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Bone or Osseous Tissue
The skeleton is an endoskeleton. It is a living structure, very metabolically active. It is capable of growth,
adaptation to bearing weight and repair. It has within it living cells, a blood supply and a nerve supply. The
endoskeleton grows at the same time as the rest of the body grows. The skeleton is made up of osseous tissue
(bone), bone marrow (blood cell forming tissue), cartilage, dense connective tissue (ligaments and tendons) and
adipose tissue.
I. Functions of Bone
A. Static Functions
1. Support
The bones of the skeleton act as a structural framework of the body and because it is rigid it can bear
weight.
It supports soft tissue and provides points of attachment for muscle via the tendons.
2. Movement
Some bones meet in moveable joints. The skeletal muscles attach to the skeleton by the tendons.
On contracting the skeletal muscles, the tendons pull on the bones to effect movement. The skeleton
plays a role in the kind and extent of body movement.
3. Protection
The skeleton protects the internal organs of the body.
a. Skull and Vertebral Column – protect the brain and spinal cord.
b. Thoracic (Rib) Cage - protects the thoracic organs - lungs, heart, blood vessels and thymus gland.
c. Pelvis - protects the lower digestive system organs, urinary bladder and reproductive organs.
B. Dynamic Functions
1. Mineral Reservoir
Ca, P, Na, K and other minerals are stored in the bones, which contribute to the strength of the bones
and the release these minerals maintain homeostatic levels of these ions in the body.
2. Hemopoiesis
The red bone marrow of certain bones is the site of blood cell formation. The marrow is housed in the
interior spaces of the bone.
3. Storage of Energy
The yellow marrow stores energy as fat.
II. Structure of Bone
Using long bone as an example. The long bones are longer than they are wide and are slightly curved. These
bones have a long axis. The shaft of the bone is the diaphysis, the ends of the long bone is the epiphysis. The
long bones have a relatively thick amount of compact bone to the outside and cancellous (spongy) bone to the
inside.
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A. Gross Anatomy
1. Shaft or Diaphysis
The diaphysis is mainly compact bone. In the center of the diaphysis is a hollow cavity, the medullary
or marrow cavity. In the adult, the cavity is filled with fat (yellow marrow cavity). In young
individuals especially newborns, the marrow is red bone marrow (hemopoietic tissue). The medullary
cavity is lined by a thin layer of connective tissue, the endosteum. It is from the endosteum that the
bone can grow in thickness from the inside by appositional growth. The cancellous or spongy bone is
covered by the endosteum.
2. Ends of the Long Bone or Epiphysis – At the proximal and distal ends of the long bone
The epiphysis has a thin layer of compact bone on the outer surface of the ends of the long bone.
The central region of the epiphysis is filled with interconnecting plate or trabeculae of cancellous bone.
The spaces between the trabeculae of the cancellous bone contain red bone marrow.
3. Metaphysis – It is the region where diaphysis and epiphysis join.
In children, the diaphysis and epiphysis are separated by an epiphyseal hyaline cartilage plate that
provides for the bone to increase in length by interstitial growth. The cartilage plates are in the
metaphysis of the long bone. In the adult, the epiphyseal cartilage plates of the metaphysis at either ends
of the long bone are replaced by bone, fusing the epiphysis to the diaphysis. This bony junction is called
the epiphyseal line.
Blood flow to the bone is by nutrient arteries that enter the bone by holes in the bone, called nutrient
foramen. Accompanying the nutrient arteries are nutrient veins and nerves.
The outer surfaces of the diaphysis are covered with a relative thick layer of connective tissue, the
periosteum. It is from the inner layer of the periosteum that the bone grows in thickness from the
outside by appositional growth. The periosteum is firmly attached to the bone by collagenous bundles
(Sharpey’s fibers) that penetrate the bone. The ligaments and tendons attach to the bone by the periosteum.
