5.1 Epithelial Tissues Flashcards
What are the main functions of epithelia?
- Barrier - between outside world and internal milieu of cell
- Protection - mechanical and chemical
- Absorption - in small intestine and kidney, large SA provided
- Secretion - in respiratory and alimentary tracts, and in glandular epithelium
What do epithelial layers protect tissues underneath them from?
- Radiation
- Desiccation
- Toxins
- Invasion by pathogens
- Physical trauma
What are the two types of epithelia?
- Covering and lining epithelium
* Glandular/secretory epithelium
What are covering and lining epithelium?
Sheets that cover the body on external surfaces and line internal surfaces - i.e. skin and gut linings
What are glandular/secretory epithelium?
Cells that have originated from invaginated epithelial cells, now arranged as 3D secretory units (exocrine will still maintain contact with outside of body, endocrine will not/are no longer in contact)
In what 4 categories are epithelia classified?
Shape, stratification, function and specialisation
What are the three shapes of epithelia?
- Squamous - flat plate like, seen in permeable surfaces as allow transport across
- Cuboidal - height and width similar, seen in lining surfaces, good absorption and secretion
- Columnar - height 2-5 times greater that width, allows protection, lubrication and absorption/secretion
What are the four types of stratification in epithelia?
- Simple epithelium - one layer
- Stratified epithelium - several layers (can be keratinised, as seen in skin)
- Pseudostratified - gives appearance of layers, but not actually present
- Transitional - several layers that may change shape (seen in urinary system)
What are the two specialisations of epithelia?
- Cilia - movement of particles, e.g. airway, mucociliary escalator
- Microvilli - increase absorption/surface area, e.g. gut
What is simple squamous epithelium?
- Structure: single layer of flat, often hexagonal cells (so flat that nuclei appear as bumps)
- Function: diffusion, filtration, secretion, absorption and some protection against friction
- Location: lining of blood and lymphatic vessels (endothelium) and small ducts, alveoli, loop of Henlé, serous membranes (mesothelium - line body cavities i.e. pleura/thoracic cavity) and inner surfaces of eardrum
What is simple cuboidal epithelium?
- Structure: single layer of cube-shaped cells - some have microvilli (kidney tubules) or cilia (terminal bronchioles)
- Function: active transport and facilitated diffusion allow for secretion and absorption (kidney), secretion by cells in glands/choroid plexus, cilia allow movement of mucus-containing particles (i.e. out of terminal bronchioles)
- Location: kidney tubules, glands and their ducts, choroid plexus (brain, produces CSF), lining of terminal bronchioles and of the surface of the ovaries
What is simple columnar epithelium?
- Structure: single layer of tall, narrow cells - some have cilia (bronchioles, auditory tubes, uterus and uterine tubes) or microvilli (intestines)
- Function: move particles out of bronchioles, partially responsible of movement of oocyte through uterine tubes (cilia), secretion by cells in glands, stomach and intestine. Absorption by cells of the intestine.
- Location: glands and some tubes, bronchioles, auditory tubes, uterus, uterine tubes, stomach, intestines, gallbladder, bile ducts and ventricles in the brain
What is stratified squamous epithelium?
- Structure: multiple layers that are cuboidal at the base layer and progressively flattened toward the surface - can be moist or keratinised. In moist layers, cells retain a nucleus, in keratinised upper layers die and do not
- Function: protection against abrasion and infection
- Location: moist - mouth, throat, larynx, oesophagus, anus, vagina, inferior urethra and cornea. Keratinised - skin
Anywhere an abrasion can occur, multiple layers will be necessary
How are stratified layers named/classified?
They are defined by the cell shape at the uppermost layer, as will contain a mix of shapes
What is stratified cuboidal epithelium?
- Structure: multiple layers of somewhat cube-shaped cells
- Function: secretion, absorption and protection against infection
- Location: much rarer than other types - found in sweat gland ducts, ovarian follicular cells and salivary gland ducts
What is pseudostratified columnar epithelium?
- Structure: single layer of cells (some are tall and thin, reaching upper surface, some do not). Nuclei of different cells are at different layers, giving the impression of stratification. Cells are almost always ciliated and are associated with goblet cells that secrete mucus onto the free surface
- Function: synthesise and secrete mucus into free surface and then move mucus/fluid that contains foreign particles over the surface of the free surface and from passengers
- Location: lining of nasal cavity, nasal sinuses, auditory tubes, pharynx, trachea and bronchi (almost exclusively respiratory system, cilia specially developed for function)
Also known as respiratory epithelium
What is transitional epithelium?
