Epithelial tissue is the basic tissue which covers surface areas: the spaces, surfaces and the cavities of the body. There are three types of epithelial tissues: Covering and lining, Glandular, Sensory The covering and lining epithelia are associated with skin, the gastro-intestinal tract, the respiratory tract, etc. The function of these cells varies according to the location and morphology: the skin protects, the glands secrete, the gastro-intestinal tract absorbs, the alveoli of the lungs allow diffusion of gases. Some epithelial cells have motile processes on their surfaces called cilia. These create wave- like activity on the cell surface and help with surface transport across the epithelium. The epithelium acts as the interface between the outside world and the rest of the body.

Therefore, the epithelium must act as a permeability barrier. They are contiguous, the cells are attached to one another. The cells rest on an extracellular matrix which they produce, called the basement membrane at arrows. The basement membrane consists of the basal lamina produced by epithelial cells, and a more fibrous reticular layer which is below the basal lamina. The fact that one part of the layer is exposed to the lumen or outside world, and the other side is attached to the basement membrane gives each cell or layer specific polarity the apical side is the side towards the lumen, the basal side is the side closest to the basement membrane In many epithelial cells, the nuclei are somewhat basal.

Some organelles, such as the Golgi apparatus, are typically supra nuclear, they are between the nucleus and the cell apex. Epithelia are a vascular. This means that blood vessels do not penetrate up between these cells, as they do in connective tissue and muscle cells. Epithelia are unlike muscle and connective tissue in that they can be derived from any of the three embryologic germ layers endoderm, mesoderm, ectoderm. A major cytoskeletal component of epithelial tissue is the intermediate filament called keratin. This differentiates epithelial cells from other cells types, intermediate filaments of connective tissue are called vimen tin and intermediate filaments of muscle cells are called des min.

The shapes are squamous are very flat cells cuboid al- taller than squamous cells, all four sides approximately equal, and columnar are cell length is greater than cell width. The Number of Layers of cells in the epithelium are; simple are a single layer of cells; all cells sit on the basement membrane. Stratified are several cells stacked upon one another; not all cells touch the basement membrane. The shape of the apical stratum determines the shape designation of the epithelium.

Pseudostratified is a simple epithelium, but gives the illusion of being stratified. All cells do sit on the basement membrane, his is not always visually apparent because the nuclei are at various levels. Surface Specializations, these classifications are used in combination to describe a particular type of epithelium. For example, the epithelium lining the trachea is ciliated pseudostratified columnar epithelium, which describes the shape of the cells, the number of cell layers, and the surface specializations.