Epithelial tissue Flashcards
What are the 4 basic tissues?
- Epithelial
- Connective
- Muscular
- Nervous
What are the 3 types of epithelial tissue? (And their subtypes)
Simple (1 layer):
- Simple Squamous (endothelium)
- Simple Cuboidal (thyroid)
- Simple Columnar (GI tract)
COMPOUND
Stratified:
- Stratified Squamous (skin)
- Stratified Cuboidal (sweat duct)
- Transitional (Urinary bladder)
Psuedostratified (trachea) → when looks like different layer because nuclei are not aligned and have different cell types
What are the characteristics of epithelial tissue?
- Rests on a Basement Membrane (that is not stained by H&E so will not see except for resp. tract and hair follicle)
- Epithalial tissue is avascular (lacks blood vessels)
- Little extracellular space
- Cells are connected with extracellular junctions
- Line body sufaces and body cavities
- Specialization → absorption, protection, secretion, contractility
- Cells are polarized
What defines a keratinized vs non-keratinized epithelial tissue?
The presence of keratohyalin granules.
Keratinized epithelium contains a surface layer of dead cells filled with keratin, making it tough and resistant to abrasion (e.g., skin). Non-keratinized epithelium lacks this layer, keeping cells moist and pliable (e.g., mouth, esophagus).
What are the 3 layers of the skin?
Epidermis
Dermis
Hypodermis
*BM lies between the epidermis and the dermis as epithelial tissue (epidermis) always lies on a basement membrane
What are the layer of the epidermis in order from top to bottom?
- Stratum Corneum
- Stratum Granulosum
- Stratum Spinosum
- Stratum Germinativum
*Basement membrane
In what type of epithelium tissue are Goblet cells often found?
in Pseudostratified epithelium (can be ciliated or not)
How is the epidermis classified as an epithelium?
It is Stratified Squamous epithelium (keratinized)
*Go from that farthest to the Basement membrane to the closest
What are the 7 functions of the skin?
- Protection → Skin resists abrasion, penetration, blocks entru of foreign material
- Containment → Skin prevent loss of body fluid
- Thermoregulation → controls transfer of heat across the body’s surface, facilitates heat retention (isolation) and dissipation (sweat)
- Immunological defense → frogs have antibacterial peptides and AA on their skin
- Sensory Abilities
- Communication → visual and pheromonal signals
- Appendages → hair, nails, sebaceous glands, sweat glands (excretion)
What are the exceptions of basement membranes we are able to see with H&E stain?
Respiratory tract basement membrane → it thickens due contact with air pollution causing irritation
Hair follicle basement membrane → Glassy membrane
Can the adjectives simple and stratified be used to describe the Stratum germinativum?
Nope
Simple and Stratified are for tissues, not cell layers
What is another word for the stratum germinativum?
Stratum basili → since it is on the bottom, at the base
*Germinativum → germinates/grows/replicates
What are some characteristics of the cells of the Stratum Germinativum?
- Stem cells
→ Can replicate into other stem cell of that same layer
→ Can differentiate into cells of the Stratum Spinosum - Columnar
What are some characteristics of the cells of the Stratum Spinosum?
- Polygonal cells
- Are stuck together by desmosomes (spikes)
- Cells eventually differentiate into cells of the stratum granulosum
*In the epidermis, there is not space, the cells are tightly connected
What are some characteristics of the cells of the Stratum Granulosum?
- Squamous (flat)
- Grains → packed with lamellar bodies and keratohyalin granules
- These cells eventually die, release their contents and are pushed to the Stratum Corneum
What are some characteristics of the cells of the Stratum Corneum?
- They don’t have a nuclei → DEAD
- They are not grainy as they have released their contents
- This layer has keratin + dead cells
*Stratum Corneum is not present in non-keratinized tissues
When we describe tissues histologically what is important?
- Start from the farthest from the epithelium to the closest
1. Simple vs Stratified vs pseudostratified
2. Shape
3. Type of tissue (epithelium, muscular, connective, neurvous)
4. Modifiers/Adjectives (keratinized, ciliated, etc.)
What is the only organ to have a transitional epithelium?
