Lecture 2: The tissues of the human body : Epithelia Flashcards
What is the function of epithelial tissue?
A group of similar cells that work together to:
* cover body surfaces
* line hollow organs, body cavities, and ducts
* It also forms glands.
3 main functions:
* Protective (especially from abrasion)
* Secretory (onto a free surface)
* Selective Barriers (limit or aid transfer)
How are epithelial cells arranged?
Arranged in continuous sheets as single or multiple layers
What are cell junctions & what are the junctions that hold epithelial tissues
Cell junctions are connection points between the plasma membranes of adjacent cells.
There are namely 5 types:
* Tight junction
* Adherens junction
* Desmosomes
* Gap junction
* Hemidesmosomes
(TADGH)
What is the apical surface?
The edge surface of epithelial tissue facing the body surface, body cavity, lumen of an internal organ, or a tubular duct that receives cell secretions. They may contain cilia or microvilli.
What is the lateral surface?
The surface of an epithelial cells which face the adjacent cells on either side - these surfaces contain junctions (gap, desomosome, tight, adherens) gdta
What is the basal surface?
The deepest layer of epithelial cells facing the basement membrane - these may contain hemidesmosome junctions.
What is the cytoskeleton?
A structure that helps cells maintain their shape and internal organization, and it also provides mechanical support that enables cells to carry out essential functions like division and movement.
What are the components of the cytoskeleton?
Microfilaments - such as actin
Intermediate filament - such as keretin
Microfilaments
Bundles beneath cell membrane and cytoplasm
strength; alter cell shape; link cytoplasm to membrane; tie cells together; muscle contraction
(mlast)
Intermediate filaments
Strength
What is ACTIN?
A type of microfilament - protein bundles beneath cell membrane and cytoplasm that determine cell shape and structure and allow for muscle contraction.
What is KERATIN?
A type of intermediate filament - structural roles in the cytoplasm of cells, also allow movement of material within cytoplasm
Where are tight junctions found?
stomach, intestines, and bladder
Describe the structure of tight junctions
- Located near the apical surface
- Individual sealing Strands
- Transmembrane
- Many proteins, but two key ones: claudins & occludins
- Join the cytoskeleton of adjacent cells e.g via ZO-1 to actin
- In a ‘sewing-like’ appearance
- No gap is left
- More strands = tighter junction
What is the function of tight junctions?
Joins adjacent plasma membranes tightly together keeping cell polarity (ie electrically tight) by preventing migration of proteins between apical and basal surfaces
What are claudins and occludins?
Transmembrane proteins that form the tight junction
Describe the structure of adherens junction
Two plaques (patches of protein tissue) on each membrane linked together by transmembrane glycoproteins (cadherin) - belt-like appearance
Cadherin links to catenins (present in membrane) which link to actin filaments
What is the function of adherens junction? (I)
- Apical, but more basal than tight junction
- Other cells are less continuous and are called “adhesion plaques”
Adherens junctions prevent cell separation from tension forces like in contractions
What is the function of adherens junction? (II)
- “belt desmosome” or “adhesion belts” (zonula adherens) in some epithelial cells
- Have a plaque layer of proteins on the inside of the cell to join actin to cadherins
Cadherin
Cadherin is the transmembrane glycoprotein that fills the gap in adherens junction and desmosome junction.
catenin
Catenin is a type of protein present in the plasma membrane that links the cadherin to the actin filaments of the cells.
Describe the structure of desmosome junctions (I)
- lateral wall
- Have “plaque” just like adherens junction
- resist shearing force
- Cadherin spans the gap and binds to desmoplakin
- Links cell surface to keratin (a cytoskeletal intermdeiate filament)
- Keratin spans from one desmosome to another on other side of the cell - structural integrity
- eg; they bind muscle cells; most common in skin epithelium and cardiac cells of the heart to prevent pulling apart
Lateral
wall
Have
Plaque just like adherens junction
Resist
shearing force
cadherins
spans the gap and binds to desmoplakin
Links
cell surface to keratin (a cytoskeletal intermediate filament)
keratin
spans from one desmosome to another on other side of the cell - structural integrity
eg;
they bind muscle cells; most common in skin epithelium and cardiac cells of the heart to prevent pulling apart
What is the function of gap junctions? (I)
*Direct communication between cells (pass chemical and electrical signals).
