Epithelium.docx

Definitions Tissue is an aggregation of cells and extracellular substances. Organs can be composed of various proportions of any of these tissue types: Muscular tissue Epithelial tissue Nervous tissue Connective tissue Mesothelium lines body cavities. Endothelium lines vessels and heart chambers. St...

Definitions Tissue is an aggregation of cells and extracellular substances. Organs can be composed of various proportions of any of these tissue types: Muscular tissue Epithelial tissue Nervous tissue Connective tissue Mesothelium lines body cavities. Endothelium lines vessels and heart chambers. Stratified means that the cell’s nucleus does not lie on the basement membrane. Squamous means that the cell is flat (and elongated) in shape. Exfoliative cytology involves harvesting cells shed (from surface tissue, mucous membranes, or body fluids) and examining them under a microscope. Examples of exfoliative cytology include skin surface cytology, vaginal smear analysis, and fine needle biopsy. Adenomere are cells of the exocrine secretary end-piece making up a secretory unit that manufactures the product of secretion. Specialized cells (myoepithelial cells) will express these secretary products into a duct system. Myoepithelial cells are contractile cells that help squeeze secretions out. Myoepithelial cells are found in salivary, mammary, and sweat glands. Ducts convey products to their needed location. Ducts can be simple (having one opening) or compound having many branches of duct systems). Ducts can be lined with simple cuboidal, simple columnar, stratified cuboidal, or stratified columnar epithelium. Epithelium: General Info Epithelium is a tissue type that is composed of closely aggregated cells with very little extracellular substance. Epithelia is closely related to supporting connective tissue. Epithelia have a basement membrane. Epithelial cells are polarized (with apical, basal, and lateral surfaces) and cohesive. The apical surface is free (not connected to anything). The basal surface is attached to the basement membrane. The lateral surface is connected to another cell. Epithelial cells are cohesive (on basal and lateral surfaces) due to cell junctions. Epithelial tissue subjected to pressure and traction have intracellular adhesion. This is commonly seen in the epidermis and urinary bladder. Epithelia use transmembrane glycoproteins (E-cadherins) for cell junctions. Epithelium types include: Lining (surface) epithelia Lining epithelia forms sheets that cover body surfaces or lines luminal organs, tubular structures, and body cavities. Lining epithelia is avascular with few nerve endings. Lining epithelia can be further classified as simple or stratified. Simple lining epithelia refers to a single cell layer. EITHER simple or stratified lining epithelia can be further classified as squamous, cuboidal, or columnar. ONLY stratified lining epithelia can also be further classified as transitional. Additionally, ONLY the stratified squamous lining epithelia can be further classified as keratinized or non-keratinized. Glandular epithelia Glandular epithelia have specialized cells which synthesize, release, and store various products. Glandular epithelia can be further classified as unicellular or multicellular. Special epithelia Special epithelia contain sensory nerve endings. Special epithelia are found in the skin, ears, and on the tongue. The nose and eyes contain modified neurons rather than special epithelia. The 3 main germ layers of epithelia are the ectoderm, mesoderm, and endoderm germ layers. Small blood capillaries NEVER enter epithelia. Stem cells have mitotic capabilities and are utilized to repair epithelium via the constant loss or replacement of epithelia cells. Stem cell location varies based on the epithelia type it is in and based on that epithelia’s function. However, stem cells can be commonly found in the basal cell layer. Epithelia: Histogenesis (Origin) Ectoderm (germ layer) The ectoderm contains the epidermis of skin. Mesoderm (germ layer) The mesoderm contains mesothelium and endothelium. Endoderm (germ layer) The endoderm contains epithelial lining and glands of most of the GI tract, including: Liver Pancreas Respiratory system Urinary bladder Epithelia: Functions Epithelia functions include: Protection Friction reduction Cleaning Diffusion Absorption Sensation Secretion Epithelia vs Connective Tissue Epithelia have high cellular density (due to it having little intracellular matrix), while connective tissue has low cellular density (due to it having lots of intracellular matrix). Both