Building tissues from cells (epithelium) Flashcards
Tissue
Collections of similar cells and the material surrounding them
How are tissues classified?
Structure of cells
Composition of extracellular
Functions of the cells
What are the major types of cell tissues?
Muscle Epithelial Nervous Connective (MENC)
Where do epithelia exist?
The cover body surface and line all of the body’s cavities
How do epithelial tissues get their nutrients
Blood vessels never pass through epithelial tissues, so they depend upon diffusion.
Basement membrane definition
Extracellular matrix of proteins outside the cell.
What is used to classify epithelial cells?
Cell shape
Layer structure
Surface specialisation
Location + function
What are the cell shape classifications of epithelial cells?
Squamous, cuboidal and columnar
What are the layer structure classifications of epithelial cells?
Simple (clear, single layer)
Pseudostratified (single layer pretending not to be single)
Stratified (multilayer, in which only basal cells attach to basal membrane)
Surface specialisation classification of epithelial cells
Ciliated (resp. tract; cilia have microtubules)
Brush border (rigid as made of actin - increases SA)
Keratinsed
Location and function classification of epithelial cells
Respiratory (contains goblet cells)
Transitional (shape of cell varies, found only in urinary tract)
How does the epithelial layer in the urinary tract show transitional behaviour?
When the bladder is empty, the epithelial layer in cuboidal; as it fills they flatten out and become squamous - but don’t separate.
What is the difference between endocrine and exocrine glands?
Exocrine secrete to free surface; endocrine secrete to blood stream.
What are the forms of exocrine secretion?
Holocrine - entire secretory cell broken down and discharged; found in sebaceous cells of skin only
Apocrine - top half of cell broken down and discharged, apical cytoplasm lost; found in mammary and prostate glands
Merocrine - normal exocytosis
(HAM)
Basal lamina
Basement membrane
What are the linker proteins in a desmosome?
Desmoplakin + plakoglobin
What are the adhesion proteins in a desmosome?
Desmoglein + desmocollin
What type of proteins are adhesion proteins in a desmosome?
Cadherin family
How do desmosomes work?
A keratin intermediate filament is connected to a linker protein (desmoplakin, plakoglobin) which is connected in turn to an adhesion protein (desmocollin, desmoglein). The adhesion molecule binds homophillically to another copy of the same adhesion molecule from the other cell; which is connected via a linker protein to the intermediate filament in the next cell.
How is the strength of a junction achieved?
Clustering of linker proteins (plakoglobin, desmoplakin)
What’s the difference between desmosomes and hemidesmosomes?
Desmosomes connect adjacent cells; hemidesmosomes connect a cell to the basal membrane/lamina.
How do adherens junctions work?
Same as desmosomes; intermediate filament is actin in this case. Also adhesion proteins are adherin dimers.
Where are adherens junctions found?
In the walls of columnar epithelial cells, actin filaments are rigid so helps pull sides together keeping width narrower - maintaining columnar shape.
What are the anchoring junctions?
Desmosomes; hemidesmosomes; adherins
What are the non-anchoring junctions?
Tight and gap
How does a tight junction work?
Brings membranes of adjacent cells so close together at spots that nothing can pass between them.
Give an example of a place in which a tight junction is important
In the lumen of the gut, prevents water from within the gut lining from moving out between cells by osmosis into the lumen of the gut where the solute potential is lower. Also in the lumen of the gut a tight junction prevents the diffusion of nutrients out of the cells back into the lumen of the gut.
What proteins are involved in the tight junction?
Claudin and occludin.
What are gap junctions
Cytoplasmic bridges between cells which allow the movement of small substances between cells.
What proteins are involved in a gap junction
6 connexin proteins come together in a hexagonal arrangement with a central hydrophilic pore - a connexon.
Why are gap junctions important in epithelial cells?
Epithelial cells need diffusion for nutrition; gap junctions allow substrates to diffuse from cell to cell so every cell gets nutrients from the blood vessels regardless of how far away it is.
What is the intermediate filament made of in desmosomes?
Keratin
Size of microvillus
0.5-1um
Describe a goblet cell
Modified columnar epithelial cell; synthesises and secretes mucus.
Size of cilia
7-10um
Describe cilia
Core of 20 microtubules arranged as 9 doublets around a central pair.
Why is keratinization beneficial?
Protection (UV, thermal, mechanical)
Prevention of dessication
