L9 Epithelia Flashcards
What are junctional complexes?
Structures involved in cell adhesion and communication
Junctional complexes include tight junctions, adhering junctions, and gap junctions.
What do tight junctions (TJ) do?
Impedes paracellular movement
Tight junctions restrict the passage of substances between cells.
What are the 3 primary germ layers?
- Ectoderm: Outermost layer
- Mesoderm: Middle layer
- Endoderm: Innermost layer
What determines the ‘tightness’ of tight junctions?
Protein strands called claudins
The ‘tightness’ can vary significantly, influencing permeability.
What is the permeability variation range of tight junctions?
10,000 fold
Permeability varies between different tissues, such as small intestine and bladder.
What are adhering junctions?
Form a belt around the cell, linked by actin filaments and cadherins
These junctions help maintain tissue organization.
What is epithelial-mesenchymal transition (EMT)?
Reversible process of epithelial assembly
EMT allows epithelial cells to disassemble and migrate.
What role do gap junctions play?
Allows laterial communication between epithelial cells
Permit small molecule diffusion between neighboring cells
They allow electrical coupling of cells.
What separates epithelial cells from underlying connective tissue?
Basement membrane
The basement membrane is composed of ECM proteins like collagen IV.
What are the main functions of epithelial cells?
Secretion - gland
Diffusion - lung
Protection - skin
Absorption - small intestine
These functions are essential in various organs.
What are the common properties of epithelial cells? (4)
- Entirely cellular - Composed of tightly packed cells
- Avascular - Lack their own blood vessels. Receive nutrients via diffusion
- Lack extracellular fibres
- Minimal extracellular space
Epithelial cells able to import substances against steep conc gradients. What properties facilitate this?
- Connected via tight junction, decrease free diffusion
- Distinct apical and basolateral domains with differing membrane properties and functions (polarised)
What do desmosomes do?
Form strong adhesion points between cells
They connect intermediate filaments to cadherin proteins.
What are the two main types of epithelial classification for number of cell layers?
Simple and stratified
Simple epithelium has a single cell layer (lung), while stratified has multiple layers (skin)
Pseudostratified epithelium is another layer
How are basic types of epithelia classified?
Classified based on:
The number of cell layers
The cell shape
What is the appearance of simple squamous epithelium?
Thin ‘scales’
This structure facilitates rapid passage of molecules.
What is the appearance of stratified squamous epithelium?
Apical cells appear squamous, basal cells are cuboidal or columnar
Common in areas of high abrasion, such as the skin.
What are goblet cells?
Unicellular glands that secrete mucus
Often found interspersed in columnar epithelium.
What types of glands are classified as exocrine?
Glands with ducts that secrete substances onto epithelial surfaces
Examples include salivary glands and sebaceous glands.
What type of epithelium allows for shape change without damage?
Transitional epithelium
Found in the bladder, it accommodates distension.
What is the role of stem cells in epithelial tissues?
Continual cell replacement for tissue homeostasis
This is crucial due to the hostile external environment.
What are the two uncommon epithelial types that do not fit into simple or stratified categories?
Pseudostratified and transitional epithelia
Pseudostratified epithelium is found in the upper respiratory tract.
What type of epithelial cells line the GI tract?
Simple columnar epithelium
This type may have cilia or microvilli for absorption and secretion.
What is a characteristic feature of stratified cuboidal epithelium?
Less common, found in glands
It typically consists of two or more layers of cuboidal cells.
True or False: All epithelial tissues are avascular.
True
Epithelial tissues lack blood vessels and rely on diffusion for nutrients.
What are the common properties of epithelial cells? (6)
Polarity
Cellularity
Junctional complexes
Avascular (lacking blood vessels)
Minimal extracellular space
Lack extracellular fibres
What does ECM proteins mean? And examples?
ECM - Extracellular matrix.
Collagens, Laminin, Proteoglycans
ECM proteins secreted by epithelial cells
What are the membranes of an epithelial cell?
Apical membrane
Basolateral membrane
Apical - Faces the lumen or external environment
Basolateral - Faces underlying connective tissue
What properties facilitate the import or expel of substances in epithelial cells?
- Polarity
- Tight junctions
- Distinct apical and basolateral domains with differing membrane properties and functio
- Connects the epithelial cells together
What are the types of junctions you need to know about?
Adhering junctions
Gap junctions
Tight junctions
How do epithelial cells regenerate?
Through stem cells
What four characteristics do all epithelia share despite their differences?
Cellularity
Polarity
Avascularity
Regeneration
Epithelial tissues composed of closely pacjed cells with minimal extracellular matrix
What is the function of a simple squamous epithelia?
Primary function is to facilitate rapid diffusion and absorption of molecules
What are the types of epithelia?
Based on number of cell layers:
Simple epithelium
Stratified epithelium
Pseudostratified epithelium
Based on the shape of the cells:
Squamous epithelium
Cuboidal epithelium
Columnar epithelium
Pseudostratified - Appears to have multiple layers but all cell touch the basement membrane
What type of epithelia is found in the small intestine?
The simple columnar epithelia
Apical surface covered with microvilli
What is psuedostratified columnar epithelia?
Despite having multiple layers of cells, it consists of asingle layer of cells of varying heights so single cell layer attached to the basement membrane.
Cilitated cells often interspersed with goblet cells
What is the most common type of stratified epithelium in human body?
Stratified squamous epithelia
What shape do the transitional epithelia cells look when relaxed?
Rounded
What are the two types of glands?
Exocrine (ducts)
Endocrine (ductless)
Secretes directly onto the epithelial surface
Secretes directly into blood