epithelia and connective tissue: Are interfaced with an extracellular basement membrane which acts as a selective barrier. Are typically not visible under a light microscope. Stain predominantly with PAS for carbohydrates. Intercellular Junctions Intracellular junction types include: Gap junctions (AKA: communicating junction) Gap junctions are communicating junctions that allow ions and small molecules to pass through for intercellular communication. To allow for the passage of these ions or small molecules, gap junctions must have specialized proteins lining these pores to aid in their passage. The cellular communication that gap junctions facilitate are especially important for heart muscles and nervous tissue, as gap junctions allow for chemical and electrical signal transmission. Gap junctions provide cytoplasmic channels or tunnels from one cell to its adjacent cell. Gap junctions consist of Desmosomes Desmosomes are anchoring junctions that bind cells together and aid in reducing internal tension. Tight junctions Tight junctions are impermeable junctions that prevent molecules from passing through intercellular space. Hemidesmosomes are NOT considered to be intercellular junctions, however, they allow for the cellular attachment to the basement membrane. Epithelia: Classification Simple Squamous Lining Epithelia Simple squamous lining epithelia are composed of a single layer of flat, elongated cells with oval to round centrally located nuclei. Simple squamous lining epithelia commonly are found: Lining body cavities (AKA: mesothelium) Alveolar walls of lungs Lining inner walls of blood vessels and lymphatic vessels (AKA: endothelium) Simple Cuboidal Epithelium Simple cuboidal epithelium is composed of a single cellular layer that have a cuboidal shape (due to all sides of the cell being relatively the same size,) with cellular limits are usually well defined, are euchromatic, and have round nuclei. Simple cuboidal epithelium sit on the basement membrane via the use of hemidesmosomes for attachment. Simple cuboidal epithelium can be found in the: Eye lens Lining follicles of the thyroid gland Choroid plexus of the brain Lining ducts of many glands Simple Columnar Epithelium Simple columnar epithelium is composed of a single layer of tall, narrow cells with ovoid nuclei that are located near the basement membrane. Simple columnar epithelium can be found lining the: Luminal surface of stomach, gallbladder, small intestines, and large intestines Uterus and uterine tubes Simple Columnar Ciliated Epithelium Simple columnar ciliated epithelia are composed of a single layer of tall, narrow cells with ovoid nuclei that are located near the basement membrane, however, they also have cilia. Simple columnar ciliated epithelium can be found in the lining of the oviduct. Simple columnar ciliated epithelium can be visualized using Azan stain. Pseudostratified Epithelium Pseudostratified epithelium is composed of a single layer of irregularly shaped and sized cells that all sit attached to the basement membrane, but do not all touch the apical cell surface. Pseudostratified epithelium s considered to be a form of simple epithelia. Pseudostratified epithelium can be either ciliated (motile) or non-ciliated (non-motile). Pseudostratified epithelium can be found in the: Upper respiratory system Nasal tract Trachea Epididymis and ductus deferens (within the male reproductive system) If there are any issues with cilia, patients can be prone to chronic upper respiratory infections (URI) or male reproductive issues such as non-motile sperm. Cilia is commonly used to move mucus up and out of the upper respiratory tract, so any damage to cilia would lead to a lack of cleansing in the upper air ways resulting in chronic URI. Stratified Squamous Epithelium Stratified squamous epithelium are composed of multiple cell layers where the superficial cells have a squamous cellular shape. Stratified squamous epithelium can be either keratinized or non-keratinized. The cells on the surface of keratinized stratified squamous epithelium have lost their nucleus, and a are composed of mostly keratin. Keratin is a protective, water-resistant protein. NOTE: The top pic is non-keratinized. The bottom pic is keratinized (L: mammary teat, R: dental pad) Transitional Epithelium (Urothelium) Urothelium contains multiple cell layers where the bottom cell layer is attached to the basement membrane and the top layer of cells have an umbrella shape. Urothelium cells have specialized plasma membranes that provide osmotic barriers between tissue and urine. Urothelium can