epithelial cells sit on the?
2 things that epithelial tissues form a barrier between
- between body and external environment - between organs
burn victims are fucked because?
easily get infections and loose water because they have lost the epithelial tissue barrier between body and external environment (skin)
why is it necessary for epithelial tissues to undergo constant and rapid repair?
because they are subject to physical breakdown and injury
reasons epithelial tissues are important
- protection - filtration - secretion - absorption - excretion - neuroendocrine functions
2 types of epithelial tissue and their features
- covering and lining epithelium: skin epidermis, lining of blood vessels, lining of respiratory, urinary and GI tract - glandular epithelium: secretory portion of glands (thyroid, adrenal, sweat)
simple = ? important for?
1 cell layer thick, important for absorption and secretion
many cell layers thick
pseudostratified = ?
1 cell layer thick, however not all cells reach the apical surface
squamous = ?
cuboidal = ?
columnar = ?
transitional = ?
shape varies with tissue streching
how do glandular epithelium form?
epithelial cells sink below the surface during development
two types of glands
1. exocrine - secretion onto free surface of epithelium 2. endocrine - secretion of hormones into blood
end pieces of glands consist of ______ cells, while the openings of glands are comprised of ________ cells. These two cell types have different ________ properties.
end pieces of exocrine glands consist of _acinar (secretory)_ cells, while the openings of glands are comprised of _duct_ cells. These two cell types have different _transport_ properties.
acinar cells function
a primary sercretion rich in organic molecules (enzymes, regulatory molecules)
duct cells function
modify the composition of the primary secretion by either absorbing or secreting specific ions (HCO3-, Cl-, K+, Na+)
what separates neighbouring epithelial cells?
paracellular space or lateral intercellular space
proteins of tight junctions
tight junctions location
most apical cell junction
how do tight junctions do their thang?
thin bands encircle the cell and interlock with thin bands from adjacent cells. The more bands, the tighter the junction.
2 functions of tight junctions:
1. a barrier to restrict movement of substance between paracellular space 2. a fence to prevent proteins from diffusing in the plane of the lipid bilayer. 2 distinct domains with different transport proteins.
2 distinct membrane domains set up by tight junctions
1. apical (or luminal or mucosal) membrane face lumen or of organ/cavity 2. basolateral membrane adheres to basement membrane and interfaces with the blood (includes paracellular space part)
amount of paracellular transport depends on...
how tight the tight junction is i.e the number of bands
band numbers vs. ion flow resistance
the more bands, the higher the resistance to ion flow
dominant transport in leaky epithelial tissues
dominant transport in tight epithelial tissues
direction of paracellular transport governed by...
laws of diffusion
tight junction resistance increases in a _________ to _________ direction in the __ tract and ______.
tight junction resistance increases in a _proximal_ to _distal_ direction in the _GI_ tract and _Kidneys_.
tight junction resistance in proximal end (duodenum, proximal tubule)
tight junction resistance in distal end (colon, collecting duct)
in transcellular transport, how do substances move across membrane
active (primary and secondary) and passive (through ion channels) trasport
from lumen to blood =
from blood to lumen =
secretion e.g. sweat
entry step for absorption
entry step for secretion
electroneutrality refers to?
movement of ions through selectively permeable membrane will attract a counter ion
under EM, tight junctions appear? in freeze fracture, tight junctions appear?
In EM tight junctions appear as though the membranes have fused. In freeze fracture, it appears as interlocking network of ridges.