Covering Epithelia:
Covering epithelia, also known as lining epithelia, are tissues composed of closely aggregated cells that form sheets covering the external surfaces and lining the cavities of organs in the body. They are essentially continuous, selectively permeable barriers between the external environment and the underlying connective tissues. This type of epithelial tissue plays a crucial role in a variety of bodily functions including:
- Protection: Serving as the first line of defence against physical damage, pathogens and dehydration. For example, the outer layer of the skin (stratified squamous keratinized epithelium) offers protection against external threats. Cilia on the epithelial cells in the intestines also protect against bacteria.
- Absorption: Facilitating the uptake of nutrients and other substances. For example, the simple columnar epithelium lining the stomach and intestines is specifically adapted for absorption.
- Secretion: Releasing substances such as mucus, enzymes and hormones. For example, glandular epithelium in glands releases enzymes, hormones, and fluids.
- Sensation: Detecting stimuli such as touch, pressure, and temperature. Taste buds on the tongue and the stereocilia in the ear are examples of sensory receptors embedded in epithelial tissue.
- Excretion: Eliminating waste products from the body. Examples include kidney epithelial tissue that excretes waste and sweat gland epithelial tissue that excretes sweat.
- Filtration: Removing particles and impurities from liquids and gases. Examples include the respiratory tract epithelium that filters air and the kidney epithelium that filters blood.
- Diffusion: Enabling selective movement of substances across the epithelium. Simple squamous epithelial cells, for instance, play a role in diffusion. This function is particularly important in areas such as the lining of blood vessels and body cavities.
All substances entering or leaving organs must cross an epithelium. Epithelial tissues are generally avascular, meaning they lack blood vessels. They receive nourishment by diffusion from the underlying connective tissue, which contains the microvasculature responsible for supplying nutrients and oxygen.
The area of contact between the epithelial tissue and connective tissue can be increased by small evaginations called papillae. Papillae are more common in epithelial tissues subject to friction, such as the skin or tongue.
The basement membrane, a specialised sheet of extracellular material, is present at the interface of the epithelial and connective tissues. It serves a vital role in supporting and anchoring the epithelial cells. It's worth noting that the terms "basement membrane" and "basal lamina" are sometimes used interchangeably.