typically be found lining urinary passages within the urinary bladder. Glandular Epithelia Glandular epithelia can be classified based on: Unicellular vs multicellular Duct/secretary unit (adenomeres) shape Type of product Mode of secretion Unicellular Glands Unicellular glands are found in the epithelial lining and glands of the intestines (and GI tract as a whole) and the respiratory tract. Example of a unicellular gland would be the goblet cells. Goblet cells have an abundance of mucinogen granules in the apical portion of the cell, causing the goblet cell to have a cup-like/goblet shape. Mucinogen granules will stain blue with “Alcian Blue” staining. Multicellular Glands Multicellular glands can be classified as endocrine or exocrine glands. Endocrine glands secrete their products into the bloodstream and lack ducts. Examples of endocrine glands include adrenal glands, pituitary glands, parathyroid glands, thyroid, and endocrine pancreas. Exocrine glands have ducts and adenomeres which are used to secrete their products onto body surfaces or internal body cavities/tracts. Examples of exocrine glands include uterine glands, salivary glands, gastric glands, and exocrine pancreas. Adenomeres Adenomeres can be: Tubular Tubular adenomeres are straight or coiled. NOTE: Pic on the left is simple tubular vs R side is simple coiled tubular. Tubular adenomeres are found in sweat glands, stomach glands, and colon glands. Acinar Acinar adenomeres are pie-shaped with a small lumen. Acinar adenomeres can be found in the pancreas and salivary glands. Alveolar Alveolar adenomeres have a large luminal space. Alveolar adenomeres can be found in mammary glands, prostate, and sebaceous glands. Compound Glands and Parenchyma Parenchyma is the collection of secretory units and ducts of a compound gland. Stroma is comprised of connective tissue and separates the large glands within the parenchyma into lobes. These lobes can be further divided by connective tissue into lobules. Interlobular ducts are intercalated and striated ducts that arise from between lobules and lead to a lobar duct. Lobules with multiple adenomeres drain into intralobular ducts. Secretary Product Classifications Serous Serous products are water secretions that contain enzymes. Sweat is an example of serous products. Cerumen is the secretion from modified sweat glands in the skin of the external auditory canals. Mucous Mucous products are slick, viscous secretions. Mucous products are found in tracheal mucus. Mixed Mixed products are from a gland producing both serous and mucous secretions. An example of a mixed product would be saliva. Sebaceous Sebaceous products are oily secretions that can also be called “sebum”. Serous glands Serous glands are cells with round nuclei, basophilic cytoplasm within the basal portion, and acidophilic in the apical part due to the presence of zymogen granules. When serous glands are attained, the red portion is near the apical surface, while the purple portion is near the basal side of the cell. Mucous Gland Mucous glands are composed of cells with a flat, heterochromatic nuclei at the base of each cell, and vacuolated (frothy) pale cytoplasm. The mucinogen granules in the mucous gland are not detectable with routine staining. Mixed Gland Mixed glands are composed of mucous and serous glands which share a common duct system where the mucous acinar units are associated with the crescent of serous cells. The crescent of serous cells is referred to as a “serous demilune”. Sebaceous Gland Sebaceous glands are composed of cells with centrally located nuclei, a pale cytoplasm that has a foamy appearance. The foamy cytoplasm appearance is due to lipid droplets being washed during processing. Mode of Secretion Classifications Merocrine Merocrine secretion involves smooth lumen where no cytoplasm is lost during secretion production, leaving the cell intact. Merocrine secretion releases secretory granules as the secretory product. The merocrine secretion method is used in salivary glands, merocrine sweat glands, and exocrine pancreas. Apocrine Apocrine secretion involves apex bulges in the cell where some cytoplasm is lost producing the secretion. The apocrine secretion method is used in predominant sweat glands of domestic mammals, prostate glands, and mammary glands. Holocrine Holocrine secretion involves the disintegration of the entire cell along with its contents which are then utilized as the secretory product. The holocrine secretion method is used within sebaceous glands. Endocrine Endocrine secretion involves product secretion into blood vessels.