What are Tissues?
In multicellular organisms, tissues are a group of similar cells, which perform a specific function. Groups of similar tissues form organs. The scientific study of tissues is known as histology. There are four types of animal tissues which are, epithelial tissue, nervous tissue, muscle tissue and connective tissue.
Types of Animal Tissues
Epithelial tissue or most commonly known as epithelium forms the lining of cavities and body surfaces. It forms glands as well. The cells of tissues are closely connected to each other through cellular junctions. There are two types of epithelial tissues; simple epithelium and stratified epithelium.
Simple epithelium is composed of a single layer of cells and it forms the lining of the tubes, ducts and body cavities.
Stratified or compound epithelium is made up of two or more layers of cells and functions as a protective covering of the body (skin).
There can be one of these three shapes of the epithelial tissue which are cuboidal (flattened, scale-like cells), columnar (rectangular with nucleus displaced towards the base) or squamous (square with the central nucleus). On the basis of the combination of layers of cells and shape of the tissue, it can be classified into the simple squamous epithelium, stratified squamous epithelium etc.
The muscle tissue is specialized for contraction and contains cells which are elongated. These cells are also known as muscle fibers. They contain actin and myosin proteins which possess contractile properties. The interaction of these proteins makes the muscles contract or relax. There are three different types of muscle tissue; cardiac, skeletal and smooth.
Skeletal muscles are found attached to bones. Body movements are caused by the contraction and relaxation of these muscles. These contain long, cylindrical and multinucleated cells. The nuclei of these cells are located towards the periphery. The striated cells in skeletal muscles contain alternating light and dark bands which are formed due to ordered arrangement of actin and myosin filaments.
Cardiac muscles are found in the heart. The cells in cardiac muscles are less striated than skeletal muscles, have shorter and often branched cells with a single nucleus. The connections between cells are visible under the microscope as dark bands called intercalated discs. These connections are necessary for cell coordination for the beating of the heart.
Smooth muscles are present in the walls of hollow organs like blood vessels, gastrointestinal tract and urinary bladder. Contractions of these muscles cause the fluid or food materials to pass through these tracts. These muscles are not striated so are named smooth muscles. They have a single nucleus with cells having tapered ends.
The nervous tissue is found in brain spinal cord and nerves. It is specialized for communication. Nervous tissue is composed of two major cell types; neurons and glial cells.
Neurons are connected to other through chemical and electrical signals. They contain a cell body which is nucleated, an elongated axon and elongated cellular processes called dendrites. The axons send signals and dendrites receive signals.
The glial cells are the supporting cells. They are of various types with many different functions, like cleaning the debris from around the neurons, maintaining proper ion concentration and generating myelin (an insulating material which surrounds axons).
There are various types of connective tissue on the basis of their form and function. They are characterized by the presence of extracellular matrix (a nonliving material consisting of a ground substance and protein fibers). The protein fibers are made up of a flexible substance elastin and a strengthening material collagen. The number and types of fibers vary between tissues. The ground substance contains interstitial fluid and polysaccharide molecules and fills the space between fibers and cells. The ground substance varies in consistency from liquid to solid. There are three types of connective tissue; dense connective tissue, loose connective tissue, and specialized connective tissue.
Loose connective tissue contains fibers and cells which are arranged loosely in a gel-like ground substance. Loose connective tissue is of two types; adipose tissue stores fat and areolar tissue is found under the skin and supports the epithelium.
In Dense connective tissue, the fibroblasts and fibers are densely packed in the connective tissue. Three types of dense connective tissue are; Ligament attaches a bone to the another bone, tendon attaches muscles to the bones and collagen is a dense irregular connective tissue found in the skin.
Specialized connective tissue forms cartilage, blood, and bones. Cartilage is a type of elastic connective tissue which acts as rubber-like padding to cover and protect the ends of bones at joints. cartilage is made up of specialized cells called chondrocytes. Cartilage is found in the outer ear, at the tip of the nose and between bones of the vertebrae. Bones are hard and made up of collagen fibers and calcium salts. Osteocytes are the cells which make up bones. The body skeleton is made up of bones. Blood is the fluid connective tissue and contains red blood cells, white blood cells, and platelets.