Concept and classification
Cell is the basic structural unit of the body. But one cell can not satisfy the requirements of the organism for function. In the multicellular organism cells differentiate to perform many special functions and form tissues. They are tissues that are responsible for maintaining body functions due to collaborative efforts of their individual cells. Tissues may be defined as aggregates or groups of cells organised to perform one or more functions.
There are four main functions (group of function), that organism must carry out: border from external environment; movement; regulation; support, nutrition and communication. All organs are made up of four basic tissue types.
The Four Basic (Fundamental) Tissue Types
1. Epithelial Tissue (Epithelium)covers body surfaces, lines body cavities, and forms glands. Creates the selective barrier between the body (internal environment) and exterior (external environment).
2. Connective tissue connects other tissue types, underlies or surrounds them. Provides support and nourishment. Produces and maintains many different intercellular substances.
3. Muscular Tissue (Muscle) is made up of contractile cells and is responsible for movement of the body and its parts. Demonstrates irritability, impulse conduction, and contraction.
4. Nervous tissue (Nerve) gathers, transmits and integrates information from outside and inside the body to control the activities of the body and its parts. Demonstrates irritability, transduces various types of energy into a nerve impulse, which is transmitted.
All of these main tissue types may be future subdivided on the basic or more specific characteristics of the various cell population as well as on the intercellular substances in those cases, where they present special characteristics.
Epithelial Tissue (Epithelium)
Epithelial tissue is the basic tissue which covers the body surfaces, lines body cavities, tubes and constitutes glands.
There are three types of epithelial tissues:
1. Covering and lining;
2. Glandular;
3. Sensory.
The covering and liningepithelium is characterised by its presence on free surface. Epithelium covers the exterior surfaces of the body (it is associated with the skin), lines internal closed cavities (pleural, pericardial, vascular system) and those body tubes that communicate with the exterior: respiratory, digestive, genitourinary tracts. Glandular epithelium forms the secretory portion of glands and their ducts (the parenchyma; connective tissue forms the stroma of glands). Sensory epithelium forms receptors of many organs.