*2 hemichannel makes up gap junction
What is the function of gap junctions? (II)
*Allows up to 1kDA small molecules through
*6 connexIN protein molecules form connexON or hemichannel
Connexins
Connexins are protein molecules present in plasma membranes that make up connexons (hemichannels).
Connexons
Connexons are circular molecules made up of 6 connexins and form gap junction.
Describe the structure of hemidesmosome
Junction formed between the basal layer and the basement membrane of epithelial tissue.
A plaque (wall of tissue) is connected to the basement membrane via a transmembrane glycoprotein called integrin
Junction
Junction formed between the basal layer and the basement membrane of epithelial tissue.
A plaque (wall of tissue)
A plaque (wall of tissue) is connected to the basement membrane via a transmembrane glycoprotein called integrin.
What is the function of the hemidesmosome?
- Connect epithelia to basement membrane
- Links cellular basal intermediate filament (keratin) to basement membrane (cbif to bm)
- hemidesmosome
- Integrin: linker protein (instead of cadherin)
- binds to laminin in the bm and to keratin intermediate filament in cytoplasm
Connect
Connect epithelia to BM
Links cellular
Links cellular basal intermediate filament (cbif, keratin) to BM
hemidesmosome
- Integrin: linker protein (instead of cadherin)
- binds to laminin in the bm and to keratin intermediate filament in cytoplasm
What are the 4 lateral junctions?
Gap, tight, desmosome, and adherens
(gdta)
What is the only basal junction?
Hemidesmosome
What is the junctional complex?
A combination of tight junction, adherens junction and desmosome
tad
What is a basement membrane?
bm two layers:
* basal lamina
* reticular lamina.
The basement membrane is thus found between epithelium and connective tissue.
What is basal lamina?
Secreted by the epithelial cells. Contains: collagen, laminin, glycoproteins, other proteoglycans
What is the reticular lamina?
Produced by cells of the underlying connective tissue known as fibroblasts
Containing fibrous proteins usch as fibronectin, collegen etc.
What are fibroblasts?
Cells of connective tissue below the basement membrane responsible for the production of proteins that make up the reticular lamina
What is the function of the basement membrane?(I)
Supports the overlying epithelium
Provides a surface along which epithelial cells migrate during growth and wound healing
What is the function of the basement membrane? (II)
Acts as a physcial barrier
Participants in the filtration of substances in the kidney
Does the epithelial layer have nerves?
Yes
Does the epithelial layer contain blood vessels? & more
No - epithelial tissue is avascular
The exchange of nutrients and wastes takes place by diffusion from vessels in the connective tissue
What are the two main types of epithelial tissue?
- Covering epithelia
- Glandular epithelia
What is covering epithelia?
(name 2 types)
Cover exposed surfaces and internal cavities often have secretory cells in them
The two types are:
* outer covering
* inner lining
Outer Covering
of the skin and of some internal organs
Inner lining
of blood vessels, ducts and body cavities, and the interior of the (rrdu) respiratory, digestive, urinary and reproductive system.
What is glandular epithelia? (Name the two types)
Secratory cells predominate
Two types include:
* endocrine
* exocrine
Exocrine
secrete onto external surfaces and into passages (ducts)
Endocrine
Hormones or precursors into interstitual fluids
Arrangement of covering and lining epithelia
Simple - single layer (secretion; absorption; filtration)
Stratified - two or more layers (protective)
Pseudostratified - appears to have multiple layers as judged by positions of nuclei
Simple
single layer (secretion; absorption; filtration)
Stratified
Stratified - two or more layers (protective)
Pseudostratified (I)
Pseudostratified - appears to have multiple layers as judged by positions of nuclei
Pseudostratified (II)
- Not all cells reach the apical surface
- All cells are in contact with the basement membrane i.e actually a simple epithelium
Shape of covering and lining epithelia
Squamous - flat and thin (helps allow passage by diffusion)
Cuboidal - same height and width (secretion; absorption)
Columnar - long and thin (secretion; absorption)
Transitional - a stratified epithelium in which the cells can change shape from cuboidal to flat shape depending on organ shape (allow stretch eg. urinary bladder)
Squamous
flat and thin (helps allow passage by diffusion)
Cubodial
Cuboidal - same height and width (secretion; absorption)
Columnar
Columnar - long and thin (secretion; absorption)
Transitional
Transitional - a stratified epithelium in which the cells can change shape from cuboidal to flat shape depending on organ shape (allow stretch eg. urinary bladder)
Features of simple squamous epithelium?
