Epithelial Tissue PDF 2024

Casey Boothe, PhD Epithelial Tissue SOM 610 10/8/24 Learning Objectives 1. Describe the difference between epithelia proper and secretory epithelia. 2. Use the morphology of epithelia to classify them into 6 categories. 3. Describe the differences between exocrine a...

Casey Boothe, PhD Epithelial Tissue SOM 610 10/8/24 Learning Objectives 1. Describe the difference between epithelia proper and secretory epithelia. 2. Use the morphology of epithelia to classify them into 6 categories. 3. Describe the differences between exocrine and endocrine glands. 4. Classify glands into categories based on morphology, mode of secretion, and product of secretion. 5. Describe the difference between classification and identification. Epithelium Muscle Aggregated polyhedral cells – tightly bound together Elongated contractile cells Small amount ECM Moderate ECM amounts Functions: Function: Covering, lining, protecting body surfaces Body movements Absorption (e.g. intestinal lining) Visceral functions (circulation, gut peristalsis, Secretion (e.g. glands) etc.) Basic Tissues All have cells + extracellular matrix Connective Nervous Numerous cell types Elongated cells – extremely fine processes Abundant ECM Little ECM Functions: Function Support and protection of tissue/organs Transmission of nerve impulses General Characteristics of Epithelia Cells show polarity Apical vs basal poles Avascular (unlike underlying connective tissue) Basement membrane found at interface between epithelium and underlying connective tissue Nerve fibers can penetrate this structure Nutrients diffuse across Lamina propria = CT immediately underlying epithelia in digestive, respiratory and urinary organs Basement Membrane Image shows renal tubules and the densely stained basement membrane at the arrows Light Microscope: unless stained for specifically, doesn’t tend to be particularly prominent Electron Microscope Basal Lamina (BL) - Type IV collagen and laminin produced by epithelial cells Reticular Lamina (RL) - Type III collagen and anchoring fibrils of VII collagen (all produced by cells of connective tissue) Epithelial cell-cell junctions Tight junctions - seals gap between epithelial cells, separates apical and basolateral junctional membrane domains (creates polarity of cell) complex Uses transmembrane proteins like claudin & occludins tight junction Adherens junctions – stronger attachment adherens points actin junction Formed by cadherins intermediate desmosome filaments Desmosomes – spot welds, strong attachment points Attach to keratin intermediate filaments gap junction Gap junctions – intercellular exchange of molecules Formed by connexins basement membrane hemidesmosome Hemidesmosomes – anchors epithelia to basal anchors intermediate filaments in a cell to lamina extracellular matrix Epithelial cell-cell junctions Tight junctions - seals gap between epithelial cells, separates apical and basolateral membrane domains (creates polarity of cell) Uses transmembrane proteins like claudin & occludins Adherens junctions – stronger attachment points Formed by cadherins Desmosomes – spot welds, strong attachment points Attach to keratin intermediate filaments Gap junctions – intercellular exchange of molecules Formed by connexins Hemidesmosomes – anchors epithelia to basal lamina Junctional complex tight junction junctional complex adherens junction desmosome Specializations of the apical domain: The core (axenome) is composed of microtubules Each cilium extends from a basal body located just beneath the apical surface of the epithelial cell Cilia Aid in the transport of material along the surface of epithelial cells Found in pseudostratified ciliated columnar epithelium in the respiratory tract & ciliated simple columnar epithelium in the oviduct Form tightly packed rows referred to as a brush border The core of each microvillus contains a bundle of actin microfilaments that are anchored to a network structure of actin filaments called the terminal web just beneath the apical surface of the cell Microvilli Increase surface area to aid in absorption Found in simple columnar epithelium lining the small intestine & simple cuboidal epithelium lining the proximal tubules of the kidney Extremely long microvilli, consisting of actin microfilaments Aid in absorption Stereocilia Characteristic of the pseudostratified columnar epithelium of the epididymis & vas deferens of the male reproductive system  Simple vs Stratified Covering/lining Shape of cells Epithelial Tissues Endocrine Glands Secretory Exocrine Glands Classification of epithelium Covering epithelia Simple are classified based on the # of cell layers and the cell squamous cuboidal columnar pseudostratified morphology in the surface layer (most Stratified apical layer) moist dry (keratinized) columnar transitional (or cuboidal) squamous Simple Squamous Epithelium Single Flattened Lining a layer cells surface Nuclei are often the most prominent feature Function: regulate passage of substances to underlying tissues Locations: lining vessels and body cavities Classification of the tissue = simple squamous epithelium Examples of when we’d ask you to identify the tissue: Endothelium = simple squamous epithelium lining vasculature Mesothelium = simple squamous epithelium lining body cavities Simple Cuboidal Epithelium Cells as tall as they are wide, typically with round nuclei and visible cytoplasm Function: active transport across epithelium, some secretion Locations: including (not limited to) kidney tubules, thyroid gland, covering of ovaries Simple Columnar Epithelium Cells are taller than they are wide, may have apical specializations like cilia or microvilli Function: often specialized for absorption or secretion Locations: lining organs with absorptive functions (i.e. intestines) [In image a) renal collecting duct, b) oviduct, c) gallbladder] Pseudostratified Ciliated Columnar Epithelium Looks stratified but it’s actually SIMPLE ALL cells in this tissue contact the basement membrane Some do not reach the free surface Function: secretion, protection, mucus transport Locations: nasal cavity, respiratory tract Structural + Clinical Correlation: Goblet cells are mucus producing cells that stain poorly. The cilia help move the mucus out of the respiratory