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Bone
∗
Tonye A. Ogele
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 3.0†
Abstract
By the end of this section you know • Bone as an organ • Bone as a tissue • The types of bone tissue
• The classication of bone • The functions of bone
3.2.3 INTRODUCTION
bone
The word
refers both to the bone tissue (also called
as the humerus, tibia, etc.
osseous tissue ) and an organ of the body such
3.2.3.1 BONE TISSUE (OSSEOUS TISSUE)
The bone tissue is a connective tissue with a calcied or mineralized matrix. In order words, bone tissue
is a connective tissue in which the matrix is hardened by deposition of calcium phosphate and other minerals.
There are four principal types of bone cells (gure 1):
1. Osteogenic cells: These are stem cells that develop from broblasts. They are found on the surfaces
of bone tissue. They continually divide to give rise to osteoblasts.
2. Osteoblasts: These are cube-shaped cells that synthesize the soft organic matter of the bone matrix.
The soft organic matter is hardened by mineral deposit. They form a single layer of cells on the surface
bone tissue. They originate from osteogenic cells. Osteogenic cells produce more osteoblasts in cases
of stress to the bones and fracture. This is to reinforce or rebuild the bone.
3. Osteocytes: Osteoblasts that have become trapped in the matrix they deposited become osteocytes.
They are found in tiny cavities called lacunae. Each osteocyte has ngerlike cytoplasmic processes that
connect it with neighbouring osteocytes and sometimes osteoblasts. The cytoplasmic processes pass
through tiny canals called canaliculi. This connection allows for exchange of nutrients, chemical signals
and metabolic wastes between the cells. Some osteocytes resorb the bone matrix, while others deposit
it. They are important for maintenance of normal blood concentrations of calcium and phosphate ions.
They also sense strain on the bone tissue and adjust the shape and density of the bone to adapt to the
stress.
4. Osteoclasts: These are bone-dissolving cells found on the bone surface. They originate from bone
marrow stem cells. Several bone marrow stem cells fuse to form a single osteoclast. Osteoclasts are
therefore large and multinucleated. The osteoclast has a
that is in contact with the
bone matrix. This border has many deep infoldings that increase the surface area and increase bone
resorption.
rued border
∗ Version
1.1: Dec 4, 2013 9:41 am -0600
† http://creativecommons.org/licenses/by/3.0/
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Figure 1: Bone cells.
The matrix of the osseous tissue consists of one-third organic matter and two-thirds inorganic matter.
The organic matter is made up of collagen and protein-carbohydrate complexes like glycosaminoglycans,
proteoglycans and glycoproteins. The inorganic matter consist mostly of calcium phosphate (85%) and
calcium carbonate (10%). Magnesium, sodium, potassium, uoride, sulphate, carbonate and hydroxide ions
are also present in lesser amounts.
There are two types of bone tissue:
and
.
compact bone
3.2.3.1.1 COMPACT (DENSE) BONE
spongy bone
This bone is a dense tissue without any spaces visible to the naked eye. It forms the external surfaces of
all bones.
The structural unit of compact bone is the
or the
. The osteon is a cylindrical
tube consisting of circular layers of matrix (concentric lamellae) around a central canal (osteonic canal
or Haversian canal). The osteons run through the shaft of long bones. The central canal of one osteon
connects to the central canal of another osteon by transverse or diagonal passages called perforating canals.
Blood vessels, lymphatic vessels and nerve bres run through the central canal and transverse or diagonal
passages.
The inner and outer boundaries of dense bone have matrix arranged in
that run
parallel to the bone surface. Also between osteons are
which are remains of old lamellae
that were broken down as the bone grew and remodelled itself.
osteon
Haversian system
interstitial lamellae
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circumferential lamellae
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Osteocytes are found in lacunae in between lamellae. The osteocytes connect with one another through
their cytoplasmic processes in the canaliculi.
Figure 2: Compact bone tissue.
1.1.2 SPONGY (CANCELLOUS) BONE
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Figure 3: Spongy bone tissue.
Spongy bone is a tissue formed by a network of slender rods and plates called trabeculae (gure 3).
Trabeculae contain irregularly arranged lamellae and osteocytes that are connected by canaliculi. There are
no central canals in the trabeculae. Between the trabeculae are spaces which give the tissue a sponge-like
appearance. The spaces are lled with bone marrow. The matrix of spongy bone is arranged in lamellae like
the compact bone, but the central canal is absent.
Spongy bone is found at the ends of long bones (gure 3) and in the middle of almost all other bones
(gure 4). It is always enclosed by the compact bone.
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Figure 4: A at bone.
