Cells are important part of our body; they are the building blocks that control all the biological and physical activities. When group of cells work together to accomplish particular function they form tissue. We have different kinds of tissues that perform different works and they help the body in one or different ways. Before going deep into the topic, it is important to study about the cell junctions that serves to connect two or more cells to form tissue or one tissue with the other. Some of the cell junctions are:
1. Tight junction: it consists of web like strand to form seal of passageway between adjacent cells, thus preventing to leak any of the substances.
2. Adherens junction: it consists of plaque in the inside of plasma membrane and cadheris helps to join adjacent cells. This junction helps to resist separation during various contractile activities.
3. Desmosomes: These cell junctions also contain plaque but they does not attach to microfilaments rather it attaches to intermediate filament. It has cadherins as a membrane protein and these are found in cells of cardiac muscle.
4. Hemidesmosomes: these are similar to desmosomes but they have integrins rather than cadherins and doesn't link adjacent cell. It links cells to basement membrane.
5. Gap junctions: it has membrane protein called connexins that forms tunnel to connect neighboring cells. Here, intercellular space is maintained for transfer of molecules.
Above are the important junctions that connect cells and tissue. Beside these important points, it is also important to know about tissues, their types and their functions. Tissues can be broadly classified into different types as per their structure and function. They are epithelial tissue, connective tissue, muscular tissue and nervous tissue. Among all, epithelial tissue is extensively found in the body and they have several functions from basic to the complex ones. Epithelial tissue is well described below whereas other tissues are only briefly summarized.
1. Epithelial tissue: This tissue is closely arranged and packed together and held in continuous pattern by different cell junctions with no or little space in between the plasma membrane. It always lies singly and never covered by any other tissue. It is avascular and exchange of material takes place through diffusion. It has the power to divide and regenerate, and this is the reason that it is subjected to stress or injury. A particular epithelial cell has an apical and a basal surface. Apical surface is the one, the faces the lumen and have cilia and microvilli, lateral surface faces the adjacent cells and basal surface is opposite to apical whose deepest layer gets attach to extracellular matrix. Epithelial tissues have different functions in the body. It provides protection against the harmful environmental conditions, it filters useful materials from unwanted ones, and it helps to secrete different fluid through the skin and many other body organs. Absorption of necessary materials required for the body and excretion of unwanted substances are all excellently done by epithelial tissues. Epithelial tissue can be divided into two categories:
Covering and lining epithelium: These epithelium tissues are involved in body covering and lining of some important organs. They can be further subdivided on the basis of their arrangement.
On basis of shape:
When these two types of epithelium are combined, it gives the characteristics of 2 and hence, double the job of protection, absorption etc. The combined forms are generally found inside the body that makes the organs function in proper way. Some of the combined forms of this tissue along with their function are:
1. Simple squamous epithelium: it consists of single layers of cells that are square in shape. The nucleus of these cells is located in the centre and that present in the places that are subject to stress and injury.
2. Simple columnar epithelium: the cell is cuboidal and the nucleus is located at the centre. It is found in organ such as thyroid gland and kidney.
3. Simple columnar epithelium: the cells of this epithelium are columnar in shape i.e. tall and their nucleus is placed near the base. It has two forms, one called nonciliated epithelium. This is again subdivided into two types: columnar cells whose microvilli are present at the apical surface and globlet cells that secrete mucus to lubricate different organ. The other is the ciliated simple columnar epithelium that has cilia at their apical surface. Therefore, simple columnar epithelium is present in digestive, respiratory, reproductive and urinary pathway.
4. Pseudostratified columnar epithelium: all the cells of this epithelium are attached to basement membrane but only few reach the apical surface. Therefore, due to the false impression of multilayered tissue, it is called pseudostratified. In ciliated form of this cell, also called pseudostratified ciliated columnar epithelium, there is presence of cilia or globlet cells whereas in the alternative case i.e. pseudostratified form no cilia is found.
5. Stratified squamous epithelium: in this case, the cells are flat and shape from cuboidal to columnar. They exist in two forms, keratinized and nonkeratinised, in case of keratinized forms, there is layer of keratin whereas the other form is nonkeratinized where no keratin is present. Keratinized stratified squamous epithelium protects skin and underlying tissue from microbes whereas nonkeratinised stratified epithelium keeps the area moist and is present in esophagus and mouth.
6. Stratified cuboidal epithelium: In this case, cells in apical layer are cuboidal and mostly carry the role of protection.
7. Stratified columnar epithelium: like, stratified cuboidal epithelium, it is also rare type of epithelium that provides the function of protection.
Glandular epithelium: this is the other type of epithelium that completes the process of secretion. These are of two type exocrine or endocrine; in case of exocrine system interstitial fluid diffuse directly to the bloodstream whereas in case of endocrine system interstitial fluids first diffuse into ducts and then into lumen of lining of respective of organs. Exocrine secretion are sweat, wax etc whereas endocrine secretion is the hormone. Glandular epithelium is also two types simple and complex.
2. Connective tissue: it is another most abundantly distributed tissue that consists of cells and extracellular matrix. Different tissue that helps in protection, defense and structural importance are found within the connective tissue. Cells like fibroblast, adipocytes, mast cell, white blood cells, macrophages and plasma cells are found here. The extracellular matrix is ground substance and fibers. This tissue can be broadly classified into different types as per their importance, function and origin. They are embryonic connective tissue, mature connective tissue. Embryonic connective tissue is further classified as mesenchyme and mucous connective tissue whereas mature connective tissue is divided into loose connective tissue, dense connective tissue, cartilage, bone tissue and liquid connective tissue. Therefore, this ways through it sounds simple but tissues with proper connection and coordination help in functioning of the body effectively.
3. Muscular tissue and nervous tissue: muscle tissue contains muscle fibers and this muscle fiber are divided into three types, skeletal, cardiac and smooth muscle fiber and these muscle fibers are present at different places as per the necessity of movements and need of the body to operate in proper way. Likewise, nervous tissue is also an important tissue that helps to transmit message from one part to the other. This way different tissues work differently to accomplish various function in human body and make them live a healthy life.
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