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 and line hollow organs, body cavities, and ducts; it also forms glands.
3 main functions: Protective, Secretory, and Selective Barriers
What are cell junctions?
Cell junctions are connection points between the plasma membranes of adjacent cells. There are namely 5 types: gap junction, adherens junction, desmosomes, hemidesmosomes, and tight junction.
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)
What is the basal surface?
The deepest layer of epithelial cells facing the basement membrane - these may contain hemidesmosome junctions.
How are epithelial cells arranged?
Epithelial cells are arranged into flat, continuous sheets and can be comprised of single or multiple layers
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 filaments - such as keratin
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
Describe the structure of tight junctions
Located near the apical surface
Strands of transmembrane proteins (claudins and occludins) link adjacent plasma membranes in a ‘sewing-like’ appearance
No gap is left
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
Where are tight junctions found?
Epithelial tissue lining stomach, intestines, and bladder
What are claudins and occludins?
Transmembrane proteins that form the tight junction
Describe the structure of adherens junction
Located near the apical surface
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?
Adherens junction links the microfilaments of adjacent cells preventing cell separation from tension forces like muscle contraction (strength and structure)
What is cadherin and catenin?
Cadherin is the transmembrane glycoprotein that fills the gap in adherens junction and desmosome junction.
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
Button-like shape
Composed of two plaques (walls of tissue) joined together by transmembrane glycoprotein (cadherin)
Cadherin links to the keratin intermediate filaments of each adjacent cell
What is the function of desmosome junctions?
Link the intermediate filaments (keratin) of adjacent cells providing strength to the junctions between cells. Allows tissue to resist shearing forces.
Where are desmosome junctions found?
Bind muscle cells
Also found in skin epithelia and cardiac cells of the heart preventing tissue being pulled apart.
Describe the structure of gap junctions
The plasma membrane at gap junctions contain connexin protein molecules which group in numbers of 6 to make connexon (otherwise known as a hemichannel). These connexon molecules move about in the cell membrane until a matching connexon is found in the adjacent membrane and form a tunnel - communication portal.
What is the function of gap junctions?
Allow the cells to communicate with one another (pass chemical and electrical signals). Also allow transfer of ions and small molecules between adjacent cells of tissue
What are connexins and connexons?
Connexins are protein molecules present in plasma membranes that make up connexons (hemichannels).
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.
Integrin acts as the linker protein between the keratin of the basal layer cell and laminin in the basement membrane
What is the function of the hemidesmosome?
Link between basal layer epithelial cells and basement membrane preventing movement of the epithelial layer over the basement membrane - fixation of cell
What is integrin and laminin?
Integrin is the transmembrane glycoprotein forming the hemidesmosome junction
Integrin forms the link between keratin and laminin (a protein) present in the basement membrane
What are the 4 lateral junctions?
Gap, tight, desmosome, and adherens
What is the only basal junction?
Hemidesmosome
What is the junctional complex?
A combination of tight junction, adherens junction and desmosome
What is a basement membrane?
A layer underlying all epithelial tissue composed of two layers - basal lamina and reticular lamina. Located in between the basal layer of epithelial tissue and connective tissue
What is basal lamina?
The top layer of the basement membrane made up of proteins that are secreted from the basal layer of the epithelial tissue. Contains : collagen, laminin, glycoproteins,
What is the reticular lamina?
The lower layer of the basement membrane made up of proteins produced by connective tissue cells below the basement membrane called fibroblasts. Contains: fibronectin, collagen, 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?
Support for overlying epithelial tissue
Formation of surface for wound healing (allows epithelial cells to move across new basement membrane reforming cuts and tears in skin)
Filtration of blood in kidneys
Protection of inner layer from passage of large molecules
Does the epithelial layer have nerves?
Yes
Does the epithelial layer contain blood vessels?
No - epithelial tissue is avascular
How does the epithelial layer receive nutrients and get rid of watse?
Diffusion across the basement membrane between the blood vessels present in the connective tissue and the epithelial tissue.
What are the two main types of epithelial tissue?
- Covering and lining epithelia
2. Glandular epithelia
What is covering and lining epithelia?
Epithelial tissue that forms the outer covering of skin and internal organs.
Also forms the inner lining of blood vessels, ducts, and body cavities, and the interior of the respiratory, digestive, urinary, and reproductive system
What is glandular epithelia? (Name the two types)
Constitute the secretory portion of glands. Two types include endocrine (hormones into blood) and exocrine (secretion onto external surfaces and ducts)
What are the further classifications of covering and lining epithelia made based on their arrangment?
