Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
18 Cards in this Set
- Front
- Back
|
Epithelial tissues |
consists of cells arranged in continuous sheets in either single or multiple layers sit on basement membrane form the boundary between body's organs or between the body and the external environment Physical breakdown and injury Constant rapid renewal process |
|
|
Epithelial cells function |
Protection - Skin Filtration - Kidneys Secretion - saliva Absorption Excretion Nueroendocrine functions - e.g. gut |
|
|
Types of epithelium |
Covering and lining epithelium - skin, blood vessels and ducts Lining of respiratory, reproductive, urinary, gastrointestinal tract Glandular epithelium Exocrine - cells that secret substances e.g. sweat, ear wax, saliva, digestive enzymes Unicellular or Multicellular Connected to surface by tubes/ducts Endocrine - secrete hormones into bloodstream Derived from epithelial cells that sink below the epithelium surface during development |
|
|
Epithelium is classified by: |
Arrangement of cells into layers: Simple - one layer thick Stratified - many layer - more barrier protection Psuedostratified - not all cells reach apical surface Shape of surface of cells Squamous - flat Cuboidal - cube-shaped Columnar - tall column Transitional - shape varies with tissue stretching |
|
|
The main thing that defines how epithelial cells work is |
Tight junctions - seperates cells from their neighbours by the lateral intercellular (paracellular) space Epithelial cells are held together at their luminal edges by tight junctions Membrane proteins - Tight junctions are composed of thin bands that encircle the cell and make contact with thin bands from adjacent cells In EM it appears that membranes are fused together In freeze fracture tight junctions appear as an interlocking network of ridges in the plasma membrane Substances let through is determined by - Doesn't let large molecules |
|
|
Gland structure |
Transport is not just at membrane level, also at organ level Different processes and properties at different parts Secretion, modification by ducts Secretory portion and duct Branched ducts End pieces of glands contain secretory or acini cells Openings of the gland (ducts) are comprised of ductile cells that have different transport properties 2 types of cells: acinar cells - create a primary secretion rich in organic molecules (enzymes or regulatory molecules Duct cells - modify the composition of the primary secretion by either absorbing or secreting specific ions (HCO3- Cl- K+ Na+) |
|
|
Tight junction function |
A barrier - they restrict movement of substances through the intercellular space between cells A fence - they prevent membrane proteins from diffusing in the plane of the lipid bilayer Without tight junction - no sidedness with proteins - some proteins on ecm side, some in intracellular side Hence they seperate epithelial cells into two distinct membrane domains Apical or luminal or mucosal membrane that faces the lumen of the organ or body cavity Basolateral membrane that adheres to the adjacent basement membrane and interfaces with the blood - 3 sides, do not cross the tight junction |
|
|
Two types of transport |
Must cross apical and basolateral memrbane Transcellular transport Para cellular transport The distinct membrane domains means that different transport proteins can be inserted in to either apical or basolateral membrane transport can occur via the paracellualr or transcellular pathway or via both - depends on what the cell is doing |
|
|
Paracellular transport |
Governed by laws of diffusion (concentration gradient) and the tightness of the junctions Electrical resistance to ion flow through tight junctions can be measure - by being peeled Proportionality: The higher the electrical resistance to ion flow, the greater the number of tight junction strands holding the cell together |
|
|
Leaky epithelium |
-paracellular transport dominates |
|
|
Tight epithelium |
- very high resistance transcellular transport dominates |
|
|
Changes in tight junction resistance |
Tight junction resistance changes in a rpoximal to distal direction in the GI tract and kidney Proximal Leaky epithelium low electrical resistance low number of strands bulk transport (paracellular) e.g. duodenum, proximal tubule Distal Tight epithelium - ability to finetuning High electrical resistance High number of strands Hormonally controlled (transcellular) - controlled to get more Na+ uptake Colon - finetuning the electrolyte balance collecting duct - fine tuning concentrated or dilute urine |
|
|
Transcellular transport |
RELISTEN Epithelial cells use primary and secondary active transport often in combination with passive diffusion through ion channels Transcellular transport can either be: Absorption: transport form lumen to blood Secretion: transport from blood to lumen |
|
|
What are the rules for transepithelial transport to occur |
1: Entry and exit steps: the entry step for absorption is the apical but for secretion is from basolateral membrane and leave the apical side 2: Electrochemical gradient: is the entry/ exit step - passive or active USUALLY ONE IS PASSIVE AND ONE IS ACTIVE 3: Electronuetrality: movement of a positive or negative ion will attract a counter ion - accompanying ion with charge - strives to maintain uniform charge across membrane 4: Water permeability - Osmosis - net movement of ions will establish a concentration gradient for water Only if aqua porins in membrane - Only if membrane is permeable to water but not ions |
|
|
Difference electrochemical gradient and concentration gradient |
. |
|
|
Exit step |
Often passive diffusion |
|
|
Entry step |
often secondary active transport Sets up electrochemical gradient |
|
|
Water and counter ions move in paracellular |
. |
