THE SKELETAL SYSTEM

The word skeleton comes from the Greek word skeleton

meaning “dried up”. It is strong yet light adapted for its

function of body protection and motion. The skeletal system

includes bones, joints, cartilages and ligaments. The joint give

the body flexibility and allow movements to occur. But from

structural point of view, the human skeletal system consists of

two main types of supportive connective tissue, bone and

Cartilage

Functions of the skeletal system:

1. Support: it forms the internal framework that supports

and anchors all soft organs.

2. Protection: bones protect soft body organs.

3. Movement: skeletal muscles attached to the skeletal

system use the bone to levers to move the body and its

part.

4. Storage: fat is stored in the internal cavities of bones.

Bone it self-serves as a storehouse of minerals. The most

important being calcium and phosphors.

5. Blood cell formation: it occurs with in the marrow

cavities of certain bones

Bone

Bone (osseous) is specialized connective tissue that has the

strength of cast iron and lightness of pinewood. Living bone is

not dry, brittle or dead. It is a moist changing, productive

tissue that is continually resorbed, reformed and remodeled.

Types of bone

Long bone, are called long as its length is greater than its

width. The most obvious long bones are in the arm and leg.

They act as levers that pulled by contraction of muscles.

Short bones are about equal in length, width and thickness,

which are shaped with regular orientation. They occur in the

wrist and ankle.

Flat bones are thin or curved more often they are flat. This

includes ribs, scapulae, sternum and bone of cranium.

Irregular bones, they do not fit neatly into any other category.

Examples are the vertebral, facial, and hipbone.

Sesamoid bones are small bones embedded with in certain

tendons, the fibrous cord that connects muscle to bones.

Typical sesamoid bones are patella and pisiform carpal bone,

which are in the tendon of quadriceps femuris and flexor carp

ulnaris muscle respectively.

Accessory bones are most commonly found in the feet. Theyusually occur in the developing bone and do not fuse

completely. They look like extra bones or broken on X-ray.

Sutural (wormian) bones are examples of accessory bones

Gross anatomy of a typical long bone

You can take Tibia (in the leg)one of the longest bones in the

body.In adults it have:

Diaphis, the tubular shaft,hallow cylindrical with walls ofcompact bone tissue. Thecenter of the cylinder is the

medullary cavity, which is filled with marrow.

Epiphysis is roughly spherical end of the bone. It is wider than the shaft. Flat and irregular bones of the trunk and limbs

have many epiphysis and the long bones of the finger and toe have only one epiphysis.

Metaphysis is the part separating diaphysis from epiphysis. It

is made up of epiphyseal plate and adjacent bony trabeculae

of cancellous bone tissue.

Epiphyseal plate is a thick plate of hyaline cartilage, which

provides the framework of synthesis of the cancellous bone

tissue within metaphysis.

• The medullary cavity running through the length of the

diaphysis contains Yellow marrow.

• The porous latticework of the spongy epiphyses is filled

with red bone marrow. The red marrow also known as

myeloid tissue

Endosteum is the lining the medullary cavity of compact

bone tissue and covering the trabeculae of spongy bone

tissue.

Periosteum: it is covering the outer surface of the bone. It

is absent at joints and replaced by articular cartilage.

Bone (Osseous) Tissue

Bone tissue is composed of cells embedded in a matrix of

ground substances and fibers. It is more rigid than other

tissues because it contains inorganic salts mainly calcium

phosphate & calcium carbonate. A network of collagenous

fibers in the matrix gives bone tissue its strength and

flexibility. Most bones have an outer sheet of compact bone

tissue enclosing an interior spongy bone tissue Compact bone tissue forms the outer sheet of a bone. It is

very hard and dense. It appears to naked eye to be solid but

not. Compact bone tissue contains cylinders of calcified bone

known as osteons (Haversion system). Osteons are made up

of concentric layers called lamellae, which are arranged

seemingly in wider and wider drinking straws. In the center of

the osteons are central canals (haversion canal) , which are

longitudinal canals that contains blood vessels, nerves and

lymphatic vessels. Central canals, usually have branches

called perforating canals /valkmann’s canal that run at right

angle to central canal extending the system of nerves and

vessels out ward to periosteum and to endosteum. Lacunae

(Little spaces) that houses osteocytes (bone cells) are

contained in lamella. Radiating from each lacuna are tiny

canaliculi containing the slender extensions of the osteocytes

where nutrients and wastes can pass to and from central

canal.

Spongy (cancellous) Bone tissue Is in the form of an open

interlaced pattern that withstands maximum stress and

supports in shifting stress. Trabeculae are tiny spikes of bone

tissue surrounded by bone matrix that has calcified.

Bone Cells

Bone contain five types of cells

a) Osteogenic (osteoprogenitor) cells: these are small

spindle shaped cell. They found mostly in the

deepest layer of periosteum and endosteum. They

have high mitotic potential and can be transformed into

bone forming cells (osteoblasts).

b) Osteoblasts are found in the growing portion of bone

including periosteum. They are able to synthesize and

secrete un-mineralized ground substance, act as pump

cell to move calcium and phosphate in and out of bone

tissue.

c) Osteocytes are the main cell of fully developed bones.

They have a cell body that occupies a lacuna. Osteocytes

are derived from osteoblasts. They together with

osteoclasts play an important role of homeostasis by

helping to release calcium.

d) Osteoclasts are multinuclear giant cell, which are found

where bone is resorbed during its normal growth.

Osteoclasts are derived from white blood cells called

monocytes.

e) Bone - lining cells are found on the surface of most bones

in the adult skeleton. They are believed to be derived

from osteoblast that ceases their physiological activity.