Animal Tissues is an important topic for class 7 biology chapters ICSC.
What is a tissue?
A tissue is a group of cells that perform the same function and have similar structure.
How many types of animal tissues are there?
There are mainly four types of animal tissues. These are -
1. Epithelial tissue
2. Muscular tissue
3. Connective tissue
4. Nervous tissue
1. Epithelial tissue - This tissue consists of one or more layers of a continuous, thin and protective sheet of cells. It mostly covers the external surface of the skin, the inner lining of the blood vessels and other organs such as windpipe, stomach and intestines. The cells present in epithelial cells are packed closely. There is no space in between these cells.
These cells are of the following three types on the basis of shape -
(a) Squamous epithelium - It consists of flat cells which are polygonal in shape and fit closely together. It is found mostly found in the inner walls of blood vessels, heart, lungs and oesophagus. It protects the internal parts of the body from injury. It prevents the internal organs from drying up and also prevents the entry of germs inside these organs.
(b) Columnar epithelium - The cells of this epithelium look like pillars. They are tall and lie side by side.
(c) Cuboidal epithelium - It consists of cuboidal cells which are almost square in shape and have equal height and width. The nucleus is present in the centre and is round in shape. It is present in the kidney tubules, thyroid gland, pancreas ,sweat glands , liver where it gives mechanical support.
What are the functions of epithelial tissue?
It protects the underlying tissue from injury. It also prevents the drying up of these tissues and prevents the germs to move inside. The epithelial lining of endocrine and digestive glands secrete useful secretions.It helps in absorbing nutrients and water from the digested food. Epithelial tissue found in kidneys and sweat glands help in the removal of waste products.
2. Muscular tissue - These tissues are found in all the parts of the body. They help in moving parts of the body such as arms, legs, eyes etc.
There are three types of muscular tissue -
(a) Voluntary muscles
(b) Involuntary muscles
(c) Cardiac muscles
(a) Voluntary muscles - These muscles are also known as skeletal because they are attached with the skeleton. They are known as voluntary muscles as they are under our control. Due to the presence of dark and light bands, they are known as striated muscles.The light and dark coloured bands are due to the presence of actin and myosin proteins.Striated muscles consist of cylindrical, narrow, long and unbranched units that have blunt ends.Cytoplasm consists of myofibrils that show striations i.e light and dark coloured bands. Each muscle fibre consists of many nuclei. These muscles are found mostly in arms, legs, face, tongue and neck.
(b) Involuntary muscles - These muscles are not under our control, so they are known as involuntary muscles. They are also known as smooth muscles as they don't have light and dark coloured bands.Each muscle fibre consists of one nucleus. They are found in blood vessels, urinary bladder, iris of eyes, alimentary canal.
(c) Cardiac muscles - These are also a type of involuntary muscles found in the heart. Their structure is similar to voluntary muscles but their function is similar to involuntary muscles. These muscles work tirelessly and continuously throughout our life. These muscles are branched, cylindrical and uninucleate. Specialized cytoplasm called sarcoplasm is present in the muscle cells which is surrounded by a thin cell membrane called sarcolemma.
3. Nervous tissue - These tissues help to control and co-ordinate the various activities in the body. Nerve cells present in these tissues are known as neurons. Each neuron consists of cyton or cell body, axon and dendrites.
4. Connective tissue - Connective tissue is the most abundant tissue found in the human body. It makes up to 30% of the body weight. Some of its main functions are packing, binding, support, protection, storage and transportation.
(a) Areolar tissue - It is a packaging tissue and is jelly like in nature. It is found around the organs under the skin such as lining of the gut. It provides elasticity.
(b) Adipose tissue - It consists of adipocytes which are fat storing cells. It is mostly found under the skin, around the eyeball, kidney and stomach.It acts like a cushion or shock absorber. It also helps in providing insulation.
(c) Tendons - They join muscles to bones. They consist of thick bundles of white collagen fibres.
(d) Ligaments - They join bones to bones. They consist of yellow elastin fibres.
(e) Cartilage - They are tough but flexible.They are found between bones, external ear, windpipe and nose.
(f) Bones - They are hard and non-elastic.
They consist of calcium, phosphorus and magnesium.
(g) Blood - It consists of RBC, WBC and platelets. RBC helps in supplying oxygen to different parts of the body. WBC helps in killing with germs. Platelets helps in the process of blood clotting.
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