Tissues
A tissue is a group of cells and intercellular substances that interact in one or more tasks. There are four types:- Epithelial tissue
- Connective tissue
- Muscle tissue
- Nervous tissue
Epithelial Tissue
In epithelial tissue, one surface is free and the other adheres to a basement membrane. Epithelial tissue lines the body surface, cavities, ducts, and tubes.
Simple epithelium is one-cell thick and may have flat (squamous), cuboidal, or columnar cells.
This animation (Audio - Important) describes the structure of epithelial tissue.
Stratified epithelium has two or more layers. It functions in protection, as in skin. Cells in the layers may be squamous, columnar, or cuboidal.
In epithelia, cells are linked tightly together with specialized junctions providing both structural and functional links between individual cells.
Tight junctions provide seals to prevent leaking across the free epithelial surface. An example is epithelia in the digestive tract that prevents attack of the wall by acids and enzymes.
Adhesion junctions are like "spot welds" that cement cells together so that they function as a unit. Skin is an example.
Gap junctions promote diffusion of ions and small molecules from cell to cell.
This animation (Audio - Important) describes cell to cell junctions.
Glands are secretory cells or organs derived from epithelium.
Exocrine glands secrete products onto free epithelial surface via ducts or tubes. They secrete mucus, saliva, wax, and milk, for example.
Endocrine glands are ductless and release hormones into the fluid that surrounds the gland.
This website has some excellent pictures of epithelial tissue. Click through the menu links on the left hand side of the page to get a quick overview.