There is no periosteum over the epiphysis, the epiphyseal bone is covered with articular cartilage.
The articular cartilage allows for movement at freely moveable joints by reducing friction and also
acting as a shock absorber when the bones move.
B. Microscopic Anatomy of Bone or Osseous Tissue
1. Compact Bone or Osteon
The basic unit of compact bone is the osteon (Haversian system). Each osteon in cross section consist
of concentric lamellae (layers) of bone surrounding a central (Haversian) canal. Between the lamellae
are lacunae that contain mature bone cells, the osteocytes. Radiating from the lacunae are tiny canals,
the canaliculi. The osteocyte processes reside in the canaliculi in living bone.
In the center of the osteon is a central canal. Each central canal contains a blood capillary. The
blood capillary in the central canal is a branch from a larger vessel. This larger vessel lies in a
canal at right angles to the central canal, the perforating (Volkmann's) canal. Nutrient blood vessels
that penetrates the surface of the bone through nutrient foramen reside in the perforating canals.
Areas between the osteons contain intersitial lamellae, which are fragments of osteons that are being
replaced as bone is remodeled or growing.
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On the surface of the bone, beneath the periosteum, there are several outer circumferential lamellae
which follow the circumference of the shaft. The inner circumferential lamellae encircle the marrow
cavity are beneath the endosteum.
2. Spongy Bone or Cancellous Bone
The basic unit of structure of the cancellous bone is the trabecula. The lamellae are arranged as a thin
plate of bone to form the trabecula. The osteocytes are embedded in lacunae with the osteocyte
processes in the canaliculi.
C. Comparison between Compact and Spongy Bone
1. Spongy or Cancellous Bone
Spongy bone has many spaces filled with red marrow for blood cell production. Spongy bone is made up
of thin plates of bone, the trabeculae. The trabeculae consist of lamellae. At the edge of the lamellae there
are lacunae with osteocytes and canaliculi with ostocyte processes. The surfaces of the trabeculae are
covered with endosteum. In the endosteum there are osteoblasts and osteoclasts.
Spongy bone is found in short bones, flat bones, irregular shaped bones and the epiphysis of long bones.
The function of spongy bone is support, and helps resist stress with a minimum of added weight. It is also
as an area for blood cell production. The name for the marrow space found in the spongy bone is the
diploe.
2. Compact Bone or Osteon
Compact bone contains few spaces. It is composed of concentric lamellae, the Haversian system or
osteon. Compact bone is deposited over spongy bone. It is thicker in the diaphysis than in the epiphysis.
The function of compact bone is to provide protection, support, resist stress of weight placed on the
bone. The surfaces of compact bone are covered with periosteum.
3. Function of Compact Bone and Spongy Bone with Regard to Weight Bearing – see p. 4.
D. Composition - Chemical Nature of Bone Intercellular Substance (Ground Substance and Fibers)
The ground substance is composed of organic chemicals and inorganic salts.
1. Organic Chemicals - the organic chemicals form a homogenous ground substance, the osteoid. The
osteoid is chemically a mixture of hyaluronic acid and chrondroitin sulfate. The collagenous fibers are
irregularly arranged in the osteoid.
2. Inorganic Salts - the inorganic salts of Ca and PO4 are embedded in the osteoid and around the fibers.
The salts are formed into crystals called hydroxyapatite crystals. The inorganic salts constitute 50%,
organic matter 25% and water 25% of the weight of bone.
The process of calcification of the osteoid occurs only in the presence of the collagenous fibers. The
mineral salts crystallize first in the osteoid and then around the fibers.
3. Collagenous Fibers - the collagenous fibers give the bone great tensile strength, i.e., resist stretching and
twisting and flexibility. The salts allow bone to resist compression by making the bone hard. The
combination of collagenous fibers and crystallized salts makes bone flexible and strong (hard) without
being brittle. The same principle is used in reinforced concrete, the steel rods gives tensile strength, the
cement gives compressional strength. These two properties of the bone can be seen by:
(1) Baking bone destroys the osteoid and collagenous fibers. The bone is brittle as only the mineral salts
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remain. (2) Placing bone in acetic acid (vinegar) dissolves the mineral salts (the inorganic material), the
bone becomes rubbery and flexible as the collagenous fibers and osteoid remain.
