Epithelia

• line body surfaces, serous cavities, vessels and "tubes"
• have absorptive, secretory and protective functions

Epithelium (singular) often forms part of a mucosa (Lt mucosa : mucus, slime): this is a complex tissue incorporating several primary tissue types and refers to the epithelia and its underlying supportive (connective) tissue and sometimes a layer of smooth muscle (muscularis mucosae)

• ectoderm gives rise to the lining of the body surface (ie skin)
• mesoderm gives rise to the lining of serous body cavities (termed mesothelium) and the lining of vessels (termed endothelium)
• endoderm gives rise to the lining of the body's "tubes" (ie intestines, bronchi)

Epithelia can be classified according to

• their type: squamous, glandular, transitional
• the number of cell layers: simple, stratified
• the shape of the cells: cuboidal, columnar, flattened
• whether they have surface specialisations: cilia, keratin

Epithelia are separated from the underlying stroma (Gk stroma : something to lie or sit on) by a basement membrane which varies in thickness depending on the type of epithelium. The basement membrane has a supportive function and is formed of ground substance and collagen types III and IV. The epithelium and stroma both contribute to formation of the basement membrane

1. Simple Epithelia

• comprise a single layer of cells which all rest on the basement membrane
• consist of cells that vary in shape from flattened to columnar according to their function
• are found:
  • lining absorptive or secretory surfaces (eg intestines)
  • where minimum barrier to passive diffusion is required (eg alveoli, vessels)
• provide little protection against mechanical abrasion due to their thinness
• may have surface specialisations such as microvilli or cilia

Simple squamous epithelium consists of a single layer of flattened cells is found lining surfaces involved in the transport of gases (eg alveoli) fluids (eg lymphatics, blood vessels) also lines the serous body cavities

	1. Small blood vessel
	2. Renal Glomerulus
	3. Serous cavity (Pericardium)

Simple cuboidal epithelium consists of a single layer of square shaped cells is found lining small ducts and tubules that have secretory, excretory or absorptive function

	4. Bile duct
	5. Renal medulla

Simple columnar epithelium is similar to cuboidal epithelium except that the cells are taller has elongated nuclei which are usually at the base of the cell is found lining absorptive surfaces (eg small intestine) secretory surfaces (eg stomach)

	6. Villus, small intestine
	7. Electron micrograph - Microvilli, small intestine
	8. Villus small intestine
	9. Electron micrograph - Villus small intestine

10. Bronchus

2. Stratified epithelia

• consist of two or more layers of cells
• have primarily a protective function; the degree and nature of the stratification depend on the type of stress to which the surface is exposed
• are not suited to absorption or secretion due to their thickness

Stratified squamous epithelium is usually several layers thick matures progressively from the basal layer which has cuboidal-shaped cells through to the surface layer where the cells are flattened degenerates when it reaches the surface and eventually sloughs off can withstand moderate abrasion does not cope well with dessication unless the surface is keratinised

	11. Cervix
	12. Skin

Stratified cuboidal epithelium consists of two or three layers of cuboidal cells is found lining larger excretory ducts such as those of salivary glands, sweat glands and pancreas does not have an absorptive or secretory function but provides a more robust lining than that of simple epithelium

13. Sweat gland,skin

Transitional cell epithelium is a special form of stratified epithelium only found in the urinary tract (also called urothelium) is specialised to withstand the toxicity of urine and to accommodate a high degree of stretching has an appearance intermediate between that of stratified cuboidal epithelium and stratified squamous epithelium, hence "transitional" in the relaxed state, the epithelium appears to be 4-5 cells thick and the cells are cuboidal; in the stretched state, the epithelium appears to be 2-3 cells thick and the cells are flattened

 

	14. Bladder

3.Glands

• are epithelial structures which discharge a secretory product
  • exocrine glands discharge their secretory product via a duct onto an epithelial surface
  • endocrine glands secrete hormones directly into the bloodstream
• are composed of highly specialised epithelial cells, the appearance of which reflects the nature of their secretory product and mode of secretion

Exocrine glands

• are formed of ducts and secretory portions
  • ducts can be straight (simple glands) or branched (compound glands)
  • secretory portions can be tubular or acinar
  
• discharge their secretory product by
  • exocytosis (eccrine secretion)
  • releasing unbroken, membrane-bound vesicles (apocrine secretion)
  • releasing whole secretory cells with subsequent cell lysis (holocrine secretion)

	15. Colonic mucosa
	16. Pancreas

Endocrine glands

• consist of clumps or cords of secretory cells surrounded by a rich vascular network
• produce hormones
• discharge their hormones directly into the bloodstream

17. Pituitary gland

4.Basement membranes

• divide epithelia from the subjacent stroma
• are formed of an amorphous matrix of ground substance and fibrils of laminin, collagen types III (reticulin) and IV
• are formed by contributions from both the epithelium and stroma
• are stained by the periodic acid-Schiff method and by the silver method
• have several functions
  • support the epithelium
  • act as a selective filter (eg glomerulus)
  • act as a selective barrier to passage of cells between epithelia and connective tissue

	18. Kidney
	19. Squamous epithelium

5. Cell Junctions (Junctional complexes)

• are specialisations of the cell membrane that allow epithelial cells to
  • adhere to each other
  • exchange information and metabolites
• there are four different types of cell junctions:

• tight junctions
  • begin just below the luminal surface
  • consist of areas of fused cell membrane alternating with areas that are not fused
  • form a continuous band around each cell, thus sealing the intercellular space from the lumen
• adherent junctions
  • are found below tight junctions
  • do not show cell membrane fusion
  • have fine filamentous material radiating from their cytoplasmic aspect
• desmosomes
  • provide strong points of cohesion between cells
  • act as anchorage points for the cytoskeleton of the cell
  • consist of paired electron dense structures, one on each side of the cell membrane, into which fibrillar elements of the cytoskeleton converge
• gap junctions
  • are broad areas of closely apposed cell membrane, but there is no membrane fusion and a narrow gap remains
  • permit passage of ions and other molecules between cells

20.Surface epithelial cells, small intestine

Key concepts:

• Epithelium lines all surfaces of the body whether the surface is external (skin) or internal (alimentary tract; lining cavities or ducts).
• The structure and specialisations of epithelial cells reflect their functional requirements (eg. absorptive cells of the alimentary tract are specialised with microvilli to increase surface area for absorption and abundant cytoplasm and organelles for processing and metabolism of absorbed materials).
• Lining epithelia are commonly classified according to their number of layers (eg. simple or stratified) and the morphology of their outermost layers (ie. squamous, cuboidal, etc). Transitional epithelium (urothelium) is a highly specialised stratified epithelium only found in the urinary tract.
• Epithelia also form exocrine and endocrine glands. Exocrine glands may be simple or compound (referring to single or branched ducts) and their secretory units may be tubular or acinar.
• Epithelia are always supported by an underlying basement membrane with important structural and functional properties. Similarly junctions between epithelial cells are specialisations of several types that contribute to the functions of the epithelium as a whole.

A typical exam question:

Oesophago-gastric junction