- Most delicate epithelium
- Where there is filtration (kidney); diffusion (lung); secretion where slippery surface needed (e.g. outer layer of serous membranes);
- Endothelium lining blood vessels; mesothelium visceral cavity lining
other location examples: in bowman’s capsule of kidney, lines cardiovascular and lymphatic systems, inside eye, alveoli of lungs, visceral cavity linings, inside blood vessels and inside heart.
Most
delicate epithelium
where there is…
Where there is filtration (kidney); diffusion (lung); secretion where slippery surface needed (e.g. outer layer of serous membranes);
Endothelium lining blood vessels; mesothelium visceral cavity lining
other examples
other location examples: in bowman’s capsule of kidney, lines cardiovascular and lymphatic systems, inside eye, alveoli of lungs, visceral cavity linings, inside blood vessels and inside heart.
Appearance of simple squamous epithelium
Thin flat and somewhat irregular like jigsaw
cells are like fried eggs or paving stones
Special subtypes
- mesothelium
- endothelium
Endothelium
lines inside of heart and the blood and lymphatic system
Mesothelium
lines pericardial, pleural, peritoneal cavities
Features simple cuboidal epithelium
Where there are secretion and absorption
Appearance of simple cuboidal epithelium
- Cuboidal or hexagonal boxes
- The distance between adjacent nuclei is approx. the heigh of epithelium
Special subtypes of simple cuboidal epithelium
Nil
Location examples of simple cuboidal epithelium
Pancreas ducts; parts of kidney tubules, smaller ducts of many glands; secretory chambers of thyroid; anterior surface of lens; pigmented epithelium at posterior of retina; secretory part of some glands like thyroid.
Microvilli
Finger-like cytoplasmic extensions from the membrane that increase surface area for absorption and exchange of materials.
Cilia
Motile projections from columnar epithelia cells that can act in a sweeping motion to move substances along surfaces
What are the modifications of the cells that make up columnar epithelia?
Cilia, Microvilli, and goblet cells
Simple columnar epithelium (features)
- More cytoplasm = more organelles
- Reflects being more metabolically active than squamous cells
What major subtypes of Simple columnar epithelium?
i. Non-ciliated simple columnar epithelium
ii. Ciliated simple columnar epithelium
Specialised Subtypes
Simple columnar epithelium can have: cilia or microvilli
Features of non-ciliated simple columnar epithelium (I)
- Single layer
- Microvilli on apical surface
- Have goblet cells interspersed
Non-motile cytoplasmic projections.
Increase surface area for absorption.
“Brush border”
Features of non-ciliated simple columnar epithelium (II)
- Can have Microvilli (e.g. small intestine)
- gastroinestinal tract from stomach to anus; duct of many glands; gall bladder
- Goblin cell is a modified columnar cell
Location examples of non-ciliated columnar epithelium
- Examples: Lines gut mucosa from stomach to anus; ducts of many glands; gall bladder.
- Function: Secretion and lubrication (mucus goblet cells); absorption;
Features of ciliated simple columnar epithelium?
- Single layer
- Have goblet cells as well
*parts of upper respiratory tract, some bronchioles, fallopian tubes, sinuses; central canal of spinal cord, ventricles of brain
Location examples of ciliated simple columnar epithelium?
- Some bronchioles, uterine fallopian tubes, sinuses; central canal of spinal cord, ventricles of brain
- Fucntion: synchronous movement assists motility of mucus and foreign objects or oocytes
What are goblet cells?
Modified columnar epithelial cells containing mucous for secretion.
Appearence of stratified squamous epithelium
- Cells form layers like plywood or pancakes in upper layers
- Lower layers may look cuboidal or columnar
- Cells furthest from nutrition are thinner, and less active.
