Epithelial Tissue

Every organ in an animal's body is built from just four basic kinds of tissue, and the first of these is epithelial tissue — the tissue that covers external body surfaces and lines internal cavities, ducts and tubes. Epithelial cells are packed tightly together with almost no space between them, held on a thin, non-cellular basement membrane that anchors the sheet to the connective tissue beneath. Because the cells sit shoulder to shoulder with very little intercellular matrix, an epithelial sheet forms a continuous, largely impermeable-except-where-needed barrier, which is exactly the property that makes it suited to covering and lining roles.

Simple epithelium is only one cell thick, and it is classified by the shape of that single layer of cells. Simple squamous epithelium is made of flat, plate-like cells, each with a flattened, disc-shaped nucleus, fitted together like tiles on a floor. Because the cells are so thin, this is the epithelium of choice wherever rapid diffusion is required — it lines the air sacs (alveoli) of the lungs, where oxygen and carbon dioxide must cross with minimum resistance, and it forms the walls of blood capillaries and Bowman's capsule in the kidney nephron for the same reason. Simple cuboidal epithelium is made of cube-shaped cells with a central, rounded nucleus; it lines the tubules of the kidney and the ducts of many exocrine glands, where its slightly greater cell volume (compared to squamous cells) supports active secretion and absorption. Simple columnar epithelium consists of tall, pillar-like cells, again with a single layer, lining the stomach and intestine; here the epithelium is frequently interspersed with goblet cells, which are modified columnar cells that secrete protective mucus. A further modification of columnar (or sometimes cuboidal) epithelium bears fine, hair-like cilia projecting from the free surface — ciliated epithelium — found lining the trachea and bronchioles of the respiratory tract and the fallopian tubes of the female reproductive tract; the cilia beat in a coordinated, wave-like rhythm that sweeps mucus, trapped dust particles, or in the fallopian tube the released ovum, steadily along the length of the tube.

Where a surface is subject to considerable mechanical wear and tear, a single layer of cells would not survive, and the body instead uses compound (stratified) epithelium, in which several layers of cells are piled one on top of the other. The outermost layer of the skin, the epidermis, is the standard example: it is stratified squamous epithelium, with the deepest cells actively dividing and pushing older cells upward, where they flatten, die and are eventually shed or worn away, continuously renewing the protective covering of the body. Because compound epithelium is built for protection rather than for absorption or secretion, its cells are packed even more densely and its permeability is correspondingly lower than that of any simple epithelium.