Cells are composed of molecules; a tissue has like cells; an organ contains several types of tissues; and several organs are found in an organ system.
A tissue is composed of specialized cells of the same type that perform a common function in the body. The science which study of tissue is called (Histology).The tissues of the human body can be categorized into four major types:
1. Epithelial tissue
2. Connective tissue
3. Muscular tissue
4. Nervous tissue
These tissues are formed by cells and molecules of the extracellular matrix. The main characteristics of these basic types of tissue are shown in Table (1). Organs can be divided into parenchyma, which is composed of the cells responsible for the main functions typical of the organ, and stroma, which is the supporting tissue.
Table (1). Main characteristics of the four basic types of tissues.
Tissue Cells Extracellulalar matrix Main functions
Epithelial Aggregated polyhedral cells Only small amount Lining of surface or body cavities, glandular secretion
Connective Several types of fixed and wandering cells Abundant amount Support and protection
Muscle Elongated contractile cells Moderate amount Movement
Nervous Intertwining elongated processes None Transmission of nervous impulses
Epithelial Tissue :
Epithelial tissues, also called epithelium (pl., epithelia), are composed of closely aggregated polyhedral cells with very little extracellular substance. These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities.
The principal functions of epithelial tissues are :
- the covering and lining of surfaces (eg, skin)
- absorption (eg, the intestines)
- secretion (eg, the epithelial cells of glands)
- sensation (eg, neuroepithelium)
- contractility (eg, myoepithelial cells)