphysis. The wall of the diaphysis is comprised of dense, compact
tissue which is thick in the middle and thinner toward the extremities. Within the
medullary canal cancellous tissue is found. The extremities or epiphyses are
expanded for articulation with other bones (they form the joints) and have broad
surfaces for muscle attachment. Here articular cartilages can be found. They are
composed of cancellous tissue surrounded by a thin layer of compact bone. The
diaphysis is connected to the epiphysis by the metaphsis which is only visible in
growing bones of children. The marrow cavity is filled with bone marrow, a type
of loose connective tissue. Yellow bone marrow is dominated by fat cells and red
marrow is responsible for forming blood cells. The periosteum, a tough connective
tissues sheath attaches to the underlying bone by Sharpey’s fiber. Lining the
medullary cavity is the endosteum which consists of one layer of bone forming
cells.
The function of flat bones is to provide protection for underlying structures or
broad surfaces for muscle attachment. Their function can be seen by their structure.
Flat bones resemble a spongy bone sandwich, composed of 2 thin layers of
compact bone covering a layer of spongy bone. Bone marrow is present however
there is no marrow cavity.
Short Bones combine strength and compactness along with limited movement.
Their structure reflects function and include a cancellous tissue covered by a thin
crust of compact substance.
Irregular bones are made of cancellous tissue enclosed in a thin layer of compact
bone.
Periosteum & Endosteum
A periosteum covers all portions of compact bone except at joint cavities. It has a
fibrous outer layer and an inner cellular layer. It provides a route for blood vessels
and nerves and participates in bone growth and repair. It is continuous with other
connective tissues that mesh with bone such as tendons and ligaments. Collagen
fibers from tendons and ligaments mesh into the periosteum. Perforating or
Sharpey’s fibers bond tendons and ligaments into the general structure of bone
making attachments strong. The endosteum consists of an incomplete cellular
layer. It lines marrow cavities, covers trabeculae of spongy bones and lines inner
surfaces of central canals. The endosteum is active during bone growth, repair, and
remodeling via action of osteogenic cells.
Blood & Nerve Supply
Osseous tissue is very vascular. Vessels pass into the bone through the outer
covering-the periosteum. Periosteal arteries enter the diaphysis via perforating
canals. These supply superficial osteons of the bone shaft. Near the center of the
diaphysis there is a large nutrient artery that passes through the nutrient foramen.
The periosteum also contains an extensive network of lymph vessels and sensory
nerves.
Bone Development & Growth
The formation of bone is called ossification or osteogenesis. The skeleton begins
to form at 6 weeks post fertilization and does not stop until around age 25. Bone
develops by two methods-intramembranous and endochondral ossification. Bone
forms from mesenchyme or fibrous connective tissue via intramembranous
ossification or by replacement of pre-existing hyaline cartilage models in a
process called endochondral ossification.
Intramembranous ossification
Intramembranous ossification produces the flat bones of the skull. Most of the
facial bones, the mandible and the medial part of the clavicle. There are several
steps to intramembranous ossification.
Step1: Development of Ossification Center
At the site where the bone is to be, chemical messages cause mesenchymal cells
(embryonic connective tissue) to cluster together into a layer of soft tissue with a
dense capillary supply. Cells enlarge and differentiate into osteogenic cells and
then in to osteoblasts. This site is called the ossification center. Osteoblasts begin
to secrete the organic matrix and eventually become trapped at which time they
become osteocytes.
Step 2: Calcification
Calcium and other salts begin to deposit on the organic extracellular matrix made
by the osteoblasts. As the trabeculae continue to grow calcium phosphate is
deposited. This causes the matrix to harden or calcify.
Step 3: Formation of Trabeculae
Osteoblasts continue to deposit matrix and it continues to be calcified producing
the struts of the bony trabeculae. Connective tissue that is present differentiates
into red bone marrow.
Step 4: Development of the Periosteum
Mesenchyme condenses at the periphery of the bone becoming the periosteum.
Trabeculae at the surface continue to calcify until spaces between them are filled in
converting the spongy bone to compact bone. The process gives rise to the
sandwich like arrangement of flat bones.
Endochondral ossification
Endochondral ossification is the way most bones are made. In this process bone
develops from a preexisting hyaline cartilage model. It begins around the sixth
week of fetal development and continues into the 20’s. There are several steps to
endochondral ossification.
Step 1: Development of the Hyaline Cartilage Model
At the site when the bone will form chemical messengers cause mesenchymal cells
to crowed together in the general shape of the future bone. The cells then develop
into chondroblasts. These cells begin to secrete cartilage extracellular matrix which
develops into a hyaline cartilage bone covered with a fibrous perichondrium.
