LO: Define epithelial tissue in general terms
Epithelial cover and line internal and external surfaces of the body.
Epithelial tissues form a barrier between the other tissues of the body and the internal and external environments, a barrier across which all the exchanges with the environment (secretion, excretion, absorption) must take place and which defends the underlying tissues.
LO: List the features common to all types of epithelial tissues
- Totally cellular; no connective tissue fibres holding the cells together; integrity depends on cell junctions
- No blood vessels (avascular)
- Stem cells lie within the epithelium (self-regenerating)
- Separated from underlying tissue by a basal lamina
- Always supported by an underlying layer of connective tissue (for mechanical support) containing blood vessels (for metabolic support)
LO: Outline the classification of epithelial tissues based on their morphology
-> Simple Epithelia
Found only on internal protected surfaces
- Simple Squamous
Appearance: Extremely flattened cells with a large surface area
- region containing nucleus bulges from the cell (fried egg)
- layer of cytoplasm can be as thin as 0.1um (below the resolution of the light microscope
Function: thin cells ideal for rapid transport across them (e.g. lining the exchange area of the lung)
- smooth surface ideal for providing low-friction covering on internal organs (e.g. lining pleural cavities and covers the enclosed lungs)
* insufficient volume of cytoplasm to be involved in secretion
- Simple Cuboidal
Appearance: As wide as they are tall (cube), central nucleus, give the appearance of beads on a necklace
Function: Commonly act as simple building blocks (e.g. forming the walls of ducts
*Exception: Form the walls of thyroid follicles AND are involved in the formation of the thyroid hormone - Simple Columnar
Appearance: Longitudinal section; columnar in shape, nuclei lying at the same level towards the base of the cells Function: (a) absorption (e.g. in the lining of the intestine) - microvilli increase SA
(B) secretion (e.g. in the lining of the intestine) - mucus secreting cells cytoplasm swells with accumulated secretory material (goblet cells)
(C) moving material over the surface of the epithelium (e.g. in the lining of the airways) - cilia - Pseudostratified columnar
Appearance: Have an apparently columnar shape but the nuclei clearly lie at different levels in the lower half of the cells, this is a true simple epithelium; a single layer of cells all of which make contact with the basal lamina (clue for correct identification) but not all reaching the free surface
Function: No obvious link between morphology and function. E.g. In the airways, the lining produces mucus and therefore goblet cells are present but the lining also moves mucus up the airways and therefore cilia are present.
LO: Outline the classification of epithelial tissues based on their morphology
-> Stratified Epithelia
-> Type 1: Squamous
- Squamous
1(a). Stratified squamous epithelium
Appearance: Composed of multiple layers of cells
- Cells in basal layer are roughly cuboidal
- Cells take on a flattened squamous appearance as they move up through the layers
Function: Form a thick layer which provides defence against mechanical damage
- Line the oral cavity and oesophagus so that these can withstand shearing forces from coarse food materials
- Demosomes between the cells maintain the integrity of the epithelium
- Rete pegs are finger like extensions of the lowest layer of cells into the connective tissue which improvement attachment of the epithelium to underlying connective tissue
1(b). Keratinised stratified squamous epithelium
Appearance: As cells move up the layers they become more squamous in shape and accumulate granules of keratohyalin (stain dark blue with H&E)
-In adjacent more superficial layer, all cellular detail is lost (no nuclei or cell membranes can be seen) and the blue staining granules disappear to be replaced by a layer of red-staining keratin
Function: Keratin is physically strong, chemically inert and semi-waterproof
- Forms the epidermis (epithelial layer of skin) where it protects from mechanical damage, chemical damage and desiccation.
1(c). Parakeratotic stratified squamous epithelium
Only found in the compartments of the ruminant forestomach
Appearance: Similar to stratified squamous (1a) but the superficial layer of cells shows cellular detail AND keratinisation
Function: Protects against coarse ingesta
- Allows absorption of useful products of fermentation
LO: Outline the classification of epithelial tissues based on their morphology
-> Stratified Epithelia
-> Type 2: Transitional Epithleium
Appearance: Variable; composed of multiple layers of cells
- As cells move up from the base oof the epithelial they get fatter and rounder
- Some may contain two nuclei
- Do NOT contain keratin
Function: Found not in the urinary system
- Accommodates a variable volume of fluid without rupturing
- Modified superficial membrane of the most superficial cells provides protection
- Balloon shape allows for greater lateral stretching of the epithelium when the bladder fills
- When stretched he cells take on a squamous shape
- Examine the epithelium in the trough of folds where it is relaxed to avoid confusion with the stratified squamous subtype