III. Factors that Affect Bone Development and Maintenance
A. Mechanical Stress and Exercise
Bone is a living tissue, it is constantly being remodeled as its strength is adjusted in proportion to stress
to which it is subjected. In response to stress there is increased amounts of collagenous fibers and
inorganic salts deposited in the bone, the bone thickens. In those areas of bone not subjected to stress,
salts are withdrawn from the bone and the bone thins.
Bone is subjected to two major stresses:
1. Mechanical Stress or Gravitational Forces
Mechanical stress results from supporting weight of the body. In weightless conditions of space travel,
bones do not grow and degenerate.
2. Exercise or Functional forces
Exercise result from pull exerted on the bones by contracting muscles. When functional forces
decrease, such when a bone is in a cast bone does not grow and degenerates.
Gravitational and functional forces alter the form of the skeleton.
Gravitational forces - as a person increases or decreases weight, skeleton must become heavier or
lighter to support the weight.
Functional forces - as muscles become stronger due to exercise, bone must become stronger, otherwise
the muscle will break the bone to which it is attached.
How does stress on the bone cause bone to thicken and thin?
The pressure or stress on the bone affects the pH of the extracellular fluids of bone and the electric
fields in the bone causing bone growth, thickening and remodeling. Alkaline pH activates the
enzyme, alkaline phosphatase in the osteoblasts to increase bone formation and thickens the bone. In
other parts of the bone not subjected to pressure, the pH of the extracellular fluids activates acid
phosphatase in the osteoclasts causing bone resorption and decreasing bone formation. When the bone
is stressed, the stress causes the mineralized crystals to generate minute electric fields which attract
osteoblasts to the area of the stress. The electric fields prevents the PTH from stimulating the osteoclasts,
thus allowing bone deposition.
B. Hormones and Bone’s Role in Ca++ Homeostasis
Bone as a Ca++ reservoir.
Many endocrine hormones influence bone development and regulate blood Ca++ levels.
Ca++ levels in the body must be maintained for muscle and nerve function and blood clotting.
The homeostatic blood Ca++ level is 8.5-11mg/100mL. The blood Ca++ is regulated by control of Ca++
resorption from the bone into the blood or Ca++ deposition from the blood into the bone depending on the
blood Ca++ levels.
Hormones that regulate the homeostatic Ca++ levels of the body.
1. Parathormone (PTH) from the parathyroid gland
PTH increases number and activity of osteoclasts in the bone, to increase resorption of bone. The
increased resorption releases Ca++ from the bone into the blood to increase Ca++ in the blood
(hypercalcemia). Under the influence of the PTH the kidney decreases Ca++ excretion but increases PO4
excretion. PTH increase vitamin D production. Low blood Ca++ stimulates PTH synthesis, high blood
Ca++ inhibits PTH release from the parathyroid by negative feedback.
2. Calcitonin from the thyroid gland
Calcitonin opposes PTH by decrease resorption of bone by decreasing the activity of the osteoclasts.
This lowers the blood Ca level (hypocalcemia). Calcitonin increases osteoblast activity to increase
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Ca++ in the bone matrix, increase bone formation and increase Ca++ loss by the kidney.
Remodeling of bone involves interaction of these two hormones.
3. Growth Hormone (GH) from the pituitary gland
GH stimulates protein synthesis, growth of skeleton and growth in general. GH enhances Ca++ ions
and amino acid absorption by osteoblasts and incorporation of the amino acids into collagen.
