CHAPTER 6
“Bones Tissue”
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Functions of the skeletal system
Classification of bones based on shape
General features of bone
Bone cells and matrix of bone
Compact and spongy bone
Bone marrows
Bone development
Overview of bone growth and remodeling
SKELETAL SYSTEM:
Defined: Includes all of the bones of the human body
(total of 206), and their associated cartilages and
joints.
Functions:
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Support – supporting framework for body
Protection – protects vital organs (brain and thoracic cavity)
Movement / levers- allows movement and flexion as well as
levers for different movements
Mineral storage- calcium and phosphate
Hematopoiesis- principal site for blood cell formation in red
marrow of flat bones (e.g. sacrum, sternum, etc.)
Electrolyte balance of calcium and phosphorus
Acid/Base balance buffers the blood calcium phosphate
Detoxification by absorbing heavy minerals
Classification based on the “shape”
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Long bones – bones that are longer than they are
wide Ex. Femur, tibia, fibula, ulna and humerus
Short bones – bones that are shaped like a cube;
there is also a special class of short bones called
sesamoid bones. Ex. Tarsal, carpal, and patella.
Flat bones – bones that are thin and flat.
Ex.Scapula, skull, ribs, and sternum.
Irregular bones – bones that do not fit in any of the
prior categories because they have irregular shapes.
Ex. Vertebrae and hip bones.
Shapes of bones
“GROSS” structure of a typical bone
• Articular cartilage: Consists of Hyaline cartilage
covering the end of the bone surface where it
articulates with another bone,
(e.g. femur and tibia, humerus and scapula).
Fibrocartilage makes up the menisci of the knee
joints.
• Epiphyses: The end of the bone. One at each
end of long bones.
• Epiphyseal line: Remnant of the cartilaginous
“growth plate” or epiphyseal plate.
Typical bone structure
Bone structure
Periosteum: Tough outer connective tissue
covering on bone. Consists of 2 layers; outside
is dense irregular CT and deeper layer lined
with osteoblast and osteoclast cells. It is richly
supplied by blood vessels and nerves and
secured to bone by Sharpey’s fibers.
Endosteum: connective tissue covering on inside
of bone cavities. Is osteogenic in that it contains
osteoblasts and osteoclasts.
Typical bone structure
Bone structure continued
Diaphysis: The shaft of the bone between the
two epiphyses. Contains the medullary cavity
and is filled with yellow marrow in adults.
Sharpey’s fibers: Bundles of collagenous fibers
that tightly attach the periosteum to bony
matrix.
Types of Bones cells
Bone is formed and metabolized by specific cells
and is in constant state of remodeling.
• Osteoclasts: Bone destroying cells
“C” means chewing
2. Osteoblasts: Bone generating cells
“B” means building
3. Osteocytes: Mature bone cells, spider shaped
and maintain bone tissue
Bone Cells
Bone matrix
• The matrix of bone is made up of organic and
inorganic matter.
• The organic portion is of collagen fibers and
various proteoglycans, glycosaminoglycans and
glycoproteins.
• The inorganic portion is calcium phosphate
salts “hydroxyapetite” and calcium carbonate
• The combination of these makes for a bone that
is very strong and yet flexible.
Chemical Composition of Bone
• In addition to bone cells, the majority of
compact and spongy bone is composed of
inorganic molecules.
• These molecules are called hydroxyapatitie
and are made of calcium and phosphate.
• The combination of these form a cement like
material that gives bone its hardness and
strength. In combination with collagen fibers
that form the matrix of bones and allows for
elasticity and flexibility.
Compact bone terms
Osteon/Haversian System: structural unit of
compact bone. Oriented parallel to shaft and
forming a group of hollow tubes through which
an artery, vein and nerve pass into and through
bone.
Lacunae: small cavities (halo’s) containing
osteocyte
Osteocyte: true bone cell, spider shaped and
found in lacunae at the junctions of the lamellae
Lamellae: layers of the collagen fiber matrix with
each layer going in opposite direction to the
adjacent layer.
Compact bone terms
Lamellae may be concentric (forming rings like a tree)
or circumferential (encircling the entire bone structure).
Canaliculi: Hair like canales that connect each lacunae
and in turn connect to the Central canal. Remove wastes
and bring nutrients into osteocytes
Volkman’s canal/perforating canal: Canals running
perpendicular to the Haversian canals, but connecting to
them. They bring in the artery, vein and nerves to the
bone structure.
Spongy Bone
• Spongy bone composes the inner portion of the bone
lining the marrow cavity. It has a honeycomb
appearance of trabeculae and spicules. Although it
looks poorly organized it is designed to withstand the
specific stresses put on each bone because of their
trabeculae.
