Epithelium Lecture Flashcards
Types of Tissue
1) Epithelium
2) Connective Tissue
3) Muscle
4) Nerve
Types of Connective Tissue
1) CT Proper
2) Cartilage
3) Bone
4) Blood
Location of Epithelium
1) Cover exterior body surfaces
2) Line cavities
3) Line tubular organs
4) Cover external surfaces of organs
5) Line vascular system
6) Line ducts
Characteristics of Epithelium
1) Typically have a free surface
2) Form a sheet by adhering to adjacent cells by junctional complexes and adhesion molecules
3) Rest on basal lamina
4) Exhibit polarity: apical, basal, and lateral
5) AVASCULAR
Functions of Epithelium
1) Absorption
2) Secretion
3) Excretion
4) Protection
5) Contraction
6) Transport of material
What is a basal lamina?
- It is like a rug
- Helps glue the epithelium to the connective tissue
How to classify Epithelial Tissue?
- number of layers
- cell shape
- THEN modifications
“Number of Layers” Classification
- Simple (one layer thick)
- Stratified (two or more layers)
- Pseudostratified (looks more than one, but all cells rest on basal lamina)
“Shape” Classification
- Squamous (thin, flat cells)
- Cuboidal (cell that is the same width as height; square)
- Columnar (tall and thin; rectangle)
Types of Modifications
1) Keratinized, non-keratinized
2) Cilia
3) Microvilli
4) Stereocilia
5) Goblet cells
Types of Epithelia
Without Modifications
- Simple Squamous
- Simple Cuboidal
- Simple Columnar
- Stratified Squamous Keratinized
- Stratified Squamous Non-keratinized
- Stratified Cuboidal
- Stratified Columnar
- Pseudostratified Columnar
- Transitional [Urothelium]
Simple Squamous
- Flat cell
- Shaped like a pancake
- Thin from side
- Large from surface
- Flat nucleus
- Endothelium: lines blood vessels and other vascular structures
- Mesothelium: covers organ surfaces
- Kidney (pariteal surface of Bowman’s, thin segment of loop of Henle)
Simple Cuboidal
- Square cells
- Height = width
- Found in glands
- Found in kidney tubules
- Nuclear location: middle or bottom of cell
- Thyroid gland
- Kidney proximal convoluted tubule (brush border), distal convoluted tubule (basal striations), collecting ducts, collecting tubules
- Eyes (lens surface, corneal endothelium)
- Gland ducts in so many locations, kidney ducts and tubules
Simple Columnar
- Rectangular cells
- Tall and thin cells
- In many organs from digestive to female reproductive
- USUALLY has apical modifications; microvilli or cilia
- Often will have goblet cells present
- Nuclear location: middle, bottom
- Gallbladder
- Gland ducts in many locations
- Surface lining of the stomach/large and small intestines, uterus, and endocervix
- Ciliated in oviducts (fallopian tubes), lungs (small bronchi and bronchioles)
Stratified Cuboidal
- found in the ducts of glands
- salivary glands (large ducts)
- sweat gland ducts
Pseudostratified Columnar
- Rectangular-ish cells
- Tall and thin cells
- Looks like someone shook a simple columnar epi.
- USUALLY have apical modifications; stereocilia, cilia
- May have goblet cells present
- Nuclear location: varies; which makes it confusing
- Ciliated in trachea and large bronchi
- Lines Eustachian tube
- Stereocilia in epididymis and vas deferens
Stratified Squamous
- flat cells on top
- Skin epidermis (keratinized )
- Non keratinized (corneal epithelium, esophagus, exocervix, oral cavity lining)
Transitional Epithelium
- Puffy round cells on top
- Cells small at the bottom
- Epi is found in the urinary system
- Allows distention for urine
- Urinary system (from renal calyx to proximal urethra)
Stratified Columnar
Large ducts of glands, urethra (male)
Basal Lamina
- Epithelial cells secrete majority of components
- Components: Collagens (Type 4 at least 50%, Type 3, and Type 7)
- Type 4 forms scaffold of basal lamina
- Type 3 and 7 anchor basal lamina to underlying connective tissue
- Type 3 is of non-epithelial origin
- Laminin: MAG that polymerizes on basal surface and interacts with Type 4 collagen to make a sheet like structure
- Fibronectin: MAG participates in binding integrins
Basement Membrane
basal lamina + fibroreticular lamina
Anchoring junctions
sites of adhesion; secure cells to the basal lamina or ECM
Focal Adhesions
Integrin-associated adhesions that anchor intracellular actin filaments to fibronectin of ECM of BM
Hemidesmosomes
- strong anchoring junction found in tissues subjected to abrasion and shearing forces (skin)
- Integrin-associated adhesion that anchors intracellular intermediate filaments to collgen type 4 and laminin in ECM of BM
What is the takeaway from these anchoring junctions?