Features of stratified squamous epithelium (I)
- Located where mechanical or chemical stresses are severe
- All forms protect against microbes
Features of stratified squamous epithelium (II)
- Apical cells are packed with keratin in places where mechanical stress and dehydration are a major issue – makes the surface tough and waterproof.
What are the two types of stratified squamous epithelia?
Keratinised and non-keratinised
Location examples of the two types of stratified squamous epithelia?
- Keratinised: skin epithelium (i.e. dry trauma areas)
- Non-keratinised: (i.e. wet trauma areas) lining of mouth, throat, tongue, esophagus, anus, and vagina
Function: Protection from abrasion, defence from microbes, require secretion from glands.
Appearance of psuedo-stratified columnar epithelium
- Nuclei at all different levels
- Appears to have several layers but it is not actually the case
Features of psuedo-stratified columnar epithelium?
- All cells contract BM but not all reach the apical surface of the tissue
*Actually a simple epithelium
Special subtypes
- Pseudostratified ciliated columnar epithelium
- Pseudostratified non-ciliated columnar epithelium
Pseudostratified ciliated columnar epithelium (I)
i) Cilia on some cells
ii) Secrete mucus, and moves it (from goblet cells)
Pseudostratified non-ciliated columnar epithelium (I)
i) No cilia
ii) No goblet cells
Pseudostratified non-ciliated columnar epithelium (II)
iii) larger ducts of glands, epidiymis, part of male urethra
iv) absorption and protection
What is the difference between ciliated and non-ciliated pseudo-stratified columnar epithelia? (I)
Ciliated cilia on some cells, secrete muscus (goblet cells ((modified columnar cells which contain mucous)).
Non-ciliated has no cilia, lack of goblet cells.
What is the difference between ciliated and non-ciliated pseudo-stratified columnar epithelia? (II)
Therefore ciliated form lines most of upper airways (secretes mucus) and non-ciliated lines larger ducts of glands, epididymis, part of male urethra (absorption and protection).
What is the features of stratified cuboidal epithelium?
Found in ducts of sweat glands and oesophageal glands, part of male urethra.
What are the features of stratified columnar epithelium?
Rare: Part of urethra; some large gland ducts such as oesophageal glands; anal mucosal membrane, part of conjunctiva of the eye
What are the features of transitional epithelium?
Bladder (some parts of ureter and urethra)
What is a gland? (I)
Glands consists of a group of cells involved in the secretion of substances into ducts, onto a surface or into blood
What is a gland? (II)
They are classified according to where they secrete their substances.
What are endocrine glands?
Secrete substances directly into blood usually via traversing interstitial fluid
eg pituitary, pineal, thyroid, parathyroid
generally distant strong effects
What are exocrine glands?
Secrete into ducts that empty onto the surface of a covering or lining epithelium
eg Sweat & salivary glands; oil glands; wax glands; pancreas;
generally local effects
What is a single cell gland? (I)
In epithelia that have independent, scattered gland cells, the individual secretory cells are called mucous cells, and they secrete mucin.
What is a single cell gland? (II)
The apical cytoplasm is filled with large secretory vesicles that look clear or foamy in a light micrograph.
What are the 3 characteristics of multicellular ducts to be considered?
1) structure of the duct
2) structure of secretory area
3) relationship between the two
Name and picture the different shapes of multicellular ducts 8 total
Simple tubular - tear shaped
Simple branched tubular -3 tears branching from single duct
Simple coiled tubular - tube that is coiled
Simple acinar (alveolar) - a single sac-like shape
Simple branched acinar (alveolar) - 3 sacs from one duct
Compound tubular (multiple branched tubular secretory portions)
Compound acinar (multiple branched acinar secretory portions)
Compound tubuloacinar (multiple branched mix of acinar and tubular secretory portions)
Simple tubular
tear shaped
Simple branched tubular
3 tears branching from single duct
Simple coiled tubular
tube that is coiled
simple acinar (alveolar)
a single sac-like shape
Simple branched acinar
Simple branched acinar (alveolar) - 3 sacs from one duct
Compound tubular
multiple branched tubular secretory portions
mbtsp
Compound acinar
multiple branched acinar secretory portions
mbasp
Compound tubloacinar
multiple branched mix of acinar and tubular secretory portions
