Epithelium Lecture Sep 25 Flashcards
What are the 4 types of tissue?
Epithelium
Connective Tissue
Nervous TIssue
Muscle Tissue
Which of the 3 main tissue types is subdivided?
What are the subdivisions?
Connective Tissue:
connective tissue proper
specialized connective tissue: cartilage, bone, blood
What are the 4 main epithelial characteristics
- THey form sheets and/or layers of cells
- They cover surfaces or line cavities
- THere is little intercellular material
- There is a basal lamina present
What germ layer does epithelia come from?
from all three!
What are the 7 functions of epithelia?
- protection
- secretion
- absorption
- excretion
- sensory perception
- sound healing
- barrier formation
What are the two basic rules for classifying epithelium?
- Number of cell layers
(simple, stratified, pseudostratified)
- Shape of the cells AT THE SURFACE!
(squamous, cuboidal, columnar)
What does it mean for an epithelium to be called keratinized?
the top layers have been keratinized - killed
this is the case for stratified squamous cells that make out skin
What are the 6 additional characteristics of an epithelium that should be described (other than number and shape)?
- ciliated?
- keratinized?
- striated? (does it have microvilli?)
- pigmented?
- glandular?
- sensory?
What are the 8 different kinds of epithelium?
- Simple squamous (this is endothelium!!!)
- Simple cuboidal epithelium
- Simple columnar epithelium (this is frequently ciliated or striated)
- Pseudostratified columnar epithelium (frequently ciliated)
- Stratified squamous epithelium (keratinized - skin, unkeritinized - esophagus)
- Stratified cuboidal epithelium (only 2 layers)
- Stratified columnar epithelium (only 2 layers)
- Transitional epithelium (more than one layer thick can be non-distended or distended. think pear shaped)
What is unusual about the surface cells of many transitional epithelium?
the surface cells can be binucleated
Transitional epithelia is only located in ONE area of the body. WHat is it?
internal lining of the urinary tract
Pseudostratified epithelia have what two types of cells?
Tall cells (touch both the basement and apical side)
Short cells (touch only the basement side)
What are three characteristics of the adhesion molecules used in epithelium?
- THey are transmembrane proteins with one or more subunits
- They mediate cell-cell and cell-extraceullar matrix interactions
- They coordinate/communicate extracellular events with intracellular materials
What are the 4 main families of adhesion molecules?
integrins
cadherins
selectins
immunoglobulin superfamily
which family of adhesion molecules mediates cell-matrix interactions?
integrins
which family of adhesion molecules mediates cell-cell interactions?
cadherins
what adhesion molecule family mediates blood cell and endothelial cell adhesion?
Important for immune response
selectins
Describe integrins
- they mediate cell-matrix interactions
- they are transmembrane proteins
- they occur as heterodimersw ith alpha and beta subunits
- 3-4 divalent cations will be bound to the alpha subunit
- They will bind to RGD (arginine, glycine, aspartic acid) sequences found on extracellular matrix molecules, especially fibronectin
What will integrins bind to on the cytosolic side?
They will bind to cytoskeleton-associated proteins that mediate attachment of the integrain domain to cytoskeletal elements like actin or intermediate filaments
What are some characteristics of cadherins?
- Homotpyic, meaning they only bind to identical cadherins from other cells
- they mediate cell-cell adhesion
- they are found at ahderens junctions, including the xonula adherens (belt desmosomes) and regulate desmosomes between adjacent epithelial cells
- they are transmembrane proteins with extracellular and intracellular domains.
5THEY ARE CALCIUM DEPENDENT!!
What do cadherins bind to on the extracellular domain?
What do they bind to on the intracellular domain?
Extracelularly, they bind to other cadherins
Intracellularly, they bind to cytoskeletal associatin proteins that mediate the interaction with the cytosolic domain and the cytoskeleon (like actin or microfilament)
Describe desmosomes.
What adhesion molecules do desmosomes use?
General AND specific
Desmosomes are attachments devices seen everywhere in epithelia.
cadherins will bind to other cadherins on other cells
including desmogleins and desmocollins
A thick, electron rich placque builds up on either side of the desmosome - this can be seen with an electron microscope
What are tight junctions? WHere are they in epithelia?
Tight junctions are occluding junctions (also called zonula occludens)
They are extremely tight connections between the apical ends of epithelial cells.
Think the plastic rink holding hexagonal coke cans together.
