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Transitional Epithelium

Link of Instagram: https://instagram.com/knowing_anatomy... Transitional epithelium is a stratified tissue made of multiple cell layers, where the cells constituting the tissue can change shape depending on the distention in the organ. When the organ is filled with fluid, cells on the topmost layer of this epithelium can stretch and appear flattened. Alternately, they can also appear cuboidal with a rounded shape when the fluid pressure is low. Structure of Transitional Epithelium Transitional epithelia are made of 3-4 layers of cells with the lowermost or basal layer staying in contact with the basement membrane. The cells of this basal layer are attached to the lamina propria through tonofilaments and hemi-desmosomes. These are among the least differentiated cells in this tissue and support the remaining cells. Cells in the intermediate layers are proliferative and can replenish cells lost due to abrasion or infection. They also have a vast Golgi network that contains a number of membrane-bound vesicles. The superficial layer of cells can change from being cuboidal to appearing flattened when the organ is distended and contain a number of actin-based cytoplasmic projections known as microvilli. A two-dimensional network of hexameric plaques covers the apical plasma membrane of these cells. These plaques are made from a protein called uroplakin and are an important feature of this epithelium, contributing to the permeability barrier for water, ammonia, urea and many other solutes of the urine. They are also likely to be involved in the ability of these cells to transition in shape. This epithelium is found lining the urinary bladder, ureters and urethra, as well as in the ducts of the prostrate gland. Function Due to its location in the excretory system, especially in the ureters and urinary bladder, one of the primary functions of this tissue is to be an extremely effective permeability barrier, impenetrable to water and most small molecules. The cells of this tissue are probably among the most resistant to osmotic pressure. Urine is hypertonic with a much higher concentration of many solutes compared to the cytoplasm of the epithelial cells. Yet, these cells are protected from desiccation even when the epithelium is fully stretched. Toxic wastes are also prevented from re-entering the blood stream. The second important function of these cells is to allow the organ to stretch and increase its volume depending on fluid pressure. For instance, when there is a lot of water that needs to be expelled from the body, a large quantity of liquid passes through the ureters, urinary bladder and urethra. The ability of the cells in the superficial layer of this epithelium to change shape (transition from a rounded cuboidal shape to a flattened squamous structure) allows these organs to stretch without exposing underlying tissue to the toxic substances in the urine