- Structure: stratified cells that appear cuboidal when organ/tube is not stretched and squamous when organ/tube is stretched by fluid - CHANGE SHAPE
- Function: accommodates fluctuations in the volume of fluid in an organ or a tube, protection against the caustic effect of urine
- Location: lining or urinary bladder, uterus and superior urethra
What are microvilli?
Fingerlike projections with microfilament cores (actin)
- What is a ‘brush border’?
A microvilli-covered surface of a simple cuboidal and simple columnar epithelium found in different parts of the body
What are cilia?
Microtubules that are arranged precisely to allow movement. Axoneme core (9x2 +2 microtubule arrangement, 9 pairs of microtubules around the edge with 2 in the middle, 20 total) Motor proteins are used to generate force (dynein, produces necessary force for bending, microtubule gliding is powered by ATP hydrolysis)
Where do exocrine glands secrete their products?
On the external or internal epithelial surface, via a duct
Where do endocrine glands secrete their products?
Secrete products into blood of lymphatic vessels (ductless)
- What organ contains both exocrine and endocrine glands?
The pancreas, secretes products used for digestion
- Are there multiple classifications of exocrine glands?
Yes, their shapes come in more than one form
What is a goblet cell?
- A unicellular glandular epithelial cell whose function is to secrete mucus - found in gastrointestinal and respiratory tracts
- Name derived from shape
- Basal region sits on basal lamina
- Apical portion, theca, faces the lumen and is filled with membrane-bound secretory droplets
- Secretes mucus to lubricate cell surface
- Is a gland in its own right, even though it is unicellular
In which type of epithelium are goblet cells usually found?
Interspersed with pseudostratified columnar cells, i.e. in respiratory tract
How do the pseudostratified columnar cells cooperate with goblet cells?
The cilia on the pseudostratified cells beat upwards and outwards to shift the mucus produced by goblet cells (which traps foreign bodies and carries them out for expulsion)
What are the layers of cells in the respiratory tract?
- Pseudostratified epithelium
- Beneath this is the connective supporting tissue, the lamina propria (specific to epithelium)
- The epithelium and the lamina propria form a mucosa
- Beneath the mucosa is the submucosa and adventitia (connective tissue with larger glands)
From which germ layer can epithelium be derived?
Any of the three:
Endoderm
Ectoderm
Mesoderm
Where is stratified or pseudostratified epithelia derived from?
The ectoderm - forms oral + nasal mucosa, cornea, skin, glands of skin and mammary glands
Where is simple epithelium derived from?
Some from the endoderm - forms lining of respiratory tract, small intestine, pancreas, gall bladder and liver
Some from the mesoderm - forms kidney, pleurae, peritoneum, pericardium, mesothelium, reproductive systems, endothelial cells of blood vessels
What is a basement membrane?
The boundary between epithelial and connective tissue - epithelia cells are usually surrounded by or lie in a supporting layer of connective tissue
What two sections does the basement membrane comprise of in epithelium?
Basal lamina
Reticular lamina
What is the purpose of the basement membrane?
Acts as a mechanical barrier and anchors epithelial to the connective tissue
What is the basal lamina?
Synthesised by epithelial cells and contains anchoring fibrils of type IV collagen which extends into the reticular lamina
In contact with epithelial tissue
What is the reticular lamina?
Produced by the connective tissue and contains fibres and collagen bundles around which fibrils are able to loop
In contact with connective tissue
What is the apical surface of the epithelium?
The surface facing outwards/towards the lumen
TOP
What is the basal surface of the epithelium?
The surface connected/adjacent to the basement membrane (basal lamina)
BOTTOM
Do epithelial cells exhibit polarity?
Yes
How are the domains distinct/epithelium polarised?
Domain structure correlates with function - they have distinct morphological, biochemical and functional differences, allowing specialised functions associated to different domains
What are the adaptations and function of the apical domain?
Rich in ion channels and carrier proteins, can have cilia, microvilli and stereocilia
Function:
• Protection
• Secretion
• Absorption
• Regulation of nutrient and water uptake
What are the adaptations and function of basal domains?
Basolateral (bottom and sides) domain has junctional specialisations Function: • Cell contact • Adhesion • Communication • Ion channels • Receptors • Basal infoldings
What types of junctions are present in the basolateral domain of epithelial cells?