They have ridges that basically form a belt around the top. THe more ridges there are, the stronger the seal
WHat adhesion proteins are used in tight junctions?
Occludins and Cloudins
they are transmembrane adhesive proteins whose extracellular domains interact with the extracellular domains of the adhesion molecules on adjacent cell membranes
What two things do tight junctions do for cells? aka what are the effects of having a tight junction?
- materials being transported across an epithelium must pass through the epithelial cells rather than between them
- Plasma membrane proteins are restriction to particular regions of the cell. i.e. apical proteins can’t diffuse down and become lateral or basal membrane proteins
Describe the zonula adherens.
What is another name for them?
how are they different from tight junctions?
also called belt desmosomes
They form belts of adhesion between adjacent cells like tight junctions do, but unlike tight junctions the cell membranes do have a small gap between them
What adherance proteins are used in adhering junctions?
Cadherins
on the extracellular surface they bind to the cadherins on other cells
on the cytoplasmic surface they bind to linker proteins bound to microfilaments (actin), which makes up the terminal web at the apical end of columnar epithelial cells
What is the terminal web?
What is it made of?
How is it connected to microvilli?
THe terminal web is a mesh of actin filaments at the apical surface of many columnar epithelial cells
the actin filaments of the terminal web than turn and bundle together to form the microvilli
Descirbe the macula adherens?
What’s another name?
How are they different from zonula adherens?
Desmosomes or spot desmosomes
They do not form “belts” of adherence around epithelial cells, they form “welds” between adjacent cells.
dense placques form on either side
WHat proteins do macula adherens (desmosomes) use?
they use the cadherins desmoglein and desmocollin (these are the transmembrane proteins)
They also use desmoplakin, which is the protein that forms the dense cytoplasmic plaque which can be seen under a microscope - tey help mediate the interaction between the desmoglein and desmocollin and the intermediate filaments of the cell’s cytoskeleon
what fibers of the cytoskeleton are involved in macula adherens (desmosomes)?
intermediate filaments
Describe hemidesmosomes.
Where are they located?
It’s basically half a desmosome that forms between the cell membrane of a basal epithelial cell and the adjacent basal lamina
What proteins are involved in hemidesmosomes?
what do they interact with on the extracellular side? the intracellular side?
Integrins!
remember integrins do the cell-matrix connections
On the extracellular side they interact with laminin found in the basal lamina
On the cytoplasmic side, they interact with intermediate filaments via linke rproteins (like normal desmosomes)
What does a junctional complex consist of?
What is this called at the light microscope level of resolution?
the three distinct junctions in a very close, serial relationship to one another:
- zonula occludens
- zonula adherens
- macula adherens
Under a light microscope these 3 structures are collectively known as the “terminal bar”
Describe gap junctions.
where do they form?
what do they allow the epithelial cells to do?
They form between adjacent cells.
They are aggreagations of groups of two connexons lined up (one from each cell involved)
there are 6 connexins in a connexon
tje aligned connexons form an aqueous channel pore that connects the cytoplasm of the two adjacent cells
This allows cells to communicate essentially - it’s important for the synchronized beating motion of cilia
How can you stain the basement membrane?
why does this work?
Stain using the PAS method
this works because the portion of the basal lamina that can be seen has glycoproteins which can be exposed and colored with Schiff’s reagent
What are the two components of the basement membrane?
- basal lamina (which has the lamina lucida and the lamina densa)
- lamina reticularis
What causes pemphigus vulgaris?
It’s an autoimmune disorder where the body inappropriately creates antibodies which have specificity for the desmogleins in desmosomes. This causes breakdown of the desmosomes, resulting in blistering
Pemphigus vulgaris attacks desmosomes, so what autoimme disease attacks hemidesmosomes? what does it attack exactly?
Bulloud pemphigus
It’s an autoimune disease that stems from antibodies to hemidesmosomes prtoines, especially against bullous pemphigoid antigen in the dense plaques of the hemidesmosomes
This results in severe blistering in the lamina lucida
What is the overall structure of the basement membrane?
nearest ot the epithelial cells is the basal lamina, which has two components:
- lamina lucida (it’s clear under the microscope, rich in glycoprotein laminin and entactin, the laminin binds type 4 collage, heparan sulfate and epithelial integrins)
- lamina densa (meshwork of type 4 collagen, sandwiched between perclan (a proteoglycan) which contains heparan sulfate side chains) it’s anchored to the reticular lamina by severl factors including fibronectin
Next there is the lamina reticularis, which includes type 1 and 2 collagent. it forms the interface between the basal lamina and underlying connective tissue. the basic groups of collagen bind to the heparan sulfate of the lamina densa. fibronectin
What are microvillin made of?
microfilaments = actin (from the terminal web)
How long are microvilli?
