BONE TISSUE
A.
PHYSIOLOGY: FUNCTIONS OF BONE
List, then briefly describe, the six basic functions of bone tissue and the skeletal
system.
Support -- Bone provides a framework for the body by supporting soft
tissues and providing points of attachment or most of the skeletal
muscles.
Protection -- Bones protect many internal organs from injury very well,
such as the brain and spinal cord. In addition, the heart, lungs, and
reproductive organs are given some degree of protection.
Movement -- Most skeletal muscles attach to bones. When the muscles
contract, they pull on bones to activate lever systems, and movement is produced.
Mineral homeostasis -- Bone tissue stores a number of minerals,
particularly calcium and phosphorus. Under control of the
endocrine system, bone releases the minerals into the blood or
stores the minerals in bone matrix to maintain critical mineral
balances.
Blood cell production -- In all bones of the infant and certain bones of the
adult, a connective tissue known as red marrow produces blood
cells by the process of hematopoiesis.
Storage of energy -- In some bones, yellow bone marrow stores lipids,
creating an important energy reserve for the body.
B.
ANATOMY: STRUCTURE OF BONE
Identify each of the following parts of a long bone:
Diaphysis -- The diaphysis of a long bone is its shaft or long main portion.
Epiphysis -- The epiphysis of a long bone is its end. The two ends
together are called the epiphyses. Each epiphysis is covered with
articular cartilage.
Metaphysis -- The metaphysis of a long bone is the region of mature
bones where the diaphysis meets the epiphysis
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Epiphyseal plate -- In a growing bone, the epiphyseal plate is formed of
hyaline cartilage divided into four zones of cells. Under the
influence of growth hormone, the plate continues to grow, giving
length to the bone. When bone growth exceeds cartilage growth,
beginning at puberty, the epiphyseal plate is slowly lost. Growth of
long bones stops when the cartilage is completely gone.
Identify each of the following parts of a long bone:
Articular cartilage -- Articular cartilage is a thin layer of hyaline cartilage
covering the articular surfaces of the epiphysis at a joint.
Medullary cavity -- The medullary (marrow) cavity is the space within the
bone containing either red or yellow bone marrow. Red bone
marrow consists of blood precursors while yellow marrow consists
of adipose tissue.
Periosteum -- The periosteum is the double-layered connective tissue
surrounding the bone except where the articular cartilage is
present. It is divided into an outer fibrous layer and an inner
osteogenic layer.
Fibrous periosteum -- The outer fibrous layer of the periosteum is
composed of dense irregular connective tissue containing
blood vessels, lymphatics, and nerves that pass into the
bone.
Osteogenic periosteum -- The inner osteogenic layer of the
periosteum contains elastic fibers and various bone cell
types, particularly osteoprogenitor cells, that give rise to new
osteoblasts when stimulated.
Periosteal functions -- The periosteum functions in bone growth,
repair, and nutrition. In addition, it provides attachment
points for skeletal muscles.
Endosteum -- The endosteum is a single layer of osteoprogenitor
cells lining the medullary cavity.
Compare spongy bone with compact bone.
Spongy bone consists of lamellae (layers) of bone matrix arranged in an
irregular latticework of thin plates of bone called trabeculae. The
spaces between the trabeculae are a part of the medullary cavity of
the bone.
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Compact bone contains very few spaces. The layers of bone matrix are
packed together tightly, forming osteons (Haversian systems). It
forms the external layer of all bones, providing protection and
support and helps the long bone resist the stress of weight applied
to them.
C.
HISTOLOGY OF BONE
Identify the following cells:
Osteoprogenitor cells -- Osteoprogenitor cells are immature quiescent
cells lining the bone surfaces. When stimulated, they enter mitosis,
giving rise to a new cell type called the osteoblast.
Osteoblast -- Osteoblasts, once differentiated, lose their mitotic ability, and
begin producing new bone matrix in a process known as
osteogenesis.
Osteocytes -- Osteocytes are mature bone cells completely embedded in
bone matrix, are incapable of mitosis, and probably do not secrete
new matrix. Their role in bone homeostasis is poorly understood.
Osteoclasts -- Osteoclasts reside scattered along the endosteal surfaces.
They function in a process known as bone resorption, the
destruction of bone matrix. This process is required for normal
bone function.