• Tight junctions (occluding, block intercellular transport)
-> zona occludens, these can have flexible tightness
• Anchoring/adhesive (link adjacent cytoskeletons)
-> adherening junctions: zonula adherens
-> desmosome junctions: macula adherens, called hemidesmosomes when at the bottom of the cell
• Communicating/gap (allow specific communication between cells through acting as channels through which molecules can move)
Through which type of microscope are tight, anchoring and gap junctions possible to be viewed?
Only through electron microscopy
- What family of transmembrane proteins are associated with tight junctions?
Cytoplasmic membrane associated with guanylyl kinase-like domain (MAGUK) protein family and zona occludens proteins (ZO-1, ZO-2 and ZO-3)
Where are tight junctions particularly tight?
In organs like the brain - vital for forming the blood brain barrier, acting as both a physical and a metabolic barrier
The two principle proteins of tight junctions and their function are:
• Claudins
• Occludins
These establish a fence junction through the formation of large multi-protein complexes
Allow for homophilic cell-cell adhesions, mediate tight, high affinity binding between adjacent cells
What proteins are seen at adhering junctions?
Cadherins
How are adhering junctions linked together?
Through cadherins binding to each other intracellularly (outside the cells), but with connections to actin microfilaments within the cell and therefore to the cytoskeleton
Filaments run into terminal webs at edges of cell, actin filaments are stabilised here by spectrin
What are the proteins at desmosomes?
Cadherins
How are desmosome junctions linked together?
Through cadherins linking to a cytoplasmic plaque made of attachment proteins - the adhesion molecules (cadherins) are also linked to the intermediate filament cytoskeleton.
- What is pemphigus?
A blistering autoimmune disease that affects the skin - antibodies produced that attack desmosomes (specific cadherins in skin known as desmogleins). Antibodies form IgA^- mediated dermatoses -> aberrant deposits of IgA (immunoglobulin A, type of antibody) on the basement membrane Affects skin (keratinocytes) and mucus membranes - junctions break down and adhesion lost, allowing skin to slough off and the formation of blisters.
- What are the three subtypes of pemphigus?
- vulgaris (blisters deep in epidermis)
- foliaceus (blisters in superficial layers)
- paraneoplastic (90% mortality, blisters in deep layers and neoplasm)
- What are the risk factors for pemphigus and treatments?
Never quite clear what the trigger is - environmental or genetic (risk factors for both, preferentially affects women)
Corticosteroids 93 plasmapheresis (exchange/transfusion of plasma to remove harmful antibodies)
- What is EB?
Epidermolysis bullosa - genetic skin disease that is caused by mutations in keratin genes. Presents as easy blistering of the skin and mucous membranes
What is the function of keratin?
Plays an important role in maintaining the integrity of the epidermis - attaches cell membrane at site of desmosome/hemidesmosome junction to nuclear membrane
What are the four main types of EB?
- Simplex - KRT5 and KRT14 mutations
- Dystrophic - COL7A1 mutation (collagen gene)
- Junctional - affects laminin and collagen, present in hemidesmosomes, causes blisters in lamina lucida
- Kindler syndrome - blistering in young children
- What treatment is possible for EB?
Stem cells - in future, regeneration using transgenic stem cells may be possible
Current is wound care, control of infection and pain, nutritional support and limiting of complications
What proteins are seen in gap junctions?
Connexins are the proteins in gap junctions, each one has many pairs that allows for chemical and electrical signals to be passed between cells
In which ways can epithelial transport occur?
It can be through (from apex to base or vice versa, transcellular/intracellular) or between cells (in the gaps between epithelia, paracellular/intercellular)
In which direction can epithelial transport occur?
Apex to base (i.e. top to bottom, absorption - seen in kidneys and intestines)
Or
Base to apex (i.e. bottom to top, secretion - seen in sweat glands)
What is the transepithelia transport of ions?
This is a selective exchange of ions
FIND SLIDE ON CANVAS
- What are the modifications to increase surface area in the intestinal lumen?
- Plicae circulares (valves of Kerckring) - transverse folds increase SA by factor 2-3, decrease velocity for movement of chyme. ‘Branches of the tree’, villi come off from here.
- Villi - epithelial covered finger-like protrusions that increase SA by factor 10, contain capillary loops and blindly ending lymphatic vessels
- Microvilli - modifications of apical plasmalemma of epithelium lining the villi, increases SA by factor 20
- Crypts of Lieberkuhn - invaginations of epithelium into lamina propria forming intestinal glands
Look at photo examples of epithelia
DO IT you WILL be tested on this.
