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Biology > Locomotion and Movement > Skeletal System
Locomotion and Movement

Skeletal System

Did you know that newborn babies have around 305 bones! A baby’s skeleton is mostly made up of cartilage. As it grows older cartilage undergoes ossification to form a complete skeletal system, having 206 bones. Let us learn about a human skeletal system.

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Parts of the Skeletal System

Human-Skeletal-System

(Source: Britannica)

The skeletal system is made up of bones and cartilage. There are two types of connective tissues called tendons and ligaments that are also considered a part of the system. Ligaments connect bones to bones whereas tendons connect bones to muscles.

The two main parts of the skeletal system, as mentioned above, are bones and cartilage.

Bones

There are 206 bones in the body which form more than 200 joints with each other. They are classified into two broad categories based on location:

  • Axial skeleton: These bones are found towards the midline of the body and include the skull, the rib cage, and the vertebral column.
  • The appendicular skeleton: These bones are found in the appendages such as arms, legs fingers, and toes.

Bones can be classified into four types based on their shape:

Types-of-Bones

  • Long Bones -They are long and slender bones found generally in the limbs. ex. humerus, femur.
  • Short Bones: They are short bones which are smaller in size and are found in the carpals and tarsals.
  • Flat Bones: they are thin and flat in nature and not all of them are completely flat. They provide surface area for muscle attachment. Ex: scapula, sternum
  • Irregular Bones: These bones do not have specific shapes and therefore cannot be put into any other group. Ex: vertebrae

Structure of a Bone

Lets now see what the structure of an individual bone looks like.

Structure-of-bone

(Source: http://lyceum.algonquincollege.com/)

Each bone tissue is made up of two types of osseous tissue: compact bone and spongy bone.

Compact bone is hard and compact in nature and always found towards the outside of the bone whereas the spongy bone which is softer and more porous is found towards the centre. The function of each bone determines the ratio in which these two types of tissues exist within it.

The connective tissue that is found on the outside of the bone is known as the periosteum. The periosteum is made up of cellular and fibrous tissue and plays a crucial role in the attachment to muscles and joints as it is this layer which contains tendon and ligament attachments. The endosteum is the connective tissue layer which lines the marrow cavity.

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The shaft of a bone is known as the diaphysis and the swollen end is called the epiphysis. The epiphyseal line demarcates the two parts. It is the diaphysis which houses the marrow cavity which is majorly composed of loose connective tissue and is responsible for producing blood cells.

The cells that form bone matrix are known as osteoblasts and the mature cells of the bone are called osteocytes. There is a special type of cells that help remove bone matrix and are found during bone remodelling known as osteoclasts. These are gigantic cells and are always found on the side of the bone where the matrix is being eaten away during growth and remodelling.

The matrix in a bone tissue is made up of two components: the organic part that contains fibres whereas the inorganic part consists of the minerals(hydroxyapatite).

Learn more about Disorders of Muscular and Skeletal Systems.

Cartilage

Cartilage is the second component of the skeletal system. It is made up of fibres that are embedded in connective tissue or ground substance. Cartilage consists of two types of fibres: Collagen and elastin fibres. The cells that form cartilage are known as chondroblasts and the mature cells of the cartilage are known as chondrocytes.

The chondrocytes lie in lacunae in the matrix. The outer layer of a cartilage is known as the perichondrium. Unlike the bone, cartilage is avascular which means that it contains no blood supply. However, the perichondrium contains blood supply. There are 3 types of cartilages namely Hyaline cartilage, elastic cartilage, and fibrocartilage.

  • Hyaline cartilage– the most abundant of the three cartilages and functions to help surfaces slide over one another. Example: found in the respiratory system.
  • Fibrocartilage– This cartilage is tough and its main function is to provide support and strength to structures. Example: found in healing tissue during bone repair(callus)
  • Elastic cartilage– This cartilage is abundant in elastic fibres and functions to maintain the shape of the area it is present in. Example: found in the middle ear.

Functions of the Skeletal System

  • The main function of the skeletal system is that it provides a framework to the body and provides shape.
  • Along with the muscular system, the skeletal system helps in the movement of the body parts of the body and locomotion of the body.
  • The skeletal system is hard and so forms a protective layer for the softer, more delicate organs from any form of injury. The rib cage protects the heart, lungs and visceral organs, the brain is protected by the skull etc.
  • It is the growth and development of bones that provides the height and width of an individual.
  • The centre of the bone consists of the bone marrow which produces blood cells and therefore hemopoietic in nature.

Solved Example for You

Q: The connective tissue on the outer surface of a bone is called as?

  1. Endosteum
  2. Epiosteum
  3. Perichondrium
  4. Periosteum

Sol: The correct answer is (d) Periosteum

The connective tissue layer on the outer surface of a bone is known as the periosteum. The connective tissue layer that is found lining the marrow cavity in a bone is known as the endosteum. Perichondrium is the outer layer of a cartilage. Epiosteum is not a structure found in either cartilage or bone.

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