In the living world, there are single-cell and multicellular organisms. Each organism has specific processes and systems that work together to perform metabolic activities that keep the organism alive. While single-cell organisms perform all these functions in a single cell, multicellular organisms have several cells specialised for specific functions. This specialisation of cells in animals is called structural organisation in animals.
Structural organisation in animals
All living things are made up of cells. In animals, cells of the same origin and function group together to form tissues, which later form organs. Subsequently, organs develop organ systems, working together to keep the organism alive and functioning. It is the structural organisation in animals. Most animals have only four types of tissues. These are as follows:
-
Epithelial tissue:
This kind of tissue usually forms some lining or covering in the body. So every type of epithelial tissue has a free surface that faces either body fluid or extra body environment. Epithelial tissues are of two types – simple and compound. Compound epithelial tissue functions as protective layers like the skin, and simple epithelium forms the lining of organs and the body cavity. Simple epithelium comprises a single layer of epithelial cells, while the compound is made up of two or more layers of cells. Since compound epithelium is not a single layer tissue, it has limited absorption or secretion utility.
Simple epithelium can be classified further on the basis of its structure in the following categories:
-
Squamous epithelium:
○ A single, thin layer of irregular cells.
○ Flattened cells.
○ Form the lining of blood vessels and air sacs in the lungs.
○ Work as a diffusion boundary.
-
Cuboidal epithelium:
○ A single layer of cuboidal cells.
○ Found on the lining of the glandular ducts and tubes of the nephron in kidneys.
○ The primary function is secretion.
-
Columnar epithelium:
○ Tall and slender cells.
○ The nucleus is near the base.
○ Sometimes the free surface has microvilli.
○ Main functions may include absorption and secretion.
-
Ciliated epithelium:
○ It is similar to the columnar epithelium, but it’s free surface has cilia.
○ The primary function is to move mucus or particles.
○ Generally found in fallopian tubes and bronchioles.
-
Glandular epithelium:
○ These are cuboidal or columnar cells that are specifically for secretion.
○ They exist either in isolation in a unicellular state, like in the alimentary canal, or a multicellular state, like in the salivary glands.
-
Connective tissue:
These kinds of tissues perform the function of lining and supporting organs in the body. All connective tissues, except blood, secrete elastin and collagen fibres that provide flexibility and elasticity to the tissues. The cells of connective tissues also secrete the matrix or the ground substance. Connective tissue is of the following types:
○ Loose connective tissue:
■ Semi-fluid.
■ Cells and fibres are loosely packed.
■ It mainly forms the support structure of epithelium tissue.
■ It contains cells that secrete fibres.
■ Adipose tissue is an example of loose connective tissue. It has cells specialised for storing fat.
○ Dense connective tissue:
■ Densely packed fibres and cells.
■ Depending on the pattern formed by the fibres, this type of tissue can be further divided into dense regular or dense irregular tissue.
■ Examples of regular dense connective tissue are tendons and ligaments.
■ Dense irregular connective tissue is mainly found in the skin.
○ Specialised connective tissue:
■ An example of this kind of tissue is cartilage.
■ Cartilage has a structure that is pliable but resists being compressed.
■ The cells of the cartilage are called chondrocytes.
■ These cells secrete the matrix and are packed in the hollow spaces created in this matrix.
○ Bones:
■ Have a hard matrix that mainly consists of calcium salts and collagen.
■ This combination of collagen and calcium compounds gives strength to bones.
■ The bone cells are called osteocytes.
○ Blood:
■ Fluid connective tissue.
■ It comprises Red Blood Cells, White Blood Cells and Plasma.
■ The main function is the circulation and transport of substances.
-
Muscle tissue:
This type of tissue is mainly responsible for the movement of the body. The muscle tissue is made of tiny fibres called myofibrils. These fibres relax and contract depending on the stimuli. Following are the types of muscle tissue:
○ Skeletal muscle tissue:
■ Striated tissues.
■ Attached to bones.
■ Responsible for the movement of the skeleton.
■ Example: biceps.
■ Voluntary.
○ Smooth muscle fibres:
■ Unstriated.
■ Tapered at the ends.
■ Held together by cell junctions.
■ Bundled by connective tissue.
■ Found in the walls of internal organs.
■ Involuntary.
○ Cardiac muscles
■ Present only in the heart.
■ Involuntary but can be controlled in some instances by the brain.
-
Neural tissue:
This tissue comprises two types of cells – the neuron and the neuroglia. The neurons transmit impulses, and the neuroglia performs a protective function. This tissue is responsible for the body’s responses to most stimuli.
Organs and organ systems:
When the tissues mentioned above group together, organs are formed, and these organs constitute organ systems. This structural organisation in animals is essential for the functioning of all the billions of cells that make up an organism. Every organ in the body contains one or more tissues that perform some functions. For example, the human heart consists of all kinds of tissues – epithelium, connective, muscle, and neural.
Conclusion
The structural organisation in animals is a fascinating study since it is the result of millions of years of evolution. There are various ways in which cells have adapted to achieve the maximum efficiency in maintaining homeostasis and ensuring survival.