- Integrins form the core of both of these junctions (transmembrane proteins found in the basal cell membrane)
- Extracellular side of these proteins contain receptors that help bind Type 4 collagen, laminin, fibronectin
Junctional Complexes in Lateral Domain
- Zonula Occludens (Tight junctions): tight seal; zip-lock
- Zonula Adherens (Adherens junctions): belt-like; six pack
- Macula Adherens (desmosome): spot weld
- Gap Junction: pipe between cells; chemically couples cells
The order of the junctions
- Tight junction
- Cell-Cell Anchoring Junctions (adherens or desmosomes)
- Channel Forming Junction (gap)
Zonula Occludens (Tight junctions)
- Acts like a zipper between cells
- Forms a tight seal between cells
- Attaches to actin
- Defines apical vs. basolateral domains of cells
- Appears as a darkened line to adjacent cells
- Does not allow material to flow between cells
- Amount of the seal depends on the tissue type
- Formed by occludins and claudins
Zonula Adherens (sticking together junctions)
- Encircles cells like 6 pack rings/belt
- Formed by e-cadherin (transmembrane) and Catenin ( intracellular)
- Cadherin: Calcium Dependent Adhesion
- Cadherins are Calcium Binding Proteins
- Calcium is needed for the Cadherins to bind
- Adheres cells together via actin of terminal web
Macula Adherens (desmosomes)
- Spot welds cells together
- Desmoglein and Desmocollin (cadherins, calcium ion dependent adhesion, calcium binding proteins)
- Catenin-like = desmoplakin
- Desmoplakin binds the intermediate filaments
- **
- On the TEM, it looks like a black fuzzy caterpillar on each cell. Those fuzzy bits are intermediate filaments (cytokeratin)
Gap Junction
- Chemically couples cells (mechanically and electrically)
- Connexins are the major protein
- 6 connexins form a connexon
- Connexons form spots that acts as channels between cells
- Allows calcium and other ions to pass from cell to cell
- On TEM, it forms a very dark and thin line
Apical Specializations
- Microvilli (non-motile, more like straws)
- Stereocilia (just very long microvilli)
- Cilia (motile, sweeping motion like little brooms)
Microvilli
- Fingerlike projections of the apical plasma membrane
- Exaggerated glycocalyx (shows up as PAS + and shows up as HOT HOT pink)
- Increase surface area
- Usually absorptive
- Actin filaments form core
- Attached to terminal web (actin net)
Cilia
- Larger finger-like projections of apical plasma membrane
- They sweep material along the surface
- 9+2 arrangement of microtubules = 9 doubles and 2 singles
- Attached via basal bodies (BB=modified centrioles; 9 triplet microtubules)
Stereocilia
- Extra looooonnngggg microvilli
- Actin core
- Passive movement by fluid
- Absorptive
- Epididymis, vas deferens, inner ear
Hair cells
stereocilia of inner ear
What makes up the glandular epithelium?
- endocrine gland
- exocrine gland
Endocrine Glands
- Secrete into blood stream (ENDOthelium)
- Do not have ducts
- Hormones are the main endocrine secretions
Exocrine Glands
- Secrete directly into the duct
- Some ducts modify he product
- Some ducts simply transport the secretion
- Product can be mucous, serous, or mixture
Functional Classification of Glands
Merocrine/Eccrine
- secrete via exocytosis
- most common
- plasma membrane is recycled
- secretory product is released; vesicle retained
**
Holocrine
- Whole cell is created
- Cells begin to die as they reach area of secretion
- Best example is the sebaceous gland around hair follicle
**
Apocrine
- portion of the apical membrane and the cytoplasm are secreted
- apical portion pinches off with product inside it
- loss of plasma membrane
- best example is lipid component of milk during lactation
Myoepithelial cells
- Contain actin, myosin, and cytokeratins; contractile
- Difficult to identify on H&E sections
- Associated with glands
- Assists with moving secretions from glands
- “Hugs” glandular elements to squeeze out contents
Renewal of Epithelium
- Most epithelial cells have a finite lifespan
- Skin turns over in 28 days
- Intestinal lining: 4-6 days
- Mitotic activity is visible in basal cell layers
- If injury or inflammation occurs, more mitotic figures are seen
- Also during dysplasia and neoplasia
