3. Introduction To Microanatomy: Epithelial Tissue Flashcards
- Identify and describe the different types of epithelial cells and give an example of where each type is found (objective)
Answer later
- Identify and describe the specialized epithelial cell junctions and the role they play in forming epithelial tissue (objective)
Answer later
Epithelial Cells: Overview (1/3)
Form continuous sheets that cover and line body’s free surfaces
- Provide protection (skin)
- Mediate transport of nutrients (intestines), gases (lungs), waste material (kidneys)
- Secrete lubricating fluid (serosal surfaces)
Epithelial Cells: Overview (2/3)
Make up functional tissues of organs (parenchyma) and are key elements of many glands
-Produce and export enzymes and hormones (pancreas, liver, salivary gland)
Epithelial Cells: Overview (3/3)
Specialized epithelial cells serve as sensory receptors
-Convert external stimuli to electrical signals (olfactory sensory neurons, auditory hair cells, photoreceptors)
Epithelial Tissue Characteristics
- Location: cover external/internal surfaces
- Organization: maintain cohesion so that they form continuous sheets
- Attached to basement membrane: cells have structural and functional polarity
- Avascular: tissue closely supported by underlying connective tissue
- Capable of repair and replacement: cells can be renewed continuously by mitotic activity and stem cell populations
Intercellular Adhesion (1/3)
Via membrane-associated structures that provide connection and communication between lateral surfaces of cells
-Tight junctions (occludens) and adherent junctions form adjacent bands encircling apical end of cell (six-pack of canned drinks)
Tight Junctions
- Opposing cell membranes held in tight contact by transmembrane adhesive proteins (claudin, occludin, junctional adhesion molecule)
- Physical/chemical barrier to control paracellular movement of substances
- Define and maintain separation between apical and basal areas of cell membrane to maintain cell polarity
Adherent Junctions
- Opposing membranes held close with thin sliver of intercellular space
- Space bridged by cadherins (Ca2+)
- Cadherins attach to cytoplasmic proteins (catenins) which in turn attach to actin filaments/microtubules.
- Transmembrane protein complexes interact with signaling molecules (tumor suppressor)
- Role in regulating cell cell contact (morphogenesis, remodeling of tissue/organs, controlled proliferation of cells)
Intercellular Adhesion (2/3)
- Desmosomes: anchoring junctions that occur sporadically (do not form band, function as “spot welds” to reinforce cellular adhesion), sheering force
- Transmembrane proteins (desmosomal cadherins) attach to intermediate filaments of cytoskeleton via cytoplasmic proteins for additional structural support
Desmosomes
- Prominent in tissues under mechanical stress (heart, skin)
- Dysfunction of desmosomal cadherins associated with cell adhesion cardiomyopathy (arrhythmogenic right ventricular dysplasia), blistering diseases (pemphigus vulgaris)
Intercellular Adhesion (3/3)
-Gap junctions: clusters of intercellular channels that allow for rapid transmission of electrical or chemical information from cell to cell, promote coordinated action
Gap Junctions
- Intercellular gap is spanned by channel-forming transmembrane proteins (connexins)
- Connexins form hexameric complexes called connexons, which align with connexons from neighboring cell (each gap junction has dozens to hundreds of aligned pairs)
Basilar Adhesion
- Basal surface of cells rests on thin felt-like sheet of macromolecules which is outside cell but secreted from cell (basement membrane)
- Basement membrane composed of 2 layers (basal and reticular lamina)
- Attached to cell via hemidesmosome
Hemidesmosome
- Transmembrane protein integrin interacts with extracellular protein laminin to link intermediate filaments of cytoskeleton to extracellular basement membrane
- Integrin plays role in cell-cell communication, modulation of cell proliferation and differentiation
Basement Membrane (not actually a membrane)
- Complex mixture of macromolecules that bind hormones and other signaling molecules
- Serves as semi-permeable filter for substances coming to cell from underlying tissue
- Epithelial cells require contact with it to function normally and establish polarity
Specializations of Apical Cell Surface (1/2)
Apical vs. Basal
- basal is by basement membrane, apical is opposite, usually facing a space
- Different in structure/function, uneven distribution of organelles and proteins
Specializations of Apical Cell Surface (2/2)
Specialized structures function to increase the apical surface area for better absorption (microvilli) or to move substances along the epithelial surface (cilia)
Microvilli- absorption organs
Villi- respiratory tract and lungs, fallopian tubes (longer projections, microtubules)
Epithelial Tissue Type I- Covering (Lining)
Classified according to cell shape and number of layers
Epithelial Tissue Type II- Secretory (Glandular)
Classified by structure
Arrangement of Cells (classification)
Simple (single layer)
Stratified (multiple layers)
Pseudostratified (single layer but looks multilayered)
Shape of Cells (classification)
Squamous
Cuboidal
Columnar
Simple Squamous
Structure: one layer with flat nuclei
Function: allow passage via diffusion/filtration, secretes lubricating substances
Location: lining blood vessels/lymphatics, alveoli/lungs , renal glomeruli, heart, serous lining of cavities (mesothelium)
Simple Cuboidal
Structure: cube like cells with large spherical central nucleus
Function: secretion and absorption
Location: renal tubules, ducts and glands
Simple Columnar
Structure: single layer of tall cells with large round nucleus
Function: protection, lubrication, absorption, secretion
Location: lining of intestine, gallbladder, female reproductive tract
Stratified Squamous
Structure: two or more layers of thin cells with flat nuclei
Function: protection, secretion, prevention of fluid loss
Location: skin, mouth, esophagus, larynx, vagina, anus
Stratified Cuboidal
Structure: 2+ (usually 2) layers of cube like cells with large central nuclei
Function: protection, secretion
Location (rare): sweat glands, mammary glands, testes, developing ovarian follicles
Transitional
Structure: top has dome-shaped (umbrella) cells with membranes that allow them to withstand hypertonic effects of urine and protect underlying cells from toxins; cells can adjust with each other as urinary bladder fills and wall is distended.
Function: protection and distensibility
Location: bladder, ureters, renal calyces
Stratified Columnar
Structure: 2+ layers of columnar (top columnar, lower usually cuboidal)
Function: protection and secretion
Location (rare): parts of male urethra, some glands
Pseudostratified
Clue is mucus white spots
Structure: single layer of cells with nuclei at varying heights (mucus in between, also look for cilia)
Function: protection, secretion, cilia-mediated transport of mucous and trapped particles
Location: lining of trachea, bronchi, nasal cavity
Glands
Begins as epithelium, proliferation of cells and growth into connective tissue.
- Exocrine: secretory portion and duct leading back to epithelium (ie acini in pancreas)
- Endocrine: no duct cells, secrete products into bloodstream (ie islets of Langerhans in pancreas)
