Epithelial Tissues: Types and Functions

• Organs are made up of four basic tissue types: epithelial tissue, connective tissue, muscular tissue and nervous tissue
• Tissue is an aggregation of cells and extracellular substances sharing a common purpose
• Cells make up tissues, which combine to form organs, which then create systems

Three Types of Epithelium

• Lining (surface) epithelia create sheets covering body surfaces or lining luminal organs, tubular structures, and body cavities
• Glandular epithelia have specialized cells that synthesize, store, and release various products
• Special epithelia contain sensory nerve endings and are found in the skin, ears, and on the tongue; the nose and eyes contain modified neurons

Functions of Epithelial Tissues

• Protection is offered, for example, epidermis protects from abrasion/injury, harmful chemicals, invading microbes, and water loss
• Friction reduction is achieved as smooth endothelial cells line the circulatory system, reducing friction between blood and vessel walls
• Cleaning is performed by ciliated respiratory epithelium that removes dust particles/foreign bodies from air passages
• Diffusion is promoted, where the endothelium of capillaries aids the diffusion of gases, liquids, and nutrients
• Absorption occurs as certain epithelial cells lining the small intestine absorb nutrients from digested food
• Secretion happens as epithelial tissue secretes chemical substances like enzymes, hormones and lubricating fluids
• Sensation is conveyed by specialized epithelial tissue containing sensory nerve endings from the skin, ears, and the tongue

Relevance of Epithelium

• An understanding of tissue organization, development and functioning
• Necessary to understand systemic histology because systemic histology concerns many organs and all glands that have an epithelial component
• Necessary to understand the terminology of tumors/neoplasia

Characteristics of Epithelium

• Supported by connective tissue (CT)
• Avascular
• Basement membrane (BM) is present
• Epithelial cells are cohesive
• Epithelial cells are polarised

Epithelial Cells are Polarized

• Epithelial cells have an apical (free) surface, lateral surfaces, and a basal surface

Classification of Lining Epithelia

• Simple epithelium consists of only one layer of cells
• Simple squamous, simple cuboidal, or simple columnar epithelium are potential classifications
• Pseudostratified columnar ciliated epithelium is another classification
• Stratified epithelium consists of multiple layers of cells
• Stratified squamous epithelium is classified as keratinized or non-keratinized
• Cuboidal/columnar epithelium is rare
• Transitional/urothelium exists

Lining Epithelia

• Simple Squamous Epithelium is composed of flat, elongated cells, with a round to oval nucleus, often centrally located
• Common locations for Simple Squamous Epithelium are the lining of body cavities (generally called mesothelium in pleural, pericardial, and peritoneal locations), alveolar walls in lungs, and the inner lining of blood and lymphatic vessels (called endothelium)

Types of Squamous Epithelium

• Stratified Squamous Epithelium composed of epithelium composed of several layers of cells
• Superficial cells determine the name = the superfical cells have a squamous shape
• It is present in two types: keratinized or non-keratinized

Simple Columnar Epithelium

• Tall, narrow cells with an ovoid nucleus located near the base of the cell
• Can be found lining the luminal surface of the stomach, small and large intestine, and gall bladder

Simple Columnar Ciliated Epithelium

• Lining of oviduct is ciliated and consists of columnar epithelium

Pseudostratified Epithelium

• Composed of a single layer of irregularly shaped and sized cells.
• All cells touch the basement membrane, but not all of them reach the apical surface
• Can be ciliated (motile)
• Examples include the lining of the nasal cavity, upper respiratory tract

Transitional Epithelium

• Lines urinary passages, for example, the urinary bladder
• The superficial cells contain a specialized plasma membrane that creates an osmotic barrier between urine and tissue fluids

Transitional Epithelium in Relaxed vs Stretched Bladder

• In a relaxed bladder, cells are cuboidal with a dome-shape
• In a stretched bladder, the lining is stretched as the cells appear flattened

Epithelial Repair

• Epithelial cells are constantly lost and replaced
• Stem cells are present which have high mitotic potential
• Location of stem cells varies depending on epithelial type and function
• Cells are pushed by new layers away from the basement membrane and are shed off

Exfoliative Cytology

• Harvesting cells shed from surface tissues, from mucous membranes, or found in body liquids and examining them under a microscope
• Skin surface cytology is used in differential diagnosis of dermatitis
• Urine sediment analysis is used when screening for lesions or neoplasia
• Vaginal smear analysis is a method of examination when looking at the estrus cycle
• Fine needle biopsy of masses is a diagnostic tool

Classification of Glandular Epithelia

• Number of cells in the gland (unicellular or multicellular)
• Shape of secretory units (adenomere)
• Type of product

Unicellular Glands

• Found in epithelial lining and glands of intestine and in the epithelial lining of the respiratory tract
• Goblet cells have a "cup”-goblet shape due to presence of abundant mucinogen granules in the apical part of the cell

Multicellular Glands

• Are modified epithelia
• Tubular glands: straight or coiled (sweat gland, stomach glands, intestinal glands)
• Acinar: pie-shaped with small lumen (pancreas, salivary glands)
• Alveolar: larger luminal space (mammary gland, prostate, sebaceous glands)

Glandular Collective Terms

• The collective of secretory units and ducts of a compound gland is termed parenchyma; the connective tissue elements comprise the stroma
• Large glands are divided into lobes, which are further subdivided by connective tissue into lobules

Classification According to Glandular Product

• Serous glands produce a watery product containing enzymes
• Mucous glands produce a slick, viscous secretion
• Mixed glands produce both mucus and serous secretion
• Sebaceous glands produce an oily secretion known as sebum