4. Testosterone and Estrogen from the gonads
These hormones promote protein synthesis, Ca++ retention, and Ca++ deposition in bone, i.e. fosters
bone deposition. However, estrogen causes closure of the epiphyseal plate earlier than testosterone.
5. Insulin, Thyroid Hormones and Adrenal Cortex Hormones
These hormones are needed for general body growth, bone growth and maturity.
C. Nutrition
Growth and maintenance of bone depend on adequate dietary intake of minerals, vitamins and sufficient
levels of the above hormones. A balanced diet must provide for the following essential substances.
Anytime there is malnutrition or severe illness during bone growth, the person will be shorter than they were
genetically determined to be.
1. Minerals – Ca, PO4, F, Mg, Fe, and Mn
2. Vitamin D (Calcitrol) – synthesized in the kidneys - necessary for absorption of Ca++ from the digestive
tract. Vitamin D synthesis is stimulated by PTH and inhibited by high Ca++ in the blood.
3. Vitamin C – necessary in the formation of the protein, collagen. If vitamin C is decreased, defective
bone formation occurs, as a low vitamin C prevents normal collagen formation.
4. Vitamin A – necessary for synthesis of chondroitin sulfate, which gives the bone’s ground substance
its solid gel-like consistency.
5. Vitamin B12 – it effects osteoblasts activities by acting as a coenzyme for oxidative enzymes in the
electron transport chain.
6. Vitamin K – necessary for protein synthesis.
IV. Bone Formation – Osteogenesis or Ossification
A. Pre-natal Bone Development
There are two methods of bone formation in the embryo beginning at 6 weeks in utero.
1. Intramembranous Ossification – The bone forms directly from mesenchymal cells which are arranged in
sheet-like layers that resemble membranes. Irregular, flat and short bones of the skeleton develop in this
manner.
2. Endochondral Ossification – The bone forms from hyaline cartilage which first developed from the
mesenchyme. The bones are modeled as cartilage models which are then replaced by bone. The long
bones of the skeleton develop in this manner.
B. Post-natal Bone Growth
1. Interstitial Growth
Growth in length of the long bone is by interstitial growth of the epiphyseal hyaline cartilage plate
in the metaphysis. The long bones grow in length from this cartilage plate. This type of bone growth
occurs from infancy to early adulthood.
2. Appositional Growth
Growth in thickness of the bones of the skeleton is by appositional growth. The bone grows on the
outside from the inner layer of the periosteum and from the inside from the endosteum. This growth is
capable of occurring throughout life. In the adult this type of bone growth serves mainly for remodeling
and repair of bones by bone deposition and bone resorption.
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Negative Feedback System for Regulation of Blood Ca++ Concentration
Some stimulus (stress)
disrupts homeostasis by
causing a decrease in
Controlled condition
Blood calcium (Ca++) level
Receptors
Parathyroid gland cells
detect lowered Ca++ concentration
Increased production
Input
of cyclic AMP
Control center
PTH gene “turned on”
Increased release of
Output
PTH
Effectors
Osteoclasts increase
Kidneys retain Ca++
bone resorption
in blood, excrete
PO4 in urine and
produce calcitiol
Response
Increase in blood Ca++ level
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Return to homeostasis
when response brings
blood Ca++ level
back to normal
Decrease in PTH or Calcitriol (Vit. D)
Rate of intestinal absorption
decreases for Ca++
Extracellular Fluid
Normal Ca++ concentration
(8.5-11 mg/100 mL)
PTH and Calcitriol
Rate of intestinal absorption
increases for Ca++
PTH
Osteoclasts release
stored Ca++ by
promoting bone resorption
Calcitonin
Kidneys allow Ca++
loss
PTH and Calcitriol
Kidneys retain Ca++
and excrete PO4
Calcitonin
Inhibits osteoclasts
Ca++ in bone matrix
increased by promoting
osteoblast activity
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Factors that increase
blood Ca++ concentrations
Factors that decrease
blood Ca++ concentrations
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