Lecture 1

Structure of Epithelial Tissues ONE CELL THICK SIMPLE MORE THAN ONE CELL THICK COMPOUND EPITHELIA

SIMPLE EPITHELIA SQUAMOUSCUBOIDALCOLUMNAR

Simple Squamous & Cuboidal Epithelia Squamous (single arrow) Formed by flattened cells whose nuclei often appear to bulge outwards. Found in places where there is movement of materials and even cells across the epithelium. Example here is from the loop of Henle in the kidney, also found lining all blood vessels, forming Bowman’s capsule in the renal cortex. Cuboidal (double arrows) In section cell profiles appear as squares with central nuclei. Found lining tubules in kidney, walls of thyroid follicles Often involved in secretory functions

l Nucleus of simple squamous cell Nucleus of simple cuboidal cell

Simple Columnar Epithelium Found extensively in the gut, glands and ducts of glands. Cells taller than they are wide although height variable. Ovoid nucleii basally located. Example here from the stomach. Cells often involved in secretion (as in the stomach) and also in absorption as in the small intestine

Pseudostratified Epithelia In a simple epithelium all of the cells are attached to a common basement membrane. Commonly in simple epithelia the nuclei in the epithelial layer all appear at the same level. The epithelium shown opposite has nuclei at different levels giving the appearance of several layers. In fact all of the cells attach to a common basement membrane and the epithelium is referred to as pseudostratified. As the cells are columnar in form this is a pseudostratified columnar epithelium. It is found in the trachea and some of the airways where it is sometimes know as respiratory epithelium and parts of the male reproductive tract.

Pseudostratified Columnar Epithelium The upper picture is from the seminal vesicle and the lower from the trachea. In both cases note how the nuclei are at different levels in the cells giving the appearance of more than one layer. In the case of the section of trachea the columnar cells carry a surface specialisation (cilia - arrowed) and also have flask shaped goblet cells between them (G)

Lecture 3 Stratified Epithelium

COMPOUND EPITHELIA Stratified columnar epithelium rare; they are sometimes found in the ducts of glands. squamous CUBOIDAL COLUMNARTRANSITIONA L

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

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

Relaxed bladder

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

Modifications of Epithelial structure

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.

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”

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

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.

Mitotic figure Keratin layer Papillae 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.

Cell Adhesion

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.

zonula Occludens Zonula adherens D esmosom