• Is the framework of the body
• Provides shape to the body and protection for
organs and soft tissues
• Consists of cartilage, bones, tendons and ligaments
•Functions
• Support: Bone is hard and rigid, bears body weight
• Cartilage provides firm and flexible support, such as
cartilage in nose, external ear, thoracic cage and
trachea
• Ligaments attach bone to bone & hold them together
• Protection: Bones of skull protects brain
• Ribs, sternum, vertebrae protect organs of thoracic
cavity
• Movement: Skeletal muscles attach to bones by
tendons
• Contraction of skeletal muscles moves the bones,
produce body movement
• Storage: Ca and P. Stored then released as needed.
Fat stored in marrow cavities
• Blood cell production: Blood cells and platelets
formation takes place in bone marrow of bones
• Basic Structure of Skeletal Cartilage
• Consists mostly of water – accounts for
resilience
• contains no nerves and blood vessels
• Surrounded by double layer of dense
irregular connective tissue – Perichondrium
• Outer layer: Contains fibroblasts
• Inner layer: More delicate, has fewer fibers,
contains chondroblasts and chondrocytes
• Blood vessels and nerves penetrate the outer
layer of pericardium but do not enter cartilage
matrix
• Nutrients diffuse through matrix to reach
chondrocytes
•Growth of Cartilage:
•
• Cartilage grows in two ways:
•Appositional growth – Add new matrix and
chondrocytes to the outside of tissue
•Interstitial growth – Chondrocytes within the tissue
divide and add more matrix between the cells
•Types of Skeletal Cartilage:
• Hyaline, Elastic and Fibrocartilage
• Hyaline Cartilage: Contains fine
collagen fibers in matrix, Include
• Articular cartilages : cover the ends of
bones at movable joints
• Costal cartilages: connect the rib to
sternum
•Respiratory cartilages: forms skeleton
of larynx
• Nasal cartilages: support external nose
Elastic Cartilage: Contains collagen and
elastic fibers - external ears and epiglottis
• Fibrocartilage: Thick bundles of
collagen fibers, compressible and tough
• Found in Menisci and intervertebral discs
• Human skeleton consists of 206 bones
• And divided into two groups:
• Axial Skeleton:
• Consists of skull bones, vertebral
column and rib cage
• Protect and support body parts
• Appendicular Skeletan:
• Consists of bones of upper and lower
limbs and girdles ( shoulder and hip
bones)
• Bones of limbs – help in movement
• Bones are classified by their shape
as long, short, flat and irregular
•Long Bones:
• Are long and thin
• Are found in arms, legs, hands,
feet, fingers, and toes
•Flat Bones
• Are thin, flattened shape, usually
curved
• Are found in the skull, sternum,
ribs, and scapula
• Irregular Bones
• Have complex shapes
• Examples:
– spinal vertebrae
– pelvic bones
• Short Bones
• Are small and thick
• Examples:
– ankle
– wrist bones
• Three
levels of structure:
•Chemical
•Gross
•Microscopy
• Consists of both organic and inorganic components
• Organic components include:
• Cells ( Osteoprogenitor cells, osteoblasts, osteocytes
and osteoclasts) and Osteoid, the organic part of the
matrix
• Osteoid (35%) consists mainly of collagen and
proteoglycans
• Inorganic components
• 65% of bone tissue is calcium phosphate crystal
called hydroxyapatites, CaPO4 crystals
• Bone Markings
• Most bones contain features
on external surface
• Depressions and openings
along bone surface, passage
for blood vessels and nerves
• Projections where tendons
and ligaments attach and at
articulations with other bones
• Include heads, trochanters,
spines etc.
• Bone tissue is classified as woven or lamellar
bone based on collagen fibers organization
within bone matrix
• Woven bone. Collagen fibers randomly oriented.
– First formed
• During fetal development
• During fracture repair
– Then Woven bone is remodeled into lamellar bone
• Lamellar bone
– Mature bone, organized in sheets called lamellae.
Collagen fibers are oriented in one direction in
each layer, but in different directions in different
layers for strength.