Bladder → goes from 5 layers when the bladder is empty to 2-3 layers when it is full and expanded
How should we proceed to classify/name a tissue?
- Simple/stratified/pseudostratified
- Shape of the outer most layer
- Adjectives if needed (keratinized, non-keratinized, ciliated, etc.)
What is a schiff reagent?
Schiff reagent = F[SO4H]2 colorless leukofuschin
PAS:
Formed by Glucose moiety + Periodic acid → Aldehyde moiety
Aldehyde moiety + F[SO4H]2 = Magenta precipitate
Schiff’s reagent + aldehydes → magenta insoluble color reaction
What is a Periodic Acid Shift (P.A.S.) Reaction?
It is the most common application of schiff’s reagent, it is a technique for the demonstration of carbohydrates in tissue sections
*Reaction forms a precipitate that that stain the carbohydrate of the BM
It allows to visualize the basement membrane → Proteoglycans, glycoproteins, collagen
The Periodic Acid-Schiff (PAS) reaction is a staining technique that detects polysaccharides, glycoproteins, and glycolipids by oxidizing sugar moieties to aldehydes, which react with Schiff reagent to produce a magenta color.
- Periodic acid - oxidizes polysaccharides to aldehydes.
- Schiff reagent - reacts with aldehydes to produce the magenta stain.
What is the composition of the basement membrane?
- Proteoglycans (Glycosaminogycans (GAGs) + core protein)
- Glycoprotein
- collagen (mostly IV)
- Layer of reticular fibers underlies basement membranes
What are proteoglycans?
(general structure/composition and function)
~ 200-300nm long
~ 50% protein/50% sugar
Structure = 1 core protein + cste link trisaccharide + variable glycosaminoglycan
*Multiple bristles around the core protein
The GAGs bind in the PA reaction to preicipitate to be stained + GAGs are negatively charged and hold water making the tissue spongy
Give an example of Proteoglycan
Heparan sulfate (synthesized by epithelial cells)
Which sugars form the link trisaccharide in proteoglycans?
Core protein (Serine) - Xylose - Galactose - Galactose - GAG
What are glycoproteins?
(composition)
10% sugar/90% protein
→ N-linked (asparagine)
Structure: N - NGluc - NGluc - Man - 2 branches of (MAN - NGLUC - GAL - SA)
- Can have just 1 side chain (or more)
What are the different glycoproteins present in basement membranes?
Laminin = cross shaped glycoprotein, Links the surface of the epithelial cells to the basement membrane (initiates heme-desmosome formation)
Nidogen-1 (formally Entactin) = small glycoprotein, essential component for BM (links laminin and collagen IV)
Fibronectin = adhesive glycoprotein binds the plasma membrane of the epithelial cells to heparan sulfate in the BM
What are the different collagens of the basement membranes?
*Collagen = fibers of connective tissue
Type IV collagen → Synthezied by epithelial cells (all basement membranes)
Type VII collagen → anchoring filaments (attaches dermis to epidermis by going through BM) Specifically in the skin
Type III collagen → reticular fibers
What are the big plain white stripes seen in the stratum corneum of a cross section?
It is an artifact (air) caused by shatter of the tissue when being cut by the microtome
What are the white parts of the epidermis with little pink spots?
They are part of the dermis that appear in the epidermis due to the plan of section and parts of the tissue that got twisted when cutting
What is a synonyme of Basement membrane?
Basal Lamina
What are the functions of the basement membrane?
- Structural attachement of epithelial cells to the CT
- Compartmentalization → isolate epithelia cells and muscle cells (muscle bundles)
- Filtration → Mvt of molecules is in part regulated by ionic charges, bc glycoproteins and other components are polar (particularly important in the kidney)
- Help to maintain the polarity of the epithelial tissues, so the Stem cells know which is up/down, where to grow (prevent over-proliferation → cancer)
- Tissue scaffolding → helps to scaffold regeneration (ex: during regenration, when skin is cut, the bleeding is due to cut through the BM)
What are the Characteristics shared by all Epithelial Tissues?
