THE SKELETAL SYSTEM
Bone tissues makes up about 18% of the total human body weight.
The skeletal system supports and protects the body while giving it shape and form.
Osteology: It is the branch of science that deals with the study of the skeletal system, their structure and functions.
COMPOSITION
The skeletal system IS composed of:
1.
2.
3.
4.
Bones
Cartilage
Joints
Ligaments
FUNCTIONS OF SKELETAL SYSTEM
• SUPPORT: Hard framework that supports and anchors the soft organs of the body.
• PROTECTION: Surrounds organs such as the brain and spinal cord.
• MOVEMENT: Allows for muscle attachment therefore the bones are used as levers.
• STORAGE: Minerals and lipids are stored within bone material.
• BLOOD CELL FORMATION: The bone marrow is responsible for blood cell production
DIVISIONS OF THE SKELETAL SYSTEM
• The human skeleton consists of 206 named bones
• Bones of the skeleton are grouped into two principal divisions:
I.
Axial skeleton: Skull bones, auditory ossicles (ear bones), hyoid bone, ribs, sternum (breastbone), and
bones of the vertebral column
II.
Appendicular skeleton: Consists of the bones of the upper and lower limbs (extremities), and the bones
forming the girdles that connect the limbs to the axial skeleton
BONES
DEFINITION
Bone is a strong and durable type of connective tissue. It is a Calcified, living, connective tissue that forms the
majority of skeletal system
It consists of:
• water (25%)
• organic constituents including osteoid (the carbon containing part of the matrix) and bone cells (25%)
• inorganic constituents, mainly calcium phosphate (50%)
Types of bones
I.Base on composed tissue, bones are classified as
 Compact – Dense bone tissue composed of osteons, which resist pressure and shocks and protect the
spongy tissue – forms especially the diaphysis of the long bones.
 Spongy – Tissue made of bony compartments separated by cavities filled with bone marrow, blood
vessels and nerves – gives bones their lightness
II. Base on shape, Bones are classified as
 Long: Greater length than width and are slightly curved for strength – Femur, tibia, fibula, humerus, ulna,
radius, phalanges
 Short: Cube-shaped and are nearly equal in length and width – Carpal, tarsal
 Irregular: Complex shapes and cannot be grouped into any of the previous categories – Vertebrae, hip
bones, some facial bones, calcaneus
 flat: Thin and composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy
bone tissue – Cranial, sternum, ribs, scapulae
 Sesamoid: Protect tendons from excessive wear and tear – Patellae, foot, hand
STRUCTURE OF BONE
 GROSS ANATOMY
A long bone is generally divided into following parts:
1. Epiphysis:
a. Present on both ends of a bone
b. composed of a spongy bone surrounded by a thin layer
of compact bone.
2. Metaphysis:
a. It is in between the epiphysis and diaphysis
b. it contains the connecting cartilage enabling the bone to
grow – disappears at adulthood
3. Diaphysis:
a. It is also called as the shaft of the long bone
b. elongated hollow central portion of the bone located
between the methaphyses
c. made of compact tissue
d. encloses the central medullary canal, containing fatty
yellow bone marrow.
4. Periosteum
a. a vascular membrane that almost completely covers the
long bone
FIG: STRUCTURE OF LONG BONE
Structure of short, irregular, flat and sesamoid bones
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These have a relatively thin outer layer of compact
bone with cancellous bone inside containing red bone
marrow
They are enclosed by periosteum except the inner
layer of the cranial bones where it is replaced by dura
mater.
FIG: STRUCTURE OF FLAT AND IRREGULAR BONE
 MICROSCOPIC STRUCTURE
 Compact Bone: To the naked eye, compact bone appears solid
but on microscopic examination several structures can be seen
 Osteons: Elementary cylindrical structure of the
compact bone, Surrounds and opens into Haversian
canal
 Haversian canal: Lengthwise central canal of the osteon,
enclose blood vessels and nerves.
