Epithelial Tissue

Occurring in sheets of tightly packed cells, epithelial covers the outside of the body and lines organs and cavities within the body. The cells of an epithelial tissue, or epithelium, are closely joined, with little material between them. In many epithelia, the cells are riveted together by tight junctions. This tight packing enables the epithelium to function as a barrier against mechanical injury, microbes, and fluid loss. Some epithelia, called glandular epithela, absorb or secrete chemical solutions. For example, the glandular epithelia that line the lumen of the digestive and respiratory tracts form a mucous membrane; they secrete mucus that lubricates the surface and keeps it moist.

Two criteria for classifying epithelia are the number of cell layers and the shape of the cells on the exposed surface. A simple epithelium has a single layer of cells, whereas a stratified epithelium has multiple tiers of cells. A "psuedostratified" epithelium is single-layered but appears stratified because the cells vary in length. The shape of the cells at the exposed surface may be cuboidal, columnar, or squamous.

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Tissue Structure and Function

Tissues are groups of cells with a common structure and function. Different types of tissues have different structures that are suited to their functions. For example, a tissue may be held together by a sticky extracellular matrix that coats the cells or weaves them together in a fabric of fibers. In deed, the term tissue derives from a Latin word meaning 'weave.'

Tissues are classified into four main categories - epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

Light Microscopes

The microscopes first used by Renaissance scientists, as well as the microscopes you are likely to use in the laboratory, are all light microscopes (LMs). Visible light is passed through the specimen and then throught glass lenses. the lenses refract (bend) the light in such a way that the image of the specimen is magnified as it is projected into the eye, ont photographic film or a digital sensor, or onto a video screen.