• Bones, whether woven or lamellar
can be classified
according to amount of bone matrix
relative to amount of space
• Compact bone:
• Contains dense outer layer, less
space
• Cancellous or spongy bone:
• Has less bone matrix & more space
•Consists of interconnecting rods or
plates of bones called trabeculae
• Diaphysis
– Shaft, long axis of bone
– Made up of Compact bone
– Surrounds central medullar or
marrow activity
– Red marrow - blood cell formation
– Yellow marrow – adipose tissue
• Epiphysis
– End of the bone
– Cancellous bone
– Joint surface of epiphysis is covered
with articular (hyaline) cartilage,
cushions the bone ends
– Epiphyseal plate: growth plate
– Growth in length occurs at E. plate
– Separates epiphysis from diaphysis
– When bone stops growing in length
becomes Epiphyseal line
• Membranes
• External surface of bone is covered
by double layer membrane called
Periosteum
– Fibrous layer – Outer fibrous layer is
dense irregular connective tissue
contains blood vessels and nerves
– Cellular layer – Inner single layer of
bone cells consists of osteoblasts,
osteoclasts, osteochondral progenitor
cells
– Periosteum is attached to underlying
bone by Perforating or Sharpey`s
fibers, made up of collagen
– Periosteum provides anchoring points
for tendons and ligaments
• Membranes
• Endosteum:
• Single layer of cells that lines all internal
spaces, such as medullar cavity
•
Contains osteoblasts, osteoclasts,
osteochondral progenitor cells
Structure of Flat, Short,
and Irregular Bones
• Flat Bones
– No diaphyses, epiphyses
– Sandwich of cancellous
between two layers of compact
bone, eg. Parietal bone of skull
• Short and Irregular Bone
– Similar to structure of
epiphyses of long bones
– Compact bone that surrounds
cancellous bone center with
small spaces filled with marrow
– Are not elongated and no
diaphyses
• Hematopoietic tissue, red marrow
is found in trabeculae of spongy
bone of long bones, diploe of flat
bones (sternum) and in some
irregular bones (hip bones)
• Four major type of Bone cells
–
–
–
–
Osteoblasts
Osteocytes
Osteoclasts
Stem cells or osteochondral progenitor cells
• Osteoblasts
• Immature bone cells that
secrete organic components of
matrix known as Osteoid
• Osteoblasts surrounded by
bone matrix, as the material
calcifies, the cell is trapped in a
space called a lacuna
• And becomes osteocytes
(mature bone cells)
• Osteocytes
• Mature bone cells that maintain the
bone matrix
• Live in lacunae
• Are between layers (lamellae) of
matrix
• Connect by cytoplasmic extensions
through canaliculi in lamellae
• Do not divide
• Maintains protein and mineral
content of matrix
• Helps repair damaged bone
• Osteoclasts
• Giant, mutlinucleate cells
• Secrete acids and proteindigesting enzymes
• Breakdown bone by
dissolving bone matrix
• Osteoprogenitor Cells
• Mesenchymal stem cells
that divide to produce
osteoblasts
• Are located in inner layer
of perichondrium, inner
layer of periosteum and
endosteum
• Assist in fracture repair
Cancellous (Spongy) Bone
• Consists of interconnecting rods or plates of bone called
Trabeculae
• No blood vessels in trabeculae
• The space between trabeculae is filled with red bone marrow:
– which has blood vessels
– forms red blood cells
– and supplies nutrients to osteocytes
Compact Bone
• The basic unit of mature compact
bone is Osteon or Haversian
system
• Osteon is a group of hollow tubes
of bone matrix, one placed
outside the next
•
Osteon consists of single central
canal and around a canal
contains blood vessels
• Osteocytes are arranged in
concentric lamellae
Compact Bone
• Perforating (Volkmann`s )Canals
• Perpendicular to the central
canal
• Carry blood vessels into bone
and marrow
• Circumferential Lamellae
• Present on outer surface of
compact bone
• Binds osteons together
•
•
Interstitial Lamellae
Present in between osteons
 Bone formation during fetal development occurs
in two patterns:
Intramembranous ossification
– Takes place in connective tissue membrane
Endochondral ossification
– Takes place in cartilage
 Both methods of ossification
– Produce woven bone that is then remodeled
– After remodeling, formation cannot be
distinguished as one or other
•
Takes place in fibrous connective tissue
membrane formed from embryonic
mesenchyme cells around the
developing brain
•
Starts at 8th week & completes by age 2
•
Forms many skull bones, part of
mandible, diaphyses of clavicles
•
Mesenchyme cell in the membrane
become osteochondral progenitor cell
•
Osteochondral progenitor cell forms
osteoblast
•
Osteoblast produce bone matrix and
collagen fiber
•
And become osteocyte and develop
trabeculae
•
More osteoblast gather around
trabeculae and produce more bone
•
Trabeculae join together and form
cancellous bone
•
Cells in the spongy cell produce red
bone marrow
•
Cells surrounding the developing bone
forms periosteum
•
Osteoblasts from the periosteum on
bone matrix produce compact bone
• Bones of the base of the skull,
part of the mandible,
epiphyses of the clavicles, and
most of remaining bones
develop through endochondral
ossification
• Mesenchyme cells develop
into chondroblasts which
secrete the matrix of hyaline
cartilage & surrounded by
perichondrium except where
joint formation takes place
• Chondroblasts becomes
chondrocytes; Chondrocytes in the
center of hyaline cartilage:
– Enlarge, calcify, and die, leaving
cavities in cartilage
• Blood vessels grow around the
edges of the cartilage and
osteochondral progenitor cells in the
perichondrium change to osteoblasts
• Perichondrium becomes periosteum
when osteoblasts begin to form bone
– Osteoblast produce compact
bone on the surface of cartilage