Step 2: Growth of Cartilage Model
Once the chondroblasts become embedded in the extracellular matrix they become
chrondrocytes. The cartilage model continues to grow longer from either end via
interstitial or endogenous growth. It also grows in diameter or thickness in a
process termed appositional or exogenous growth. Here new cartilage is laid on
the outside of the model by chondroblasts. As the model continues to grow
chondrocytes in the area get larger in the mid-region area and the cartilage matrix
begins to calcify. The enlarged chondrocytes are deprived of nutrients due to their
size and calcification and diffusion cannot occur. They die and disintegrate. As
they die they leave spaces which merge into small cavities called lacunae.
Step 3: Development of the Primary Ossification Center
Ossification continues inward from the surface of the bone to the inside in the
middle of the model- the primary ossification center. A nutrient artery penetrates
the perichondrium. This stimulates osteogenic cells there to become osteoblasts.
Once this occurs the perichondrium is termed the periosteum. In the primary
ossification center most of the cartilage will be replaced with bone. Osteoblasts
begin to deposit a thin collar of boney matrix around the middle of the cartilage
model forming the trabeculae of spongy bone. This primary ossification spreads
from the central area toward both ends of the cartilage model.
Step 4: Development of the Medullary Cavity
As the primary ossification center grows to the ends of the bone, osteoclast cells
break down some of the newly formed spongy bone trabeculae. This leaves a
cavity. Capillaries and fibroblasts migrate to the inside of the cartilage and take
over the spaces left by the dying chondrocytes. As the center is hollowed out and
filled with blood and stem cells, it becomes the primary marrow cavity. The region
of transition from cartilage to bone at the end of the primary marrow cavity is
called the metaphysis.
Step 5: Development of the Secondary Ossification Centers
When branches of the epiphyseal artery enter the epiphyses the secondary
ossification centers form. Bone formation is similar to as described in the center of
the bone. Here however spongy bone remains in the entire part of the epiphyses.
Secondary ossification proceeds outward from the center of each epiphysis toward
the outer surface of the bone.
Step 6: Formation of Articular Cartilage & Epiphseal Growth
Hyaline cartilage covering the epiphyses develop into articular cartilages. During
infancy and childhood the epiphyses fill with spongy bone; cartilage is limited to
the articular cartilages covering each joint surface. Prior to adulthood there is
some hyaline cartilage that remains between the diaphysis and the epiphysis. This
is called the epiphyseal or growth plate. This is the area where bone will continue
to grow in length until it becomes adult sized.
Bone Growth
Ossification continues throughout life with growth and remodeling of bone. Bone
increases in length and width. Bone increases in length at the epiphyseal plate. This
is called interstitital growth. Two events are involved in the growth in length of
bones. 1) interstitial growth of cartilage on the epiphyseal side of the epiphyseal
plate and 2) replacement of the cartilage on the diaphyseal side of the plate by
endochondrial ossification.
The epiphyseal plate consists of four zones: zone of resting cartilage, zone of
proliferating cartilage, zone of hypertrophic cartilage and zone of calcified
cartilage. The result is a plate consisting of hyaline cartilage in the middle with a
transitional zone on either side. The epiphysis makes cartilage and ostoblasts try to
overtake it by making bone. In the zone of resting cartilage there are small
chondrocytes present which do not participate in bone growth. These cells anchor
the plate to the epiphysis. The zone of proliferating cartilage contains slightly
larger chondrocytes that undergo interstitial growth. The cells divide replacing
those that die on the diaphysis side of the plate. The zone of hypertrophy contains
large, maturing chondrocytes arranged in columns. The zone of calcified cartilage
contains few cells. The cells here are mostly dead due to the extracellular matrix
around them having been calcified and no blood or nutrients can reach them.
Osteoclasts dissolve the cartilage matrix and osteoblasts and capillaries from the
diaphysis invade the plate and osteoblasts begin to make boney matrix.
Chondrocytes proliferate on the epiphyseal side of the plate. New chondrocytes
replace the ones destroyed by calcification. Bone on the diaphysis side increases.
Osteoblasts cannot catch up with the proliferating chondrocytes and so bone gets
longer. At puberty rising levels of sex and thyroid hormones cause the balance to
shift and osteoblasts begin to outpace the manufacture of cartilage at the
epiphyseal end. The epiphyseal plate gets narrower. The growth plate eventually
fuses shut, leaving an epiphyseal line and completes the length of the bone.
The diameter of the bone can still increase through appositional growth. In this
type of growth new tissues is deposited at the surface of the bone. At the bone’s
surface periosteal cells differentiate into osteoblasts. These begin to secrete the
organic parts of the matrix. The oteoid tissue is calcified and as the osteoblast
becomes trapped they become osteocytes. These lay down matrix in layers parallel
to the surface and produces the circumferential lamellae of bone.