Unlike the other connective tissues, the matrix of bone contains an abundance of
mineral salts embedded into an homogeneous frame work of extracellular
materials.
Identify the three main components of bone matrix and briefly describe the
process of ossification (mineralization or calcification).
1.
2.
3.
Tricalcium phosphate (hydroxyapatite--50% of total matrix)
Ground substance (25% of total matrix is water)
Collagen fibers (25% of total matrix)
The predominant mineral salt is tricalcium phosphate (hydroxyapatite)
(50% of total mineral) (there is also calcium carbonate, magnesium
hydroxide, fluoride, and sulfate). As these salts are deposited into the
framework of ground substance and collagen fibers, they crystallize and
the tissue hardens or ossifies.
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Although the hardness of the bone depends upon the crystallized mineral salts,
without the collagen the matrix would be very brittle (ex: an egg shell). Why?
Collagen fibers provide pliability and tensile strength to resist being
stretched or torn apart. The mineral salts are crystallized onto the
collagen fibers, giving bone its hardness.
Did you ever see the experiment in which a chicken bone is placed into
vinegar for a few weeks? When the bone was pulled from the vinegar, it
could be bent and twisted, even tied into a knot. Why? Because the
acetic acid in the vinegar dissolved the mineral salts from the bone,
leaving only the collagen framework.
We tend to think of bone as a solid mass of calcified matrix, but it is instead
riddled with microscopic spaces through which blood vessels pass and fluids
percolate.
Define each of the following:
Volkmann’s canal -- A Volkmann’s canal is a minute passageway by
means of which blood vessels and nerves from the periosteum of a
bone penetrate into compact bone.
Haversian canal -- An Haversian (central) canal is a circular channel
running longitudinally in the center of an osteon of mature compact
bone. It contains blood and lymphatic vessels and nerves.
Concentric lamellae -- Concentric lamellae are rings of calcified bone
matrix surrounding the Haversian canals of compact bone.
Lacunae -- A lacunae (“little lake”) is a small hollow space within bone
matrix wherein resides an osteocyte. They are located between
concentric lamellae.
Canaliculus -- A canaliculus is a small channel or canal connecting two
lacunae in compact bone. Each canaliculus contains a cellular
process of an osteocyte.
Osteon -- An osteon (Haversian system) is the basic unit of structure in
adult compact bone. Each consists of a central canal with its
concentrically-arranged lamellae of matrix, lacunae, osteocytes,
and canaliculi.
Interstitial lamellae -- Interstitial lamellae are fragments of older compact
bone found between newer osteons. They have been partially
destroyed during bone replacement.
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D.
BONE HOMEOSTASIS
1.
REMODELING
What is the process of bone remodeling?
Remodeling is the ongoing replacement of old bone tissue by new
bone tissue. It occurs as a delicate balance between bone
resorption by osteoclasts and bone formation by osteoblasts.
What does bone remodeling accomplish?
1.
2.
3.
2.
Changes the way bone matrix resists stress
Removes worn or injured bone
Provides a reservoir for body calcium
BONE’S ROLE IN CALCIUM HOMEOSTASIS
Consider the role of bone in calcium homeostasis. How is it hormonallycontrolled to either store or release calcium dependent upon the body’s
needs at the moment.
Blood calcium levels are very tightly controlled between 9.5-10.5
mg%.
The hormones parathyroid hormone (PTH) and calcitonin (CT), as
well as Vitamin D, are the principal regulators of blood calcium
concentrations.
This control is regulated by negative feedback mechanisms that are
related to the amount of calcium in the blood. What happens if
blood calcium drops too low?
The controlled condition is blood calcium concentration. In this
case it has dropped below 9.5 mg%.
Parathyroid gland cells detect the lowered calcium concentration.
This serves as input into the control center for the feedback system.
The parathyroid gland cells respond to the input from the receptors
by secreting parathyroid hormone into the blood. This is the output
of the system.
PTH has three targets (effectors):
1.
increase bone resorption
2.
increase calcium reabsorption by the kidneys
3.
increased absorption of calcium by the gut (in
conjunction with vitamin D)
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The response to these effects is an increase in blood calcium
concentration.
The final result is a return to homeostasis as blood calcium levels are
brought back into the 9.5 – 10.5 mg% range and the feedback system
turns off.
Calcitonin has just the opposite effects.
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