0.1 to 0.5 mm long
What characteristics does the cell membrane have when it covered the microvilli?
It is covered with a very thick glycocalyx which is PAS positive since it’s comprised of glycosaminoglycans
What cells often have microvilli?
What are these cells called if they DO have microvilli?
THey are particularly common in absorptive epithelia, such as the intestinal epithelia
THey give the apical border of the epithelium a striated appearance, so they are said to be striated - also brush border
What is the function of microvilli?
They greately increase the surface area of the cell for absorption purposes
they contain enzymes for the hydrolysis of sugar phosphates and splitting of disaccharides to monosaccharides
what are stereocilia?
where are they located?
They are unusually long microvilli that often have the apical ends twisted together. they’re non-motile
they’re found primarily in ducts of the male reproductive system
they increase the absorptive surface of cells
Describe cili. How are they different from microvilli and sterocilia?
Cilia are long (5-10 mm), MOTILE processes that extend from the apical surface of epithelial cells
they may occur singly or in large numbers
the shaft is covered by plasma membrane
what cytoplasmic structure do cili arise from?
basal bodies
What is the general structure of cilia?
Plasma membrane enclosed
there is an internal core structure called an axoneme which has two single, central microtubules (which terminate where the ciliary shaft joins the basal body) and nine peripheral pairs of fused double microtubules (with each doublet joining a triplet in the basal body)
The dublets all include subfiber A (a complete tubule) and subfiber B( and incomplete tubule)
Subfiver A has dyneins attached
How do cilia bend?
Cilia move based on dynein arm movement - the dyneins attached to a nearby subfiber A use ATP hydrolysis to “walk” upward along the subfiber B, causing a curving motion of the cilia
THe reason the “walking” results in a curving motion and not just a sliding motion is because the negative ends of the doublets are connected at the bottom by a nexin link
What usually happens in people who have immotile cilia syndromes?
They have defects in microtubules, denien arms, or other axonemal structural proteins.
they are often sterile
Describe Kartagener’s syndrome.
They have absent or aberrant dynein arms in cilia and flagella
They tend to be plagued with chromic sinusitis and bronchiectasis
intertile
and occasionally situs inversus (no one knows why)
What are the two types of ciliary rhythem? Which is more common?
Isochronal - cilia beat together
metachronal - sequential beat
This one is much more common
It results in waves that sweep across the epithelial surface
Describe flagella
they have the same axial structure as cilia, but they’re much longer than cilia
there are usually only 1 or 2 per cell
they beat in a more random, whiplike motion
Describe the vascular supply ot epithelia.
Epithelial is avascular, thus nutrients need to diffuse into the epithelial cells.
In order to do this in thicker epithelial layers, the underlying connective tissue will form vascular papillae - they’ll buldge up into the epithelial layer to get the blood vessels closer
Describe the nervous supply of epithelial tissue.
THe terminal breaches of sensory nerves may penetrate the basal lamina to terminate among epithelial cells. this is particularly iportant for skin.
What is the epithelial response to damage?
The cells will become ameboid and greatly increase in motitic rate - this is super important in wound healing.
What is a serous membrane?
What is a mucous membrane?
Serous membrane: mesothelium (simple squamous epithelium) plus basement membrane plus underlying connective tissue.
They line closed cavities in the body
Mucous membranes: surface epithelium, + basement membrane + underlying connective tissue (lamina propria)
They line cavities and canals which connect with the exterior (urinary system, reproductive system, esophagus, etc)
Define epithelioid
term for an epithelium-like appearance of non-epithelial cells
Define metaplasia
term for an epithelium-like appearance of non-epithelial cells
Define neoplasia
- an abnormal change from one distinctive kind of cell to a tumor cell
a. benign
b. metastatic
- carcinoma: a malignant neoplasm of epithelial origin
Define hyperplasia
- an increase in the number of cells in a tissue or organ, excluding tumor
formation - increases the bulk of the part or organ
Define hypertrophy
overgrowth or general increase in bulk of a part or organ
Define anaplasi
- loss of structural differentiation
- particularly prevalent in malignant neoplasms
