Anatomy & Physiology

Anatomy & Physiology Study Guide

Chapter 4: The Tissue Level of Organization For the human body to work efficiently, several different types of cells must coordinate their efforts. Cells organized into layers create tissues —collections of specialized cells and cell products that perform a relatively limited number of functions. The study of tissues is called histology . Histologists recognize four basic types of tissue: epithelial, connective, muscle, and nervous. 4.1 Epithelial Tissue Epithelial tissue includes epithelia and glands. Epithelia are layers of cells that cover internal or external surfaces. Epithelia cover every exposed surface of the body. Epithelia form the surface of the skin, and they line all passageways that communicate with the outside world, such as the digestive, respiratory, reproductive, and urinary tracts. Glands are structures composed of epithelial cells that produce fluid secretions. Epithelia have several important characteristics: • Cellularity: In epithelia, cells are tightly bound together at specialized attachment sites called cell junctions . The three major types of cell junction are occluding junction (tight junctions), gap junctions, and desmosomes (macula adherens). Cells also attach to other cells or to extracellular protein fibers by using cell adhesion molecules (CAMs). • Polarity: An epithelium has an exposed surface that faces the exterior of the body or some internal space and a base that is attached to adjacent tissues. • Attachment: The base of epithelium is bound to a thin basal lamina or basement membrane. The basal lamina is a complex structure produced by the basal surface of the epithelium and the underlying connective tissue. • Avascularity: Epithelia lack blood vessels. • Regeneration: Epithelial cells that are damaged or lost at the exposed surface are continuously replaced through the divisions of stem cells in the epithelium. Epithelial cells have several structural specializations that distinguish them from other body cells. For the epithelium to perform the functions listed, individual epithelial cells must be specialized for the movement of fluids over the epithelial surface. They provide protection and lubrication, which helps the movement of fluids through the epithelium, or the production of secretions that provide physical protection. There are many specialized types of epithelia that are classified based on their cell shape and the number of cell layers between the base and the exposed surface of the epithelium. The three epithelial cell shapes are squamous, cuboidal, and columnar. In a sectional view, squamous cells appear thin and flat, cuboidal cells look like little boxes, and columnar cells are tall and relatively slender rectangles. The number of cell layers will determine if the tissue is simple (one layer) or stratified (multiple layers).

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