Lecture 3 Stratified Epithelium

2 SQUAMOUSCUBOIDALCOLUMNARTRANSITIONAL COMPOUND EPITHELIA Stratified columnar epithelia are rare; they are sometimes found in the ducts of glands.

Stratified squamous epithelium In stratified squamous epithelium the lower cells are roughly polygonal in shape. As cells migrate towards the surface they become flattened. Found in areas requiring protection such as oesophagus, anal canal and vagina.

Note how cells at the surface are very flattened as opposed to the nearly columnar basal cells. Surface cells are continuously lost and replaced by cell division in deeper layers

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Transitional Epithelium Also called urinary epithelium as it is only found in the urinary system. Received its name because early microscopists thought it represented a transition stage between stratified squamous and stratified cuboidal epithelium. The two figures illustrate one of the basic properties of the epithelium which is that it is capable of being stretched. The upper section is from a relaxed bladder whilst the lower is from a distended bladder. Note how the shape of the surface cells can change when the the epithelium is stretched. Epithelium also capable of protecting underlying tissues from osmotic damage 22

25 Relaxed bladder

27 Stretched Bladder Compare the shape of the surface cells in relaxed and stretched bladder.

Modifications of Epithelial structure 28

Cilia Cilia are surface projections from cells which permit movement of materials / objects over the surface of the epithelium. In this section of (part of the lining of the fallopian tubes) it can be seen that not all of the cells are ciliated. 29

30 Thin epoxy section of the trachea showing cilia (C) on a pseudostratified columnar epithelium (arrows). Note how the goblet cells (G) between the epithelial cells lack cilia. C G

Microvilli Microvilli (MV) are finger-like projections from the apical surface of (usually columnar) epithelial cells. They increase the surface area of the cell surface and are found in areas involved with absorption such as the small intestine and proximal convoluted tubule of the kidney. In these two areas they are often referred to as a “brush border” 31 Brush Border

Stereocilia Stereocilia are very long, modified microvilli and concerned with absorptive functions. They are chiefly found in parts of the male reproductive tract.

35 Stereocilia on the epithelial lining of the epididymis (arrow). Note the pseudostratified columnar epithelium.(double arrow)

Keratinisation Characteristically found in the skin, this adaptation is for protection. The thickness of the keratin layer varies (depth of it here indicated by two arrows) in different sites. It is thickest in the sole of the foot and thinnest on the outer surface of the lip.

37 Keratinised stratified epithelium from the skin. Note how the basal layers are folded forming papillae. These serve to attach the epithelium to the underlying tissues. In inset note the mitotic figures; cells lost at the surface of both forms of stratified squamous epithelium are replaced by division of cells in the basal layers. Keratin layer Mitotic figure Papillae

Cell Adhesion 39

Function of Cell / Cell Junctions OCCLUDINGPrevent diffusion of substances between adjacent cells. ADHERENT Link actin filament network between adjacent cells FOCAL CONTACTSLink actin filaments of a cell to extracellular matrix DESMOSOMESLink intermediate filament networks of adjacent cells HEMIDESMOSOMES Connect intermediate filament network of a cell to t he extracellular matrix COMMUNICATINGAllow selective diffusion of molecules between adjacent cells

Junctional Complexes The EM shows some of the cell / cell junctions found between epithelial cells. Where different junctions occur close together as between these two intestinal epithelial cells they are known as junctional complexes.

42 Zonula Occludens Zonula adherens Desmosome