- Cells are closely associated with little/no extracellular space
- Cells are strongly attached via intracellular junctions (desmosomes, hemidesmosomes)
- Line body surfaces and body cavities
- Specialization according to location of the body → absorption (GI tract), secretion (epidermis), sensation (epidermis), contractility (eusophagus)
- Cells are polarized (differentiate top vs bottom, where to grow)
- Epithelial tissues rest on a BASEMENT MEMBRANE
What is an electron dense vs electron lucent region in EM?
Electron lucent appears clear → lots of electrons passed, no tissue
Electron dense appears black → electrons bounced on lead (bound to proteins), tissue region
What are keratohyalin granules?
They exist mostly in keratinocytes of the stratum granulosum and a bit in stratum spinosum
keratin gives toughness to the skin
*When cell dies, keratin forms sheets in the stratum corneum
How long does it take for cells of the stratum germinativum to peel of from the skin (full cycle)?
~ 30 days
→ This is how long a tan stays
Describe melanocytes.
Ovoid shaped cells with a centrally located nucleus and projections extending to the stratum spinosum
Melanocytes are intercalated in the stratum germinativum, above the BM
Melanin granules start in the Golgi as clear granules and get darker as they make their way up in the fingers to the stratum spinosum → budding of melanin granules by phagocytosis of the cells of the stratum spinosum (eat them) → granules enter keratinocytes and sit over the nucleus and make an umbrella to protect DNA from UV light
What are the 2 types of melanin?
Eumelanin (brown → most common form of melanin)
Pheomelanin (red/pink → pigment responsible for producing red hair color)
What are the stages of development of melanin?
- Clear melanosome produced in Golgi of melanocytes (they uptake Tyrosine as melanogenic substrate)
{Tyrosinase, as they migrate up the projections} - 3,4dihydroxyphenylalanine
{Tyrosinase} - Dopaquinon
- Melanin (melanin granules)
*Melanin granules are phagocytosed by keratinocytes or the stratum spinosum
What are the functions of hair?
- Thermoregulation
- Filtration (Nasal hair)
- Protection
- Sensory (whiskers on a cat)
- Communication (eyebrows or hair of a cat goes up when mad)
What explains hair color?
There are melanocytes in the hair bulb producing melanin → taken up by keratinocytes. Keratinocytes produce the hair matrix with incorporated melanine
Where does the hair shaft start?
What are the layers of the hair shaft?
(human and mice)
Hair shaft = sebaceous gland and higher
From center → outside:
Humans: Medulla → Cortex → Cuticle
Mice: Cortex → Cuticle
*this explains why our hair is softer
What are the layers of the hair follicle?
Outside → Inside:
Glassy membrane (BM) → Exertenal Root Sheath → Internal Root Sheath
Internal root sheath:
Henle’s layer → Huxley’s layer → Cuticle of IRS
Which part of the hair is continuous with the BM of the epidermis?
BM is continous → ERS and IRS and continuous with epidermis
Stratum germinativum and spinosum are continnuous with External root sheath
Describe the Sebaceous gland as a tissue.
Non-squamous stratified epithelium
- Lays of BM which is a continuation of the epidermis BM
What are sebaceous glands composed of?
Composed of lipid dropeletes containing cholesterol, phospholipids, triglycerides, glycogen
(appears white because the processing of the tissue dissolved the fat)
Example if holocrine secretion:
These substaces are released onto the haire shaft when the cell dies → called sebum
What regulated sebaceous glands?
Stimulated by androgens
Inhibited by estrogens
*No myoepithelial cells
What type of secretion is sweat?
What is its composition?
Merocrine secretion:
- Water, urea, ammonia, NaCl
Describe the cells of the External Root Sheath.
1-2 layers of cuiboidal non-keratinized cells (keratinocytes)
*Confirm for keratin
Describe the cells of the Internal Root Sheath.