and forms the Bone collar
 Blood vessels enter the cartilage
 Connective tissue surrounding the blood
vessels bring osteoblasts and osteoclasts
 Osteoblasts secrete bone matrix and
changes the calcified cartilage of diaphysis
into cancellous bone
 Bone formation area – Primary ossification
center
 Osteoclasts remove the bone from center
and forms medullar cavity
– Forms red bone marrow

Capillaries and osteoblasts enter the epiphyses
creating secondary ossification centers

Epiphyses fill with spongy bone:
–
All the cartilage is replaced by bones except in
epihhyseal plate and an articular sufaces

In mature bone, compact and cancellous bone are
fully developed and epiphyseal plate becomes
epiphyseal line
• Bones increase in size only by Appositional growth
•
Formation of new bone takes place on the surface of
older bone or cartilage
• Growth in Bone Length
• Growth in bone length occurs at the epiphyseal plate
• Growth at epiphyseal plate involves the formation of
new cartilage by
– Interstitial cartilage growth
– Followed by Appositional bone growth on the
surface of the cartilage
• Epiphyseal plate is organized into 4 zones:
•Zone of Resting Cartilage:
• Nearest to epiphysis, contains randomly
arranged chondrocytes
• Do not involve in bone growth
•Zone of Proliferation:
• Contains actively dividing chondrocytes
• Chondrocytes produce new cartilage through
interstitial cartilage growth
• As the cells divide, the epiphysis moves
away from the diaphysis. This in turn
produces length growth in bone
• Zone of Hypertrophy:
• Contains mature and enlarge
chondrocytes
• Zone of Calcification:
• Matrix is calcified, chondrocytes die
• Calcified cartilage is
replaced by bone
• Growth at articular cartilage increases the
epiphyses size
• Also growth at articular cartilage increases
size of bones with no epiphyses: e.g., short
bones
• Articular cartilage persists throughout life and
does not ossified as epiphyseal plate
• Bones increases in thickness or long bones increase
in width, because appositional bone growth takes place
beneath periosteum
 Size and shape of a bone determined genetically but
can be modified and influenced by nutrition and
hormones
Nutrition
– Lack of calcium, protein and other nutrients during growth
and development can cause bones to be small
Vitamin D
– Necessary for absorption of calcium from intestines
– Can be eaten or manufactured in the body when skin is
exposed to sunlight
– Rickets: lack of vitamin D during childhood
Have bowed bones
– Osteomalacia: lack of vitamin D during adulthood leading to
softening of bones
Vitamin C
• Necessary for collagen synthesis by
osteoblasts
• Scurvy: deficiency of vitamin C, causes
hemorrhage because of lack of collagen
deficiency
• Lack of vitamin C also causes wounds not to
heal, teeth to fall out
• Bone Remodeling: Old bone is replaced with new bone
• Osteoclasts remove old bone and osteoblast forms new bone
• Bone remodeling converts woven bone into lamellar bone
• And involve in bone growth, changes in bone shape, adjustments in
bone due to stress, bone repair, and Ca ion regulation
• Caused by migration of Basic Multicellular Units
– Groups of osteoclasts and osteoblasts that remodel bones
• Bone constantly removed by osteoclasts and new bone formed by
osteoblasts
 Bone undergo repair after damage
 Has four major steps:
 Hematoma formation:
 Bone fracture damages blood
vessels in bone & periosteum and
hematoma forms
 Hematoma - Localized mass of
blood released from blood vessels,
Clot formation stop the bleeding
 Inflammation and swelling occurs
after injury
 Callus formation. Callus is mass of tissue
that forms at a fracture site and connects the
broken ends of the bone
 Internal Callus – Forms between the

ends of broken bones
Several days after fracture blood vessels
grow into clot
• Macrophages clean up debris,
osteoclasts break down dead bone
tissue, fibroblasts produce collagen and helps in
put the bone together
• Osteoprogenitor cells from the periosteum of
healthy bone tissue produce
• Chondroblasts - which secrete cartilage tissue
• Ostoeblasts - which secrete bone matrix
• New bone is formed
 External Callus – Forms collar around opposite
ends of bone fragments
 Periosteal osteochondral progenitor cells 
osteoblasts and chondroblasts
 Produce bones and cartilage
 Callus ossification:
Callus replaced by woven,
cancellous bone
 Bone remodeling:
Replacement of woven bone
and damaged material by
compact bone
• Bone is major storage site for calcium
• The level of calcium in the blood depends upon
movement of calcium into or out of bone
– Calcium enters bone when osteoblasts create new
bone; calcium leaves bone when osteoclasts
break down bone
– Two hormones control blood calcium levelsparathyroid hormone and calcitonin
• Parathyroid hormone (PTH)
• Major regulator of blood Ca+ level
•If blood Ca+ level decreases, secretion
of PTH increases, increases
osteoclast no.
• Causes increase bone
breakdown and increases blood
Ca+ level
• Calcitonin
• Increase in blood Ca+ level
stimulates thyroid gland and secrete
calcitonin and inhibits osteoclast
activity
• Bone matrix decreases and bone is more brittle due to
decreased collagen production; but also less
hydroxyapatite
• Bone mass: Highest around 30. Men denser due to
testosterone and greater weight. African Americans
and Hispanics have higher bone masses than
Caucasians and Asians. Rate of bone loss increases
10 fold after menopause (estrogen production
decreases), Cancellous bone lost first, then compact
• Increased bone fractures
• Bone loss causes deformity, loss of height, pain, stiffness
– Stooped posture
– Loss of teeth