1/54
Looks like no tags are added yet.
Name | Mastery | Learn | Test | Matching | Spaced |
---|
No study sessions yet.
Many tissues contain
epithelial cells embedded in the extracellular matrix populated with other cell types/connective tissue
All ducts, lumens, and tubes are lined with
epithelial cells
Epithelium/epithelia are
sheets of cells covering an external surface or lining of an internal body cavity
Some epithelia act mainly as
a protective barrier
some have complex biochemical functions
Epithelial sheets are
polarized and rest on a basal lamina
An epithelial sheet has two faces
the apical surface
basal surface
The apical surfae of an epithelial sheet is
free and exposed to the air or to a bodily fluid
The basal surface of an epithelial sheet is
attached to a thin, tough sheet of extracellular matric called the basal lamina
Laminin provides
adhesive sites for integrin molecules in the basal plasma membranes of epithelial cells
serves as a linking role
The basal lamina supports
a sheet of epithelial cells
Epithelial cells are
polarized
apical (top) and basal (bottom) surfaces have different structures and functions
In the small intenstine, the epithelium contains two main cell types
absorptive cells (polarized)
goblet cells (polarized)
Absorptive cells
take in nutrients apically from the gut and pass them to underlying tissues basally
Goblet cells
secrete mucus from their apical side
polarized in a way that supports mucus secretion
Cell polarity in absorptive and goblet cells relies on
junctions between neighboring cells and the basal lamina
Tight junctions make an epithelium
leakproof and separate its apical and basolateral surfaces
Epithelial cell junctions have different functions,
some seal cells tightly to prevent leakage, others provide mechanical support, and some allow communication between cells
Tight junctions are very important in
vertebrates because it forms a barrier that prevents water-soluble molecules from passing between cells
Without tight junctions,
functions like nutrient absorption would not work properly
What are the main functions of epithelial cell junctions?
prevent leakage between cells (tight seal)
provide strong mechanical attachments
allow cytosolic exchange between cells
What type of junction provides a barrier function in vertebrate epithelial sheets?
tight junctions
What proteins form tight junctions?
claudins and occludins
How do tight junctions prevent leakage fo water-soluble molecules?
they seal neighboring cells together so that molecules cannot pass between them
What would happen without tight junctions in absorptive epithelia like the gut?
absorptive pumping would be ineffective, and extracellular fluid composition would equalize on both sides of the epithelium
How do tight junctions help maintain epithelial cell polarity?
they prevent the diffusion of membrane proteins between the apical and basolateral surfaces
they serve as sites where protein complexes assemble to organize apical-basal polarity inside the cell
What are the main types of cell junctions that provide mechanical strength in epithelia?
adherens junctions
desmosomes
hemidesmosomes
What do adherens junctions and desmosomes connect?
they connect one epithelial cell to another
What do hemidesmosomes connect?
they attach epithelial cells to the basal lamina?
How do mechanical junctions provide strength to epithelial sheets?
by linking junction to cytoskeletal filaments, forming a continuous network across cells
What family of transmembrane proteins are adherens junctions and desmosomes built around?
cadherins
What kind of binding do cadherins exhibit?
homophilic binding
Homophilic binding is
cadherin-to-identical-cadherin
What ion is required for cadherin-mediated adhesion?
calcium (Ca2+)
To what cytoskeletal filaments are cadherins linked at adherens junctions?
actin filaments
What structure do adherens junctions often form in epithelial cells?
a continuous adhesion belt near the apical end of the cell, just below tight junctions
What is the function of the actin-myosin network in adherens junctions?
it allows contraction of the epithelial sheet, enabling it to change shape
How can contraction of adherens junctions shape epithelial sheets during development?
1 axis contraction → sheet rolls into a tube
multiple axes contraction → sheet forms a cup that can pinch off into a sphere
What embryonic structures are formed by epithelial sheet movements?
the neural tube (brain and spinal cord) and the lens vesicle (lens of the eye)
What type of cytoskeletal filaments are connected at desmosomes?
keratin filaments
a type of intermediate filament
What type of cadherins are involved in desmosomes?
a different set of cadherins specific to desmosomes (not the same as those in adherens junctions)
How do desmosomes provide strength to epithelial sheets?
by linking keratin filaments between neighboring cells, forming a strong, interconnected network
What type of epithelium especially relies on desmosomes for strength?
tough, exposed epithelia such as the epidermis of the skin
Why do epidermal cells need to be anchored to underlying connective tissue?
to prevent separation and blistering
What proteins mediate the attachment of epithelial cells to the basal lamina?
integrins in the basal plasma membrane
What are hemidesmosomes?
structures that anchor epithelial cells to the basal lamina
What do integrins bind to on the outside of the cell?
laminin in the basal lamina
What do integrins bind to on the inside of the cell?
keratin filaments via linker proteins
What is the main function of gap junctions?
to allow direct transfer of ions and small molecules between the cytoplasms of adjacent cells, enabling electrical and metabolic coupling
What protein complexes form gap junction channels?
connexons
How are connexons arranged between cells?
in adjacent cells theyre aligned end-to-end to form water-filled channels across the membranes
Are gap junctions able to open and close?
yes because they respond to extracellular and intracellular signals
What structure in plant cells is functionally similar to gap junctions?
plasmodesmata
How do plasmodesmata differ structurally from gap junctions?
theyre lined with plasma membrane and connect the cytoplasm of adjacent cells through the cell wall
What types of molecules can pass through the plasmodesmata?
small molecules, some proteins, and regulatory RNAs
Why is regulated movement through plasmodesmata important in plants?
it helps control development by transporting transcription regulators and regulatory RNAS between cells