Epithelia Flashcards
What is the Epithelium in its most general definition?
Epithelia is a layer of contiguous cells (I.e touching and sharing a common border), which of varied embryonic origin (e.g. Urothelium is mesoderm, skin is ectoderm and GIT is endoderm), which lines the external and internal surfaces of the body.
How does the serosa differ from the mucosa?
Both the serosa and the mucosa are types of epithelium, however the mucosa liens surfaces which are exposed to the external environment, whereas the serosa encloses internal body cavities, which are potential spaces, for instance the pericardium, peritoneum and pleura. The mucosa consists of the epithelium, the lamina propria Dan the muscularis mucosa (in the GIT). The serosa consists of a layer of mesothelium and an underlying layer of connective tissue.
What is the function of the epithelium?
The epithelium has many functions, not least the formation of a barrier between two areas thus allowing regulated and selective movement of substances. This is particularly important when it comes to regulating fluid loss during heat regulation, while also acting as a physical barrier to pathogens. This latter function is aided by the normal flora of the skin, which acts as a biological barrier. The former function of heat regulation is aided by the arteriovenous anastomoses (AVAs) in the apical skin. Other functions of epithelium can include absorption, secretion and facilitation of smooth laminar flow.
What are the epithelial derivatives of the endoderm, mesoderm and ectoderm?
Endoderm - gastrointestinal tract epithelium, respiratory tract epithelium, the lining of the bladder (apart from the trigone), specialist glands such as the thymus, thyroid and salivary glands.
Mesoderm - trigone of the bladder, urogenital tract (I.e. vas, epididymis, uterus, ceviche, uterine tubes etc.), kidney tubules, serosal layers, blood vessels.
Ectoderm - skin, cornea.
Describe the structural specialisation of simple squamous epithelium
Simple squamous cells for a thin layer of smooth cells whose functions can generally include gas exchange, for instance at the lungs, substance exchange, for instance at the kidneys, and a surface for smooth laminar flow of blood, which is the endothelium. The cells are ideal for exchange due to their large surface area and short diffusion distance. They also form a surface for movement and lubrication. Locations therefore include the lungs as type I pneumocytes, endothelium and mesothelium.
What is the basement membrane?
The BM is consists of two layers, the basal lamina (containing lamanin) and an underlying layer of reticular connective tissue. It is secreted by the epithelial cells and functions to anchor them to the underlying lamina propria, while also acting as a cellular and molecular filter. The cellular filter is relevant to the invasion of neoplasms, while the molecular filter is significant to renal filtration, particular of negatively charged proteins.
Describe the structural specialisation of simple columner epithelium
Simple columnar epithelium mainly functions in absorption of substances. These cells may have functional adaptations, for instance micro villi and stereocilia. These adaptations both function to facilitate absorption in the gut and efferent ducts/rete testes respectively. In order to maintain the selective nature of absorption, tight junctions exist between the cells in order to polarise them and limit paracellular transport. In the uterine tubes motile cilia are present, which allow transport of the ovum and are under the control of oestrogen signalling.
Describe the structural specialisation of simple cuboidal epithelium
Simple cuboidal epithelium is mainly suited to the function of secretion and as such is mainly found in glands such as the thyroid, but almost have an absorptive function in the kidney tubules, where they are involved in the active transport of substances, as well as secretion of waste products into the ultrafiltrate, for instance potassium. Specialist functions in the thyroid include absorption of iodide and synthesis of thyroid hormones. In the PCT they can also synthesise bicarbonate and ammonia from glutamate.
Describe the structural specialisation of stratified squamous epithelium
Stratified squamous epithelium consists of multiple layers of epithelial cells which are suited to enduring abrasion. As such, such epithelium is generally a labile tissue type, repeatedly undergoing replacement. Such examples include the epidermis, vaginal mucosa and oesophogus. The skin is keratinised, whereas other locations are generally non-keratinised. Keratinisation refers to the layer of stratum corneum formed by keratinocytes, which is rich in keratin protein and lack nuclei. In the vagina the cells are rich in glycogen, a substrate for lactic acid production by lactobacilli in order to create a chemical barrier.
Describe the structural specialisation of pseudostratified columnar epithelium.
Pseudostratified epithelium consists of a single layer of cells which all have contact with the basement membrane, but whose nuclei are located at different levels due to the variable placement of cells. They are mainly found in the respiratory tract, where they have functional specialisations of cilia and are integrated with goblet cells. Together they form the mucocilary escalator, a type of physical innate immune barrier found in the teaches and bronchi.
Describe the structural specialisation of uroepithelium
Urothelium is the transitional epithelium of the urinary tract, present in the bladder, ureters and parts of the urethra, which can exist in two forms. In a non-stretched state the epithelium exists as a columnar cells, however in the distended state they exist as cuboidal cells. This enables the bladder to distend during fill in and allow continence. It also functions to protect the underlying tissue from the toxic components of the urine.
What is the difference between cilia, stereocilia and microvilli?
All ar types of cellular specialisations but each have different structures and functions.
Cilia - motile structures which enable movement of substances along the lumen which the cells line. Movement is created by the presence of microtubules and motor proteins which run along it.
Stereocilia - similar to cilia but supported by actin and, as such, are non-motile. Differ from microvilli in that they are much longer. These exist in the hair cells of the maculae inner ear semicircular canals and organ of corti.
Microvilli - non motile extensions of the plasma membrane which function to increase the surface area for absorption. Supported by actin microfilaments.