FIG: MICROSCOPIC STRUCTURE OF BONE
 Lamellae: concentric rings or plates of bone
 Lacunae: They are present Between the lamellae and are tiny spaces, containing tissue fluid and
spider-shaped osteocytes (mature bone cells)
 Canaliculi: Thry link the lacunae with each other and with the central Haversian canal.
 Cancellous bone: To the naked eye, cancellous bone looks like a honey comb. Microscopic examination
reveals the following
 a framework formed from trabeculae (meaning 'little beams'), which consist of a few lamellae and
osteocytes interconnected by canaliculi
 The spaces between the trabeculae contain red bone marrow that nourishes the osteocytes
 Bone cells
 Osteoblasts
o These are the bone-forming cells that secrete collagen and other constituents of bone
tissue.
o they later mature into osteocytes
 Osteoclasts
o Their function is resorption of bone to maintain the optimum shape.
 Osteocytes
o As bone develops, osteoblasts become trapped and remain isolated in lacunae.
o They stop forming new bone at this stage and are called osteocytes.
DEVELOPMENT OF BONE
 At an early stage of development, the skeletal structure appear as dense concentration of mesenchymal
cells that tend to take shape of particular bones
 These localized areas of bone formation are called Ossification Centres. They may be endochondral or
intramembranous
 Ossification may be endochondral or intramembranous depending on the environment in which the
bone formation takes place
 INTRAMEMBRANOUS OSSIFICATION
 Mesenchymal cells differentiate into osteoblasts
 Osteoblasts begin to secrete intercellular substance of the bone
 intercellular substance replaces amorphous fluid and encloses the osteoblasts and are called as
osteocytes
 Actively secreting osteoblasts, line the surface of newly formed spicules of the bone and continue
to grow in a radial manner producing trabeculae
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Branching and forming of different trabeculae result in scaffolding
The bone formed by the scaffolding trabeculae is termed as cancellous bone
The continued deposition of fresh bone lamellae on trabeculae & appositional bone growth forms
the compact bone
The surface connective tissue becomes the periosteum
 ENDOCHONDRAL OSSIFICATION
 The process by which the bone is formed in a cartilaginous environment is called endochondral
ossification
 Mesenchymal cells differentiate into chondroblasts
 Cartilage model increases in length and width by interstitial as well as appositional
 It involves:
1. Proliferation
2. Maturation
3. Enlargement of chondrocytes
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The earliest chondrocytes of the centre mature and becomes enlarged
They secrete alkaline phosphatase into the intercellular substance and becomes calcified
A new vascular environment is created which with the invasion of perichondrium by blood vessels
It brings change in the behaviour of pluripotential cells
These cells differentiate into osteoblasts and a thin layer of bone is laid around the cartilage
Membrane enclosing the cartilage model is called periosteum.
The formation of secondary ossification centre helps in the growth of bone
The young cartilage at each end of the model continues to grow and extend by interstitial growth to
increase the length of the model
 The cancellous bone in the central part is reabsorbed to form a medullary cavity and later on filled with
myeloid tissue
FUNCTIONS OF BONE
Bones have a variety of functions. They:
a) Provide the framework of the body
b) Give attachment to muscles and tendons
c) Permit movement of the body as a whole and of parts of the body, by forming joints that are moved by
muscles
d) Form the boundaries of the cranial, thoracic and pelvic cavities, protecting the organs they contain
e) Contain red bone marrow in which blood cells develop: haematopoiesis
f) Provide a reservoir of minerals, especially calcium phosphate.
THE SKELETON
• The human skeleton consists of 206 named bones
• Bones of the skeleton are grouped into two principal divisions:
I.
Axial skeleton: Skull bones, auditory ossicles (ear bones), hyoid bone, ribs, sternum (breastbone), and
bones of the vertebral column
II.
Appendicular skeleton: Consists of the bones of the upper and lower limbs (extremities), and the bones
forming the girdles that connect the limbs to the axial skeleton
 THE AXIAL SKELETON
The axial skeleton consists of: 1. Skull. 2. Vertebral column (spinal column). 3. Thoracic cage. 4. Sternum