Explains covering epithelia and their roles in protection, absorption, secretion, sensation, excretion, filtration and diffusion, then covers features such as avascularity, papillae, the basement membrane, cell polarity and regeneration. The note moves on to simple epithelia, classifying them as squamous, cuboidal or columnar by cell shape, and ends with ciliated and microvillous surface specialisations and where each is found.
Topic 41 of 97 in the MU Sofia syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 16 sections, about 251 words (5 min of reading), with 12 flashcards.
In this note
Covering Epithelia: An Overview
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.
Epithelial cells are characterized by polarity. They have an apical pole facing the external environment or the lumen of an organ, and a basal pole that attaches to the basement membrane.
Epithelial tissues exhibit a high regenerative capacity and can quickly repair and replace damaged cells. This regenerative ability is particularly important in areas exposed to constant wear and tear, such as the skin and the lining of the digestive system. For example, the human epidermis is renewed approximately every 15 to 30 days.
Epithelial cells also possess specialised cell contacts, such as tight junctions and desmosomes. These junctions help maintain the integrity of the epithelial layer and regulate the passage of substances between cells.
Simple Epithelia
Simple Epithelia
Epithelia are tissues composed of closely aggregated cells with strong adhesion to one another and a thin layer of extracellular matrix (ECM). They are cellular sheets that line the cavities of organs and cover the body surface.
Simple epithelia are tissues that are composed of a single layer of cells that all rest on the basement membrane.
They can be classified by the shape of their cells into:
Simple squamous: Thin cells where the nucleus is the thickest and most visible structure. They regulate passage of substances into the underlying tissues. Simple squamous epithelia are found lining vessels, body cavities, the heart, and lymphatic vessels.
Simple cuboidal: Cells that are roughly as tall as they are wide. They are generally thicker than simple squamous epithelia and contain a higher density of organelles, like mitochondria, to carry out active transport and other functions. Simple cuboidal epithelia are commonly found in the lining of kidney tubules and covering the ovary and thyroid.
Simple columnar: Cells that are taller than they are wide and often specialised for absorption. Simple columnar epithelia are typically found lining the stomach, intestines, and gallbladder.
Simple epithelia can also be classified by the presence of surface specializations such as cilia and microvilli. For example, simple columnar epithelium with cilia is found lining the trachea, bronchi, and nasal cavity, where the cilia help to remove particles from the air passages. Simple columnar epithelium with microvilli is found in the small intestines, where the microvilli increase the surface area for absorption.
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