Epithelium

• Contains no blood supply
• Cell are arranged to form cohesive sheet with no or little extracellular matrix in between
• Displays a free surface – apical surface
• Opposite layer adheres to extracellular basement membrane, basal surface
• Display polarity – apical, luminal and lateral surfaces display structural specialization
• Epithelial cell are specialised for absorption, secretion or to act as barrier
• Lateral surfaces display junctional complexes for intercellular cohesion and communication
• One type of epithelium may change into another type – metaplasia eg replacement of the squamous epithelium of the oesophagus by columnar epithelium (Barrett's oesophagus) may result from reflux oesophagitis

• Coetzee et al p43
• Stevens and Lowe p33-34
• Wheater p80

Lateral surface

1. Cell membrane proteins acting as specialised cell adhesion molecules (CAMs)
2. Specialised areas of the cell membrane incorporated into cell junctions.

   Three types are recognized: occluding junctions, anchoring or adherence junctions and communicating junctions.

   • Occluding junctions bind cell together to form an impermeable barrier
     • Zonula occludens or tight junction

   • Anchoring junctions link the cytoskeleton of cells to each other and two underlying tissues
     • Zonula adherens provides mechanical strength
     • Macula adherens or desmosomes provides mechanical strength in tissues where there are tensile or shearing stresses, eg skin

   • Communications junctions allow direct cell-cell communication
     • Gap junction or nexus allow rapid communication for coordinated action

Luminal surface

• Microvilli – short finger-like projection of the cell membrane to increased surface area, visible with LM as brush border
• Cilia – hair-like surface projections of cells involved in transport, visible with LM
• Glycocalyx – thin extracellular layer consisting of protein glycoprotein and sugar residues; stains PAS positive; can act as enzyme, CAM or for cell recognition

Basal surface

• Basal interdigitations or folds – greatly enhance surface area

• Coetzee et al p43-46
• Stevens and Lowe p35-38
• Wheater p80

• Protein-secreting epithelial cells have large nuclei and abundant rER.
• Mucin-secreting epithelial cells have an expanded Golgi system
• Steroid-secreting epithelial cells have an extensive sER
• Ion-pumping epithelial cells have many mitochondria and a large surface area

According to number of cell layers

• Simple – single layer
• Stratified – 2 or more layers
• Pseudostratified – single layer but nuclei situated at different levels in the cell. All cells are in contact with the basement membrane, but not all cells reach the apical surface. Both conditions create the illusion of several cell layers.

According to cell shape in sections at right angle to the surface

• Squamous – Multiple layers of flattened thin cells with little cytoplasm and prominent nucleus
• Endothelium – squamous epithelium in cardiovascular system
• Mesothelium - squamous epithelium of mesodermal origin lining serous membranes and cavities
• Cuboidal - cell height, width and depth are the same, round centrally placed nucleus
• Columnar - cell height greater than width, nucleus elliptical or cigar shaped
• Transitional - stratified, top layer dome or umbrella shaped

• Coetzee et al p47 & 57
• Stevens and Lowe p33-34
• Wheater p80

Cells, tissues and organs can be identified using a few simple and straightforward criteria. Try and construct a mental flowchart: are all the cells the same type (tissues) or are there different cell types specifically arranged (organ): If you decide on an organ, is it hollow or solid?

Let us have a look at epithelium more specifically:

• Decide if there is one or more than cell layer
• Decide what shape the cells are
• Decide what shape the nuclei are
• Decide where the nuclei are situated
• Are there specializations visible on the apical surface?

Remember:

• Start with lowest possible magnification
• Never rely on colour, look at the morphology
• Never memorise images from the atlas, microscope or computer