• Trabeculae are tiny bone struts or plates that form
very strong support structure for the spongy bones.
They are irregularly arranged lamellae and
osteocytes, but contain no osteons per se as it
receives it nutrients from the marrow tissue.
Spongy bone histology
Bone marrow
• Yellow marrow is found in medullary cavity of
long bones and is not hematopoietic in adults.
Yellow marrow replaces red marrow as we
mature and is made up mainly of fat.
• Red marrow is found in the axial skeleton and
girdles and in the epiphyses of the femur and
humerus and is very active hematopoietically.
Classification based on bone formation
• During embryonic development a “blue-print”
for each of our bones is formed from fibrous
membranes and/or hyaline cartilage.
• Cartilage is replaced (the cartilage does NOT
magically become bone) by bone in one of
two ways: 1). Endochondral ossification and
2). Intramembranous ossification
Bone development
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Ossification or osteogenesis
- is the process of forming new bone
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Two methods of ossification:
1. Endochondral ossification
2. Intramembranous ossification
Endochondral ossification
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The process by which bone is formed from hyaline
cartilage
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Most bones in the body are formed by this method
(including the vertebrae, pelvic bones and limb
bones).
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Consists of 3 sites of ossification.
1. The primary ossification center
2. The metaphysis
3. The secondary ossification center
Endochondral ossification
• During the first 8 weeks of fetal development,
hyaline cartilage forms a model of future bone
formation. The center (diaphysis) of the
cartilage is “primary ossification site” which
contains numerous chondrocytes in lacunae.
Surrounding the cartilage model is an outer
layer chondrocytes called the “perichondrium”
which differentiate into “osteoblast cells” and
begin to lay down a bony collar around the site.
• Once the collar is formed the perichondrium
becomes periosteum.
Endochondral ossification
Endochondral ossification
• Buds of connective tissue grow from the periosteum
into the caritlage and transform the primary
ossification site into the primary marrow space. This
space is lined with spongy bone.
• The metaphysis forms between the marrow space and
the cartilaginous epiphyseal end.
-It is considered as a transitional zone where cartilage
is formed into bone at each end of the epiphysis. The
metaphysis is just beneath the epiphyseal plate where
bone growth continues until after adulthood.
The metaphysis
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Consists of five distinct zones:
Zone of reserve cartilage
Zone of cell proliferation
Zone of cell hypertrophy
Zone of calcification
Zone of bone deposition
The secondary ossification center
• Begins at the time of birth.
• Forms in the epiphysis and develops similarly
to the primary ossification center.
• The bone formed in the secondary ossification
site persists as spongy bone and growth occurs
beneath the outer covering of hyaline cartilage
which persists as articular cartilage within the
joint cavity on each end of the epiphysis.
Intramembranous ossification
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Bone formed by replacing a fibrous membrane
and not from cartilage. Ex. Skull and clavicle
• Basic Overview of Process
– during the first 8 wks of embryonic development,
fiberous membranes (CT) form in the areas of future
flat bones
– beginning around 8 wks, an “ossification center”
forms in the membrane. This center is composed of
osteoblasts.
– the osteoblasts begin to secrete hydroxy apetite
– the internal spongy bone forms
– the external compact bone forms
Bone Growth
• Once the cartilage models of embryonic
development are replaced by bone, they must
continue to grow through infancy, childhood
and adolescence.
– Increased length: bones continue to lengthen
because hyaline cartilage remaining in the
epiphyseal plates continues to grow. As
adulthood approaches, this cartilage becomes
less active and is eventually replaced by bone.
Bone growth
Appositional growth - Increased Width: bones
continue to widen as osteoblasts form more
layers of bone around the outside and
osteoclasts break down some of the bony
matrix inside.
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Why would bone need to be broken down
inside as it grows outside?
Control of Bone Growth
• Bones increase in length and width because of
the influence of minerals, vitamins, and
hormones in the body.
• Calcium and phosphate are necessary for
calcification
• Vitamins A, C and D promote bone growth.
• The specific hormones which affect growth are
growth hormone (GH), thyroid hormone (T3 and
T4), and the sex steroids (estrogen and
tesstosterone).
BONE REMODELING
• Even though the bones in an adult do not
continue to grow as described above, they are
constantly being remodeled. This means that
bone is always being broken down by
osteoclasts and reformed by osteoblasts (really
no different from remodeling ones home).
• Each week we turn over about 5% of our bone
mass.
Control of Remodeling
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Two major factors influence remodeling.
Calcium levels - our bodies need a
homeostatic level of calcium in the blood for
all cells to function properly.
Mechanical stress - the varied activities of life
puts different stresses on each bone as we
age, which requires slight adjustments to
compensate for these stresses.