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Page 43 44 of 135 Chapter 4: Histology tive unit of many very similar cells that perform a special function. Histology = study of tissues. There are 4 major groups: epithelial, connective, muscle, and nervous. EPITHELIAL TISSUE Structure (Morphology): Epithelial tissue occurs in sheets of cells. e Upper free surface, is called the apical surface e Deeper surface is attached to tissue beneath, called a basement membrane apical surface basement membrane L] e Avascular — because there are no blood vessels...blood supply reaches these cells by diffusion. e Innervated — a nerve runs through the tissue (notice how your arm hairs stand on end when cold!) Where would you find this tissue? e Covering outside of body = skin e Lining internal cavities that open to the outside = lungs, where urine forms in our kidneys, e Organs of our digestive tract (esophagus, stomach, intestines) e Lining both endocrine and exocrine glands Function: cover and line body surfaces; built for secretion, absorption, filtration, and diffusion How is epithelial tissue classified? 1. Number of cell layers (1 or more) a. simple epithelium has 1 layer of cells b. stratified epithelium has 2 or more layers of cells 2. Shape of most of the cells composing them a. : (shaped like floor tiles)& --nucleus is flattened b. . (shaped like dice) --nucleus is round C. . (shaped like bricks or cones on end) |:| --nucleus is oblong Cell junctions associated with tissues, such as ET: 1. Tight junctions (occluding junctions) = form an impermeable barrier to the passage of fluids. 2. Gap junctions (communicating junctions) = connection between cytoplasm for ions/molecules to flow. Form the intercalated discs of the heart. 3. Desmosomes = structure used to adhere types of squamous ET together (prevents stresses). Desmosomes account for the reason our skin peels off in sheets.