Simple - single layer of cells
Stratified - multiple layer of cells
Pseudostratified - single layer of cells however not all cells reach the apical surface
What are the further classifications of covering and lining epithelia made based on the shape of the cells?
Squamous - flat and thin
Cuboidal - same height and width
Columnar - long and thin
Transitional - always stratified where the cells are able to change shape from cuboidal to flat depending on shape of organ
What are the features of the simple squamous epithelium?
A single layer of thin, flat cells
Present at sites of filtration, diffusion, and secretion in serous membranes (soft tissue)
E.g - lining cv and lymphatic system (known as endothelium)
What is endothelium?
Layer of simple squamous epithelium lining the vessels of the cardiovascular system and lymphatic system.
What is mesothelium?
Specialised type of simple squamous epithelium that lines the serous membranes of the heart, lungs, and abdominal areas.
What are the features of simple cuboidal epithelium?
A single layer of cuboidal cells
Present at sites of secretion and absorption
Found in the secreting portion of glands like the thyroid gland
What are microvilli?
Finger-like cytoplasmic extensions from the membrane that increase surface area for absorption and exchange of materials.
What are 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
What are the features of non-ciliated simple columnar epithelium
Made up of a single layer of rectangular cells. These epithelial cells can have microvilli on the apical surface and goblet cells which are modified columnar epithelial cells containing mucous.
Increased surface area means this type of epithelia tissue exists at sites of secretion, lubrication (mucus goblet cells) and absorption.
Found in the ducts of many glands
What are the features of ciliated simple columnar epithelium?
Single layer of rectangular cells containing goblet cells. These cells have cilia which are motile projections that can move in a sweeping motion to move substances along surfaces.
The cilia allows for synchronous movement assisting with the movement of mucus and foreign objects
Found in bronchioles, sinuses, uterine fallopian tubes, etc.
What are goblet cells?
Modified columnar epithelial cells containing mucous for secretion.
What are the two types of stratified squamous epithelia?
Keratinised and non-keratinised
What are the features of keratinised stratified squamous epithelium?
Multiple layers of epithelial cells where the top layer is thin, and flat cells (bottom layer could be made up of columnar and cuboidal cells)
Contains keratin making surface tough and waterproof - therefore structural and strength roles
Found in skin
What are the features of non-keratinised stratified squamous epithelium?
Layers of multiple cells with squamous epithelial cells on the top layer.
These cells do not contain keratin therefore lines wet surfaces such as the mouth, oesophagus, vagina, and pharynx, tongue.
They are moistened by mucous.
What are the features of psuedo-stratified columnar epithelium?
Made up of a single layer of rectangular epithelial cells that all touch the basement membrane however do not all reach the apical surface of the epithelial tissue
There are two types : ciliated and non-ciliated
What is the difference between ciliated and non-ciliated pseudo-stratified columnar epithelia?
Ciliated has goblet cells (modified columnar cells which contain mucous)
Non-ciliated has no goblet cells.
Therefore ciliated form lines most of upper airways (secretes mucous) and non-ciliated lines larger ducts of glands, epididymis, part of male urethra (absorption and protection).
What is the features of stratified cuboidal epithelium?
Made up of cuboidal cells that are arranged into multiple layers.
Usually for protection - limited secretion and absorption
Found in ducts of sweat glands and oesophageal glands, part of male urethra.
What are the features of stratified columnar epithelium?
Apical layer cells are columnar with irregular shaped cell layers below them.
Protection and secretion
Found in part of urethra, large excretory ducts of some glands, small areas in anal mucous membrane.
What are the features of transitional epithelium?
Apical layer has large round cells and the cells below them are subject to change in shape (resembling cuboidal and squamous and different states)
Allows urinary organs to stretch and maintain protective lining while holding variable amounts of fluid without rupturing.
Found in the lining of the urinary bladder
What is a gland?
Glands consists of a group of cells involved in the secretion of substances into ducts, onto a surface or into blood
What are endocrine glands?
Glands that secrete substances directly into blood usually via traversing interstitial fluid
eg Hormonal glands
What are exocrine glands?
Glands that secrete substances into ducts that empty onto the surface of a covering or lining epithelium
eg Sweat glands and salivary glands
What is a single cell gland?
In epithelia that have independent, scattered gland cells, the individual secretory cells are called mucous cells, and they secrete mucin. 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)