Bone Dynamics
Bones constantly adapt to demands placed on them and are continually remodeled
throughout life. Organic and mineral components are continuously recycled and
removed through remodeling. This process continues through life as part of
normal growth and maintenance. Remodeling can replace matrix leaving the bone
undamaged or it can change the shape, internal architecture or mineral content of
the bone. The turnover rate for bone is high. 10% of skeleton tissue is replaced
each year. Remodeling gives bone the ability to adapt to new stresses. Wolff’s law
states that bone’s structure is determined by the mechanical stresses places on it.
One such stress that can change bone is exercise. Exercise is a type of stress which
builds more bone. When bone is stressedmineral crystals generate small
electrical fields which attract osteoblasts which make bone. Such electrical fields
are being used in medicine to stimulate repair of severe bone fractures. Heavily
stressed bones become thicker and stronger. The bony landmarks or bumps and
ridges on the surface of bone where tendons attach may become more pronounced
as muscles work to withstand increased forces. Regular exercise is needed to
maintain normal bone structure. Bone degeneration results from inactivity. Non
stressed bones become thin and brittle. A few weeks of immobility such as
weightlessness conditions or being bedridden or paralyzed can cause up to 1/3 of
the bone’s mass to be lost. Bone will rebuild just as quickly upon reuse. It should
be noted that changes in mineral content does not necessarily change the shape of
bones because boney matrix contains protein fibers. Bones may look okay but may
be soft due to no mineral deposition. This condition is called osteomalacia. One
form of this disorder is rickets which is typically due to a vitamin D3 deficiency.
Bones, not properly mineralized are flexible so the legs will bend under the weight
of the body. Children with rickets often have bowed legs.
Hormonal & Nutritional Effects
Growth of bone requires several nutritional and hormonal elements. Normal bone
growth and maintenance requires calcium and phosphate salts as well as
magnesium, fluoride, and manganese. The hormone calcitriol made by the
kidneys is needed for absorption of calcium and phosphate from the GI tract which
is needed for normal growth and maintenance of bone. Synthesis of calcitriol
depends on Vitamin D3; therefore Vitamin D3 is also needed for proper bone
growth. Vitamin C is another component essential for proper bone growth. It is
needed for enzymatic reactions needed for collagen synthesis and to stimulate
osteoblast differentiation. Without vitamin C there is a loss of bone strength and
mass, a condition called scurvy. Vitamin A stimulates osteoblast activity. It is
especially important for bone growth in children, Vitamins K, and B12 are needed
for protein synthesis.
Growth hormone and thyroxine stimulate bone growth. A proper balance is
needed to maintain normal activity at the epiphyseal cartilage until puberty. Sex
hormones-androgens in males and estrogens in females are responsible for growth
spurts seen in teens at puberty. These sex hormones are involved in closing the
epiphyseal plates. They stimulate osteoblasts to produce bone at a rate faster than
epiphyseal cartilage can expand. The time of epiphyseal closure differs from bone
to bone and individual to individual. For toes it is 11 years and for the pelvis and
wrists, it is 25 years. Estrogens close the epiphyseal plate faster than androgens
which is usually why females are shorter than males.
Calcium Balance
Bones serve as mineral reservoirs and have a primary role in homeostasis. Calcium
is the most abundant mineral in the body. 90% of it is found in bones. Calcium is
crucial to membrane functions and to activities of neurons and muscle cellsespecially cardiac muscle cells. Calcium concentration affects sodium permeability
of excitable membranes. A 30% increase in calcium will cause sodium
permeability to decrease making membranes less responsive. As calcium levels
decreases, sodium permeability increases making cells extremely excitable. A 35%
drop in calcium will cause nerves to be so excitable that a person may have
convulsions or muscular spasms. A 50% drop results in death. Parathyroid
hormone made by the parathyroid gland is the most important hormone needed for
homeostatic control of calcium levels.
Parathyroid hormone has several targets including the bones, kidneys and the
intestines. Homeostatic regulation is via a negative feedback mechanism. When
calcium levels fall, parathyroid glands (embedded in the thyroid gland) secrete
parathyroid hormone. Parathyroid hormone raises blood calcium levels through 3
effects: 1) it stimulates osteoclasts and increases release of calcium from bones, 2)
it promotes calcium reabsorption by the kidneys and 3) it increases uptake of
calcium by the intestines by stimulating the formation of calcitriol (the active form
of vitamin D.
Another hormone form the thyroid gland, calcitonin is used to lower blood calcium
levels. As blood calcium levels rise, parafollicular or C cells in the thyroid gland
release calcitonin. Calcitonin inhibits osteoclast activityslowing rate of calcium
release from bone and stimulates osteoblasts which encourages calcium to be
deposited into bones. This hormoen is more important during childhood. It is also
important in reducing the loss of bone mass during prolonged starvation and during
late stages of pregnancy. Its role in healthy adults is unknown.