HISTOLOGY The study of Tissues

Overview of Tissues All the organs of the body are made up of four basic tissues: (1) Epithelial, (2) Connective Tissue, (3) Muscle Tissue and (4) Nervous Tissue.EpithelialConnective TissueMuscle TissueNervous Tissue

Epithelial Tissue Cells that line surfaces: Epithelium is the name given to the cells that line a surface (surface of the body- epidermis; surface of body cavity and internal organs; inner surface of ducts and tubes and glands (sweat, salivary, etc.) are also formed by epithelial cells.

Functions of Epithelial Tissue Form barriers - are always contiguous with one another and are usually joined by special junctions. (tight gaps, etc.) ; intercellular space between epithelial cells is very limited. Permeability – some are much more impermeable than others. The epidermis of the skin is an impermeable barrier; the epithelium lining the intestine, while providing a barrier, must also absorb nutrients. Secretory Function- (eg. stomach, glands)

Terminology Free surface: All epithelia have a free surface, that is to say a surface to which no cellular or extracellular formed elements adhere. The free surface may contain specialized modifications, such as microvilli (increase the absorptive surface) or cilia (move secretions). Basement Membrane: All epithelia rest on a basement membrane, through which it is attached to the underlying tissue.

Types of Epithelial Tissue Epithelia are classified according to the number of layers of cells they contain and the shape of the cells. LayersShape of Cell Simple – 1 layerSquamous – flattened cells Stratified – 2 or more layersCuboidal – same height, width, depth Columnar – height exceeds their width Note: if there are multiple layers and they don’t look similar you comment on the outermost layer regarding the shape

Simple Squamous Epithelium Figure 1 shows the simple squamous epithelium lining within the capsule of the kidney

Simple Squamous Epithelium Figure 2 shows the aorta lined by a layer of flattened cells called endothelial cells (arrows).

Simple Cuboidal Epithelium Figure 4 shows parts of several thyroid gland with cuboidal cells in a roughly circular pattern.

Simple Cuboidal Epithelium Figure 5 shows parts of the kidney tubules consisting of simple cuboidal epithelium.

Simple Columnar Epithelium Figure 6 shows the tall simple columnar epithelium of the duodenum with bears a brush border formed by microvilli which greatly increase the absorptive surface.

Stratified Squamous Epithelium This type of epithelium is found on tissues likely to encounter friction. Figure 7 shows the stratified squamous epithelium of the esophagus (the shape of the cells can be inferred from the shape of the nuclei and that they are identifiable in all layers)

Stratified Squamous Epithelium

Figure 8 shows another example of stratified squamous epithelium of the epidermis of skin. The cells can be seen to become more flattened toward the deep-staining red layer. The deep – stained area are cells filled with keratin (protein) through a process known as keratinization. More specifically this epithelium is known as keratinized stratified squamous cell epithelium or cornified stratified squamous epithelium associated with the soles and palms.

Stratified Squamous Epithelium

Stratified Cuboidal Epithelium Figure 9: demonstrates stratified cuboidal epithelium associated with ducts of various glands (sweat glands, sebaceous glands)

Stratified Columnar Epithelium Figure 10: Found associated with the largest ducts of glands (submandibular).

In addition to the above, there are two special types of epithelium called Pseudostratified and Transitional.

Pseudostratified Epithelium Figure 11 demonstrates Pseudostratified epithelium which has the appearance of being stratified, because it has cell nuclei at different heights within the epithelium. This is because it has some low cells which do not reach the free surface, and some tall cells, which do. However, all the cells rest on the basement membrane, so it is in fact a simple epithelium. In sections of pseudostratified epithelium, it is quite often difficult to discern that all the cells sit on the basement membrane. The distribution of pseudostratified epithelium is fairly limited. It is found in parts of the respiratory and male reproductive systems.

Pseudostratified Epithelium

Transitional Epithelium Figure 12 shows Transitional epithelium found in parts of the urinary system (kidney, ureter, urinary bladder urethra) that can be distended by the pressure of urine. The thickness of transitional epithelium depends on the state of distension (full) or contraction (empty). When the ureter or bladder is empty, the epithelium can be six or more layers deep, when it is distended, the epithelium is only two or three layers deep.

Transitional Epithelium