Henle’s layer → Squamous cells
Huxley’s layer → Cuboidal cells with trichoyline ganuales (acidophilic)
Cuticle layer → squamous partly keratinized cells
Describe the epithelium of the Excretory duct and Secretory portion of the sweat glands.
Excretory duct → Stratified cuboidal epithelium
Secretory portion → simple columnar epithelium (we can’t see the myoepithelial cells responsible for contractility)
What are the different junctional complexes in hepatocytes/SI from the most apical to the most basal?
Microvilli
- Zonula occludens / Tight junctions
- Zonula adherens
- Macula adherens / Desmosomes
- Hemidesmosomes
(- Gap junctions)
What proteins are involved in Zonula occludens / Tight junctions ?
Membrane proteins:
Occludin, Claudin → form homodimers
Cytoplasmic Proteins (link to actin cytoskeleton):
ZO-1, actin, etc.
*very little space between cells, almost touching
What proteins are involved in Zonula adherens?
Membrane proteins:
Cadherins → form homodimers
*15nm between cells
Cytoplasmic Proteins:
a-actinin, vinculin, Actin
*polarity?
What proteins are involved in Desmosomes (macula adherens)?
Membrane proteins:
Desmocollin and Desmoglein → form homodimers
*25nm between cells
Cytoplasmic Proteins:
Desmoplakins I and II, Plakoglobin, Desmocalmin → form plaques inside the cell, close to cell wall
Intermediate filaments (keratin) pass through plaques as horse-shoes and bind elsewhere in cytoplasm → renforces the plaques
*IM don’t have polarity
*Strongest junctions
What proteins are involved in Hemidesmosomes?
TM proteins:
a6 Integrin, b4 Integrins, BP180
Type VII Collagen in the BM binds the TM proteins
Plaque contains → BP 200
What does BP stand for in BP 180 proteins (TM protein in hemidesmosomes)?
Bullous Phemphigoid → a disease in which the skins peels off due to a deficiency in the formation of hemidesmosomes
What proteins form Gap Junctions
2nm between cells
Gap junctions are not a proper junctional complex, but allow formation of tunnels for communication between cells
1 Gap junction = 2 Connexons
1 Connexon = 6 Connexins
*1.5nm in Diameter
What does it mean for an epithelium to be at steady state?
of newly originated cells by the stem cells are = to the # of dead cells on the surface of the epithelium
What is the definition of the turnover time?
The time required to change the whole population of epithelial cells
~30 days for the epidermis
~3 days in the GI tract
What is Radioautography?
It provides a means to study the turnover of epithelial cells and of other cell types
It is the experiment that allowed discovery of stem cells?
- Dip tissue section (1/time point) in 3H-thymidine → incorporated during S-phase in DNA
- At wanted time point, fix the tissue
- Tissue section coated with photographic emulsion (AgBr) → Decay of isotopes will emit b-particle → reduce the Ag+ ions to Ag → dark silver grains
- Develop tissue slice in a dark room → visualize under a light microscope to see the dark spot
→Depending on time points, differentiated keratinocytes will be in different layers
Who discovered Stem Cells?
Y. Clermont and C.P. Leblond
What are the 3 different types of growth?
What tissue is associated with which?
Static growth → neurons, skeletal muscles
Expanding growth → liver
Renewing growth → epidermis and most epithelia
*Growth is achieved by cell division
What are the phases of the cell cycle? How long do they last?
G1 → Presynthesis of DNA; 25 hours; 2n
S → Synthesis of DNA; 8 hours
G2 → Post DNA duplication; 3 hours; 4n
Mitosis → 2.5 hours (2n restored)
What are the steps of mitosis in the eukaryotic cells?
(0. Preprophase → Intranuclear condensation of chromosomes)
1. Prophase → Visualization of chromosomes and mitotic spindle ~ 1h
2. Metaphase → Chromosomes in equatorial plane, lost of nuclear envelope and nucleolus < 1h
3. Early Anaphase → Longitudinal splitting of chromosomes
4. Late Anaphase → Migration of chromosomes to the poles <0.5h
5. Telophase → Nuclear restitution, nuclear envelope and nucleolar formation ~ minutes