1.2 BONE
The bone is made up of bone (osseous) tissue, brous connective tissue, cartilage, bone marrow, blood
vessels, nervous tissue and adipose tissue.
All bones have an outer thin layer of compact bone around a central mass of spongy bone or medullary
cavity (marrow cavity). A brous connective tissue called the periosteum surrounds the bone tissue
except at the points covered by cartilage. The brous connective tissue covering the cartilage is called the
perichondrium. The periosteum is supplied by blood vessels and nerve bres. It nourishes the external
parts of the bone, and provides attachment for tendons and ligaments. The endosteum covers the trabeculae
of spongy bone and lines the canals that pass through the compact bone (e.g. medullary canal). It is made
of reticular connective tissue.
The bone marrow is the soft tissue that occupies the medullary cavity and the spaces between the
trabeculae (trabecular cavity) of spongy bones. There are two types of bone marrow:
and
.
The red marrow produces blood cells. It is therefore a
. The red marrow looks like
blood but is thicker. It lls the marrow cavity of nearly every bone in childhood. In adulthood however,
most red marrow turns into fatty yellow marrow. Yellow marrow is not hemopoietic but in severe or
chronic anaemia, it can transform back to red marrow. In adulthood, red marrow is found only in the skull,
vertebrae, ribs, sternum, pelvic girdle, proximal heads of the humerus and femur.
Blood vessels enter the bone through minute holes called nutrient foramina.
yellow marrow
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red marrow
hemopoietic tissue
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Figure 5: A long bone.
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1.2.1 CLASSIFICATION OF BONES
According to their shape, bones are classied into the following:
• Long bones which are tubular e.g. humerus, ulna, radius, metacarpals, phalanges, femur, tibia, bula,
metatarsals.
• Short bones which are cuboidal e.g. tarsal bones and carpal bones
• Flat bones which are at e.g. cranial bones, ribs, sternum, scapula, pelvic bone.
• Irregular bones which have various shapes aside long, short and at e.g. bones of the face, vertebrae
• Sesamoid bones which develop in tendons e.g. patella
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Figure 6: Classication of bones.
1.2.3 TYPICAL STRUCTURE OF LONG BONE
diaphysis
epiphyses
Most long bones have a tubular
(shaft) and
at both ends. The diaphysis (shaft)
is made up of an outer coat of compact bone surrounding a central medullary cavity (marrow cavity). The
marrow cavity contains yellow marrow in adults and red marrow in children.
The epiphyses are often more expanded than the diaphysis. They are made up of an outer coat of compact
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bone surrounding spongy bone. The articular surface of the epiphysis is covered by a thin layer of hyaline
cartilage.
Between the diaphysis and each epiphysis in childhood, is a hyaline cartilage called the epiphyseal
plate. This is the point of growth of the bone and is therefore also called the growth plate. In adults it is
replaced by the epiphyseal line and the bone no longer grows. The epiphyseal plate or epiphyseal line is
sometimes called the metaphysis.
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Figure 7: The structure of a long bone.
TYPICAL STRUCTURE OF SHORT, FLAT, IRREGULAR AND SESAMOID BONES
Short, at, irregular and sesamoid bones are made up of compact bone on the outside covering spongy
bone within. Bone marrow is found between the trabeculae. There do not have epiphyses or diaphysis.
1.2.2 FUNCTION OF BONES
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•
•
•
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Support: Bones provide the framework that supports the soft tissues.
Protection: Bones protect some tissues of the body e.g brain, spinal cord, heart, lungs.
Movement: The bones cooperate with the skeletal muscles to move the body and its parts.
Formation of blood cells: Blood is produced in the red bone marrow that is found in certain bones
of the body.
• Storage of Minerals: The bone stores minerals like calcium and phosphate and makes them available
when needed.
• Fat storage: Fat is stored in bone cavities.
PICTURE ATTRIBUTIONS
Figure 1 5 Openstax College (2013). Bone structure. Retrieved from the Connexions Web site:
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Figure 6 7 Openstax College (2013). Bone. Retrieved from the Connexions Web site: http://cnx.org/content/m44789/1.3
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Human anatomy & physiology (8th ed.) San Francisco: Benjamin
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Anatomy and physiology: The unit of form and function. (4th ed.) New York:
Essentials of anatomy and physiology (4th ed.) New
Marieb, E. N. & Hoehn, K. (2010)
Cummings.
Moore, K. L., Dalley, A. F., & Agur A. M. (2010)
Lippincott Williams & Wilkins
Saladin, K. S. (2007)
McGraw-Hill.
Seeley, R. R., Stephens, T. D. & Tate, P. (2002)
York: McGraw-Hill.
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