Epithelia Flashcards
What is epithelia?
Epithelia are the boundary between internal environment and external environment. they are able to import or expel substances, sometimes against steep concentration gradients
They cover surfaces, line cavities and form glands
What are the types of germ layers?
Endoderm - e.g. GI lining
Mesoderm - e.g. lining of CV system
Ectoderm - e.g. epidermis
What are some functions of the epithelia?
*Epithelia are present in every organ
Protection-Skin
Barrier- blood brain barrier
Diffusion- lungs
Absorption- small intestine
Secretion- glands
What are the common properties of epithelia?
Apical surface- free surface
Basolateral membrane- anchored surface (baso- bottom, lateral- side)
Basement membrane
Dynamic barrier- so they can change shape
Connected via tight junctions (decreases free diffusion)
Different membrane properties (polarised)
AVASCULAR- no blood vessels
Minimal extracellular space
What’s the function of basement membranes?
Separates cells from underlying connective tissue AND provides structural support and organisation
Which ECM proteins are secreted by epithelial cells?
Collagens
Laminins
Proteoglycans
What are desmosomes?
Desmosomes form strong adhesion points between cells
where are the basal lamina and reticular lamina located?
Basal lamina is right below the epithelia cells and reticular lamina are under the basal lamina.
The reticular lamina anchor the basal lamina to underlying connective tissues like collagen
what is the purpose of reticular lamina?
Reticular fibres anchoring Basal Lamina to underlying connective tissue (collagen/elastin)
What is the primary function of tight junctions?
They impede paracellular movement (movement between cells) and their permeability varies based on claudin proteins
What do transmembrane claudins do?
They determine tightness between cells and tissues.
They also have a High barrier function – e.g.
renal thick ascending limb – most ions and water
*A high barrier function means that a material or substance is very effective at preventing the passage of other substances through it
In some cases they can be Leaky – e.g. renal proximal
tubule – permitting transepithelial movement
what is the permeability in epithelial tissues like?
Permeability varies 10,000 fold
bladder is tighter than the small intestine as reabsorption occurs in small intestines
why is there a variety of different claudins throughout the body?
Because there are different permeability properties depending on the area
What are adhering junctions?
Adhering junctions form belt around the cell (under
TJ) - linked actin filaments + cadherins
why can disruption of organisation in adhering junctions be related to cancer
Adherens junctions are primarily responsible for maintaining cell-to-cell adhesion. The loss of cell-cell adhesion enables cancer cells to migrate more easily. They can squeeze through gaps in the tissue and invade surrounding structures, such as blood vessels and lymphatic vessels. This invasion allows cancer cells to travel to distant sites in the body and form new tumors (metastases)
where can the mesenchymal layer be found?
It is a layer of tissue found under the epithelia
what are the 4 key junctional complexes in epithelia?
Adherning junctions
Tight junctions
Gap junctions
Desmosomes
what are gap junctions and what do they do?
Gap junctions are specialized intercellular channels that directly connect the cytoplasm of neighbouring cells. They allow for the rapid exchange of small molecules and ions between cells
Why do we say gap junctions are electrically coupled?
Because they move ions
what are desmosomes and what do they do?
Desmosomes form strong adhesion points between cells Link the intermediate filament cytoskeleton of adjacent cells, further enhancing tissue stability.
why is cell replacement important in epithelia?
The more hostile the external environment is, the higher cell death is. HOWEVER there is continuous cell replacement from stem cells ( so there is some sort of tissue homeostasis)
what 4 properties do epithelia share despite their differences
Polarity
Basement membrane
Junctional complexes
Cell replacement
what are the 2 categories of epithelia?
(classification)
Simple- single layer of cell (lung)
Stratifies- many layers of cells (skin)
What do we call epithelia that don’t fit into the 2 categories?
Pseudo-stratified- tall but 1 layer (upper respiratory tract)
Transitional- change as they go over the layers (urothelium)
what are the 2 basic types of epithelia?
simple squamous
simple cuboidal
key ideas about simple squamous epithelia
Thin
Quick diffusion
Secretion/absorption of molecules via active transport
key ideas about squamous cuboidal
Small but cuboid
Secretion or absorption of molecules via active transport
What do we call epithelial cells that are stretched into columns?
Columnar epithelia
Key ideas about columnar epithelia?
Can be with or without cilia/ microvilli
Absorption/secretion via active transport
Line majority of GI tract
Can be ciliated surfaces
what type of epithelia’s change shape?
Give an example
Transitional epithelia and stratified columnar epithelia change shape
Transitional epithelial cells in the bladder go from being rounded when bladder is empty, to being DISTENDED when bladder is filling up
What are glands?
Secretory epithelial tissues that can either have DUCTS or be DUCTLESS
what is the role of glands with ducts?
They secrete onto the external surface (exocrine)
What is the role of ductless glands?
They secrete directly into the blood
what gland secretes proteins?
serous glands
what gland secretes sebum?
sebaceous glands
