Epithelia Flashcards
Function of Epithelia
- Covers surfaces
a. Provides protection against damage and desiccation
b. Creates barriers to free movement of molecules and pathogens - Lines passageways through tissues
- Forms glands responsible for secretion
- Selective absorption
- Serve as sensory receptors by receiving and transducing external stimuli
Subcategories of Epithelia
- Mesothelium - lines body cavities (e.g. peritoneum)
- Endothelium - lines blood vessels and lymphatic vessels
- Epithelioid - epithelial derivatives that lack a free surface (e.g. hepatocytes and Leydig cells)
Cilia
Short, finger-like extensions of apical plasma membrane with core of microtubles in 9 + 2 arrangement (9 outer doublets + 2 central singlets) that that beat to establish flow of fluid over surface of cells
In microscope basal bodies of cilia may form dark line beneath apical surface.
Maintenace of Epithelial Cells
Replaced by division of other epithelial cells
- Simple epithelia
a. All cells potentially capable of dividing into functionally differentiated
b. Population of undifferentiated cells is retained as source of new cells (e.g. in crypts of intestinal glands) - Stratified epithelia
a. Only basal cells retain ability to divide (contact with basement membrane required for mitosis)
Control is complex and largely unknown (steady state, diseased, cancerous)
Microvilli
Short, finger-like extensions of apical plasma membrane with core of actin microfilaments that increase surface area for absorption.
In microscope:
- Striated border/brush border (microvilli)
- Terminal bars (apical junctional complex)
- Terminal web (horizontal network of cytoskeletal filaments under apical surface)
Stratified Squamous Epithelia
- Keratinized (e.g. epidermis of skin)
a. Hard, flattened dead surface cells that are filled with keratin and lack nuclei (squames); provides protection against friction, desiccation, and is waterproof - Non-keratinized (e.g. lining of esophagus and vagina)
a. Surface cells remain alive and nucleated; provides protection against friction
Types of Columnar Epithelia
- Unmodified - found where function is to protect a wet surface or form a non-leaky channel (e.g. ducts of glands)
- Secretory - goblet cells function in mucous secretion (e.g. surface of stomach)
- Mixed secretory and absorptive cells (e.g. small/large intestine)
- Ciliated - found where function is to move something relative to epithelial surface (e.g. respiratory tract and oviduct)
Types of Adherens Junctions
- Zonula adherens
- Macula adherens
- Hemidesmosomes
- Fascia adherens - associated with intercalated discs of cardiac muscle
Non-surface Epithelia
- Cells do not cover a surface, although they may have free surfaces facing a sinus or other cavity (e.g. hepatocytes and Leydig cells)
Apical Junctional Complex
Consists of zonula occludens, zonula adherens, and macula adherens
Forms the boundary between apical and basolateral domains of epithelial cells; typically found at apical margins of columnar epithelia, visualized as terminal bars in LM
Basement Membrane
Layer of extracellular matrix that separates epithelia from underlying connective tissue.
Not visible with H&E; stained blue with aniline blue.
Made up of two layers:
- Basal lamina
a. Produced by epithelial cells
b. Further divided into lamina lucida and lamina dense, which are only visible by TEM
c. Consists of laminin, Type IV collagen, and proteoglycans - Fibrous/reticular lamina
a. Produced by connective tissue cells
b. Consists of reticular fibers (Type III collagen) and fibronectin matrix
Types of Occludens (Tight) Junctions
- Zonula occludens
- Focal tight junction
Zonula Occludens
Transmembrane proteins form series of anastomosing ridges that forms a leak-proof seal between cells (i.e. barrier to intercellular diffusion)
Outer leaflets of opposing plasma membranes appear to fuse in EM.
Epithelial Junctions and Their Functions
Specialized regions of plasma membrane where cells make contact with other cells or with fibers or extra-cellular matrix.
- Occludens junctions - forms barrier to intercellular diffusion
- Adherens junctions - cell attachment
- Gap junctions - cell communication
Surface Folds
Folds of plasma membrane (frequently found on lateral surface of epithelia) that increases surface area
Pseudostratified Columnar Epithelia
- Multilayered appearance (due to different cell height), but ALL contact basement membrane
- Found where more protection is needed than provided by simple columnar cells
- Often has surface modifications
a. Ciliated (e.g. Trachea and Oviduct)
b. Stereocilia (e.g.Epididymis)
c. Unmodified (e.g. Vas deferens)
Types of Cell Surface Modifications
- Microvilli
- Cilia
- Stereocilia
- Surface Folds
Macula Adherens (Desmosome)
Forms spot adhesions between lateral plasma membranes of epithelial cells, and is associated with cytoplasmic intermediate filament, cytokeratin.
Transmembrane protein = cadherin
Transitional Epithelia
- Stratified epithelia with apical cells dome to squamous shape, which allows epithelium to expand and contract without tearing
- Unique to urinary system (e.g. lining of bladder, ureter, renal pelvis,etc.)
Classification of Epithelia
Classification based on cell shape and number of layers:
- Shape
a. Squamous - width greater than height
b. Cuboidal - about equal width and height
c. Columnar - height greater than width - Number of Layers
a. Simple - single layer of cells in contact with basement membrane
b. Stratified - two or more layers of cells; classification based on morphology of cells at apical surface
Stereocilia
Long, finger-like extensions of apical plasma membrane with core of actin microfilaments
Found in epididymis and sensory cells of inner ear
Focal Tight Junction
Morphologically similar to zonula occludens junctions, but occur only at focal points of cell contact and therefore do not form a seal.
Hemidesmosome (Half Desmosome)
Forms attachment between basal plasma membrane of epithelia and basal lamina of basement membrane
Associated with cytoplasmic intermediate filaments (cytokeratin)
Transmembrane protein = integrins
Gap Junctions
Form small diameter channels that facilitates communication between adjacent cells by allowing movement of small molecules between them
Responsible for electrical coupling (ions) and chemical coupling (small metabolites)
Zonula Adherens (Belt Desmosome)
Forms continuous belt around apical margin, below zonula occludens
Associated with cytoplasmic fibers which are seen as terminal web in LM
Transmembrane protein = cadherins
Properties of Epithelia
- Contiguous, closely apposed cells that adhere by special junctions, with little to no ECM in between
- Morphologically and functionally polarized cells
a. Distinct surface domains with different protein/lipid composition (apical and basolateral domains separated by apical junctional complex)
b. Internal organization related to surface domains - Avascular (i.e.blood vessels don’t cross basement membrane; cells receive metabolic support via diffusion)
- Basal surface of bottom layer of cells attached to basement membrane
