Skeleton and Type of Skeletal Materials

What is Skeleton?

Plants and animals need skeleton for their shape and support. If humans had no skeleton for instance, they will be imagined to be like a lump of skin on the floor, without any form, shape and support. Also if plants had no skeleton, they would not be upright as we have known them to be. For this reason;

In animals, skeleton may be defined as a bony framework of the body that provides support, shape and protection.

Note: Plants have also got their own skeleton.

Unicellular organisms do also have a form of skeleton.

Skeleton provides support to humans in the sense that our bodily muscles are attached to various bones in our body via the tendons. For instance, the bone of the ribs serves as an attachment to the intercostal muscles while some of the the arm muscles attaches to the scapula bone.

Animals and plant have got different shapes and this is a function of the skeleton. The ability to stand in an upright position is one of the characteristics of primates; and the nature of their skeleton is responsible for their uprightness. A snake assumes its shape because of the kind of skeleton it possesses. Also, the shapes of crocodiles, horses and lizards are derived from the nature of their skeleton.

Skeleton offers protection to the delicate internal organs. Examples of these organs are the heart and lungs which are protected by the rib cage or thoracic cage. Also, the brain is protected inside the skull and this is one reason why footballers can perform heading skills in the game of football without damaging their brain.

The rib cage consists of the 24 rib bones and the sternum. Together with the intercostals muscle and diaphragm, these bones support respiration in humans.

 

Skeletal Materials

The term “skeletal materials” should not be confused with “types of skeleton”.

Think of skeletal material as the kind of substance in which a skeleton is composed of.

The materials that make up the skeleton are different in animals. For instance, the skeleton of humans is composed of both cartilages and bones while that of a cockroach is composed of a material called chitin.

Note: Some fishes have their skeletal material composed entirely of cartilage.

 

The skeletal materials in animals can be put into 3. These are CHITIN, CARTILAGE and BONE:

• Chitin

Chitin is a skeletal material found in insects and anthropods. Cockroaches and grasshoppers are examples of insects while shrimps and crabs are examples of anthropods. This skeletal material (chitin) is usually present outside the organism and this makes it an exoskeleton.

Note: Exoskeleton is a type of skeleton

Chitin is not a living tissue and thus does not grow along with the organism. It is a structural polysaccharide composed of repeating units of modified glucose molecules. In order for organisms (with chitin as skeletal material) to grow, they must shed or put off their chitin to the environment. After their growth, these organisms will regenerates a new chitin. This is why you may see the shed chitin materials of cockroaches in the corners of homes or inside a box of old clothes.

The process by which living organism grows by shedding off their chitin is called moulting or ecdysis.

 

• Cartilage

The skeleton of some complex vertebrates are composed entirely of cartilage as its material. Cartilage consists of living cells called chondroblasts. They may also contain very little or no amount of calcium and phosphates, in addition to collagen fibres.

It’s the calcium in bones that actually give the bone its great strength.

 As a tissue, a cartilage is a tough and flexible. This mix of toughness and flexibility in cartilages play an important role in a developing fetus (developing child in the womb). In fact, the skeleton of the fetus is composed mainly of cartilage, most of which will ossify (become bone) as the fetus come of age. Generally, cartilages do not have a direct blood supply in humans. Cartilaginous fishes like sharks, skates, stingrays and rays are examples of animals that have cartilage as their entire skeletal material.

The skeleton of bony fishes is composed entirely of bone.

Most animals including humans will usually have cartilage alongside bone as their skeletal material. In this case, the cartilages will also act as a shock absorber, cushioning the effects of bones moving against bones during locomotion. They are always seen where two long bones meet or articulate.

Image Credit: Quora

A joint is formed where two bones meet or articulate

Mammals generally have 3 types of cartilage: these are

1. Hyaline cartilage: A major characteristics of the hyaline cartilage is the fact that it has very few fibres in its matrix. For this very reason, hyaline cartilage may also be referred to as a pure cartilage. This cartilage is the major component of embryonic skeleton (that is, the skeleton of a developing child in the womb). It is found in the following parts of the body:

1. Trachea and Bronchi (as rings of cartilage around them, keeping them open)
2. The protruding part of the nose
3. The nasal septum (or the wall separating both nasal cavity)
4. On the surface of all moveable joints; for instance, the shoulder joint and hip joint
5. Between the ribs and the sternum.
6. It also lines the end of growing bones

2. Fibrocartilage: They are tougher than hyaline cartilage because they contain numerous collagen fibres. Fibrocartilages are found in following regions in human:

1. The disc between each vertebra that makes up the vertebral column.
2. Menisci that cushions the joint of the knee and elbow
3. It’s present between the two pubic bones of the hip (pubic symphysis)
4. Where tendons and ligaments are present

Tendons attach muscle to bones. Muscles, bones and tendons are responsible for movement in humans.

Ligament attaches bone to bone. They generally help to stabilize the joint.

3. Elastic cartilage: As the name suggests, an elastic cartilage can be easily stretched because they are elastic. The elasticity of this cartilage is as a result of a large amount of a protein called elastin present in its matrix. Elastic cartilage is present in the eternal ear (pinna) and the epiglottis.

The epiglottis is a cartilaginous material found just above the pharynx; and it flaps during swallowing in order to prevent food from getting into the trachea and ultimately the lungs.

Note: It’s generally a good idea not to be carried away by talking when eating; else, one might end up coughing out food substance from the mouth and nose. This coughing action is another mechanism of preventing food from going into the lungs.

Ossification is the natural process that leads to the formation of bone.

Oftentimes, cartilages are ossified into bones.

 

• Bones

Bone is the major skeletal material present in human beings and most mammals. I have written a separate article on it because its quite a broad topic.

Please read on bones here.

 

Differences between cartilages and bones

The table below shows the differences between cartilages and bones

Cartilage	Bone
Composed mainly of living cells	Composed of both living and non living cells
They are usually replaced by bone through a process called ossification	They cannot be replaced by cartilage
It contains a very little amount of mineral salts (calcium and phosphates salts)	It contains a high amount of calcium and phosphate salts. They make up about two third of the bone mass
It is not as strong as a bone although it is more flexible	It is stronger and more rigid than a cartilage
It is made up of living cells called chondroblast	It is made up of living cells called osteocytes
Cartilage do not have a direct supply of blood	Bones have a direct supply of blood
A cartilage grows from only one of its end	Long bone grows at both ends (epiphysis)
Bone marrow is absent	Bone marrow is present. Blood cells are formed in the bone marrow. Please read on Red blood cell here
They are of 3 types: Hyaline cartilage, Fibrocartilage and Elastic Cartilage	They are various kind of bones: Compact bone and Spongy bone, Long bone, Cartilaginous bones, Sesamoid bones and so on
Cartilage is made up of a protein called chondrin	Bone is made up of a protein called ossein
Cartilage is completely made up of organic materials	Bone is composed of both organic and inorganic material
Haversian system is absent within the cartilage cells	Haversian system is present  within the bone cells

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This content is written by Alfred Ajibola