What are properties of epitheal tissue?
- Tight cohesive sheet
- Little extracellular matrix (ECM)
- Covers surfaces & forms glands
- Avascular (no direct blood supply)
- Can regenerate
- Sits on basement membrane
- Has nerve supply
- Derived from all 3 germ layers
What does the Basement membrane consist of?
Extracellular matrix which is formed by secretions of both epithelium and connective tissue.
It attaches to the underlying connective tisse.
It comprises of the basal lamina & reticular lamina.
How can epithelia be classified?
- Number of layers
- Shapes of cells
- Surface specialisations
How can epithelia be classified according to the number of layers? What are the different classifications used for?
Simple: - Diffusion/filtration/secretion/absorption
- Minimum protection
- Stratified: - Protect underlying tissues, wear & tear
- Little absorption/secretion
- Pseudostratified: - Nuclei lie at different levels
- Not all cells reach surface
How can epithelia be classified according to the shapes of cells? What are these different classifications used for?
Squamous: - Allow rapid movement of substances through them
Cuboidal: - Produce secretions or absorption
- Columnar: - Protect underlying tissues
- Transitional: - Readily change shape, accommodates stretching
- Due to distension/expansion/movement of body
Why are epithelial classifications important?
Cancer - cells often change number/layering/shape
How can biopsies be taken?
- Solid tumour biopsy
- Needle biopsy
- Fluid biopsy = ascites
What are the 2 different cell types in the cervix and what do they do?
Glandular Columnar Cells at superior endocervical canal - Make mucus
Stratified Squamous Epithelial Cells - Protective
What do Pap tests look for?
Cell changes under microscope
Why may cells be precancerous?
If changes to cells are caused by human papillomavirus & not treated.
Define Cell Polarity
Having opposite characteristics at 2 extremeties.
Fundamental feature of many types of cells.
Why are epithelial cells polarised?
They have different specialisations at different edges/surfaces:
- Apical - Separated by a tight junction from the basolateral surfaces
What domains is the plasma membrane divided into?
Apical & Basolateral domains
In what ways do the plasma membrane domains have distinct compositions and why is this a good thing?
- The protein & lipid components differ giving disctinct properties
- Allows directional transport of molecules
- Permits structural & functional specialisations
What is unclear about plasma membranes?
How they generate & maintain polarity
What are the 2 free surfaces if epithelium, not including specialisations?
Smooth - Reduce friction
Folds - In epithelium where organ needs to change shape
What is the role of the Cytoskeleton?
- Network of proteins in cytoplasm
- Supports cell & holds organelles in place
- Responsible for cell shape
What are the 3 components of the cytoskeleton and what are they made of?
Microtubules: Hollow tubes of tubulin
Microfilaments: Actin filaments in bundles
Intermediate filaments: e.g keratin, vimentin, desmin
Name 3 types of Apical Specialisations
Describe the structure, composition and location of Microvilli.
- Finger-like cytoplasmic protrusions
- Contain actin filaments
- Increase surface area of plasma membrane
- Known as brush border
- Found in small intestine
- Vary in size
What is the function of stereocilia, what are its' limitations and where is it found?
- Closely related to microvilli
- Sensation & Absorption is its uses
- Lacks mobility unlike cilia
- Limited distribution
Describe the structure, distribution and location of cilia.
- Hair-like processes containing tubulin microtubule
- Can be several hundred per cell
- Found in lining of trachea to move dust and mucus away from lungs.
- Also found in Fallopian tubes & are said to "beat" as they move the ovum from the ovary to uterus
- They "beat" due to an internal structure
What classification of epithelia is in the lower airways?
Pseudostratified ciliated columnar epithelium.
What is the Baso-lateral domain specialisation?
Junctions between cells or with basement membrane.
What are uses of the Baso-lateral domain specialisation?
- Maintain polarised state
- Joins cells together to form cohesive sheet
- Allows exchange of info & metabolites between cells
What do the junctions of the baso-lateral domain form?
Junctional Complex - Stable barrier to movement of substances between lumen & intercellular space
What are the 3 cell junction types?
- Occluding/Tight junctions
- Anchoring junctions
- Communicating/Gap junctions
What are basic features of occluding/tight junctions?
- Seals cells together
- Prevents leaking between cells
What are basic features of anchoring junctions?
Mechanically attaches cells to neighbours
What are basic features of communicating/gap junctions?
Mediate passage of signals between cells
Name a type of Occluding/Tight junction, and give details on its structure and function.
Zonula Occludens Junctions:
- Forms continuous band around each cell
- Apical part of lateral surface
- Cells in close contact, appear fused
- Joins cytoskeleton together
- Involves occludins & claudin proteins
Give an example of an Anchoring junction and give details on its' structure and location.
- Just below tight junctions on lateral membrane
- Actin filaments running through cell attach to adherens junctions
- Strong junction
- Many along lateral membrane
- Involves cadherin & catenin proteins
Give details on the structure, function and location of Desmosomes (macula adherens).
- An Anchoring junction
- Rivets cells together
- Distributes tensile forces through cells
- Achoring sites for intermediate filaments
- Transmembrane linker proteins in hemidesmosomes = Cadherins
- If no desmosomes present in skin = Pemphigus
Decribe the structure and function of a Gap Junction.
- Directly connects cytoplasm of 2 cells
- More basal part of lateral membrane
- Uniform 2-4nm gap between adjacent cells
- Not just a gap - spanned by channel-forming proteins such as connexins
- Allows molecules & ions to pass freely between cells without leaking into extracellular space
- Allows electrical communication between cells
Descibe an Anchoring junction found in the basal domain in terms of function and structure.
- Connects epithelia to underlying basal lamina
- Connection site for intermediate filaments
- Many keratin filaments attached to desmosomal plaque also attach to hemidesmosomal plaque
- Resemble desmosomes but chemically distinct
- Transmembrane linker proteins in hemidesmosomes = Integrins
In what disease are junctions often disrupted and what effect does this have?
Allows cells to break off & migrate in invasive metastatic cancers.
What do intestinal epithelia do?
Transport nutrients from lumen to connective tisse & BVs
How do Intestinal epithelia maintain polarity?