tract (along with whatever get stuck in it). In smokers the number of goblet cells increases, leaving fewer ciliated cells. In some smokers, the pcce of the bronchi transforms into stratified squamous epithelium via metaplasia…generally not a good thing! Stratified Squamous Epithelium Multiple Flattened layers cells most apically Function: protection, prevents water loss Locations: Skin, esophagus, outer covering of cornea Keratinization – when the surface cells lose their nuclei and organelles and become filled with keratin (image “a” to the right) You’ll need to specify keratinized vs non-keratinized stratified squamous epithelium on lab practicals Stratified Squamous Epithelium nuclei no nuclei Keratinized Nonkeratinized Anucleate cells in surface layer Nucleated cells in surface layer Epidermis of skin Found lining mouth, esophagus, larynx, vagina, & anal canal Stratified Cuboidal or Stratified Columnar Epithelium Stratified cuboidal and columnar epithelia are typically only 2 layers thick, found in larger ducts Transitional Epithelium Stratified epithelium - also called urothelium Superficial cells are large and dome-like (umbrella cells) Function: protect underlying tissues from hypertonic urine, which is potentially cytotoxic; also accommodates distention of the bladder Locations: urinary tract (ureter, urinary bladder, urethra)  Simple vs Stratified Covering/lining Shape of cells Epithelial Tissues Endocrine Glands Secretory Exocrine Glands Gland = specialized organ that produces and secretes a particular product (proteins, lipids, etc) Glands develop from covering epithelium Exocrine glands maintain connection to surface, endocrine glands lose connection Exocrine glands are classified by: Mode of Morphology Product Secretion Unicellular Merocrine Serous Multicellular Apocrine Mucous Holocrine Seromucous (mixed) Sebaceous  Stroma = support tissue; parenchyma = functional tissue Multicellular Exocrine Glands Secretory portion - Supported by connective tissue (CT) - In large glands the CT may form septa Duct - Connects secretory portion to surface Morphological classification of multicellular exocrine glands is based on structural arrangements of the ducts and secretory units ducts No: SIMPLE – are they branched...? Yes: COMPOUND secretory portion – are they branched...? – what is the overall shape of the secretory units? tubular coiled tubular acinar/alveolar tubulo-acinar/alveolar...?  Structural classes of exocrine glands (a) Simple (exocrine) glands ducts are not branched = simple secretory portion may be branched have different shapes: tubular, coiled tubular, acinar/alveolar Junqueira F4-20a  Structural classes of exocrine glands (b) Compound (exocrine) glands ducts are branched = compound secretory portion may be all tubular all acinar/alveolar tubulo-acinar/alveolar MODE of Secretion Merocrine Secretion: Serous Product Most exocrine glands are merocrine! Serous cells synthesize proteins (e.g. digestive enzymes) Which organelles do you expect to be prominent in these cells? Merocrine Secretion: Mucous Product Cells have RER and Golgi BUT the proteins that are synthesized are highly glycosylated mucins - Once secreted these proteins become hydrated and form mucus - Mucins usually washed away during slide prep PAS stain – sufficient carbohydrates still present to stain mucin granules Holocrine Secretion As cells differentiate they accumulate product Product is released via total cell disruption Example = sebaceous glands of the skin Classify the gland based on its secretory product. Apocrine Secretion Secretory product accumulates in apical end of cells Apical portion blebs off – takes part of cell membrane and cytoplasm with product (usually lipid) Example = mammary gland secretes lipid components of milk this way Mammary glands also utilize merocrine secretion! Goblet cells Specialized secretory cells scattered within the lining of the small intestine and respiratory tract Morphology: unicellular gland Product: mucous Mode of secretion: merocrine Disclaimer: Morphology is typically not obvious when looking at the slides in lab. The following slides will walk you through what you can expect to see and which glands you should know the morphology of for test purposes. Simple Glands Simple – meaning that the duct is not branched Slide Set Skin Stratified squamous epithelium Simple branched alveolar gland Simple coiled tubular gland (these are sweat glands) Morphological classification: Simple branched alveolar gland  the duct is not branched but the secretory portion is! Mode of secretion: holocrine Secretory product: sebaceous Identification: sebaceous gland Compound Glands Compound branched alveolar gland Duct is branched Multiple secretory units empty into each duct Secretory units are rounded/sac-like Mode of secretion: holocrine Secretory product: sebaceous Slide Set Morphological Submandibular gland Mixed sero-mucous gland classification: compound branched tubuloalveolar Secretory product: Serous and mucous mucous Mode of secretion: units merocrine serous acini serous “demilunes” UMC-103 Pancreas has endocrine and exocrine components! Endocrine = Pancreatic Islets (Islets of Duct Langerhans) which have no ducts but are Exocrine well-vascularized Gland Exocrine = synthesizes and secretes digestive enzymes (stored as zymogens) Morphological classification (exocrine): compound branched tubuloacinar Endocrine Gland Mode: merocrine Product: serous Slide Set  mucous lipid (sebaceous) Compare glands with mucous vs sebaceous products basally displaced centrally located nuclei nuclei Questions? Questions to ask yourself as you practice classifying epithelia Is there one layer of cells or more than one? Are all cells touching the basement membrane? What is the shape of the most apical layer of cells? Draw the 8 different types of covering epithelia discussed. List their function and one example of where you would expect to find that epithelium Study tip: reorganize the material in a way that makes it easy to compare/contrast – fill these in, modify them however you like Mode of Secretion Mechanism Example Merocrine Exocytosis… Holocrine Apocrine Secretory Product Description of secretory cell appearance (basophilic? Eosinophilic? Hardly stains? Etc) Serous Mucous Lipid/Sebaceous Mixed