#13336 - Epithelia - Organisation of the Body

Epithelia

Define: planar sheets of cells that are held together by tight junctions, adherens, desmosomes, gap junctions and are associated with connective tissue. The epithelium supported on the basal side by a basement membrane called the basal lamina which the epithelial cells synthesise. Basement membrane, made up type 4 collagen and give epithelia structural integrity. Below the basal lamina lies the capillary bed, which provides epithelia with required nutrients and disposal of waste. The nucleus in epithelial cell is found closer to the basal surface than the apical surface

Found

-lining all surfaces of the body, line cavities, vessels

-act as interfaces between different compartments in the body

-line the internal and external surfaces of the body

-sepearte the external environment from the internal milieu

-major functional component of organs

-mediate selective transfer of substances

Functions of epithelial tissue

-seperate the internal and external environment

-selective diffusion, absorption, secretion and protection

-regulate the movement of solutes and water to and from the body

-skin, linings of the repiratory tract, alimentary canal and kidney tubules

1. Boundary and protection

-cover the inner and outer lining of body cavities- stomach and urinary tract

-barrier to contaminants

-cells replicate often to replace to damaged or dead cells- skin

2. Sensory function

• Epithelial cells are avascular but are innervated- signals for sensory transduction

3. Vectorial transport and absorptive function- intestinal epithelia

-ability to translocate ions from one compartment to another- unidirectional transport

-achieved by the asymmetry of cell membranes at the two faces- cell is polarised- apical and basolateal membranes

• Active transport of sodium ions

• Ussing model

  • Sodium concentration is kept low by the basolateral Na-K ATPase

  • Na ions move down the electrochemical gradient into the cell via the apical membrane

  • Transepithelial movement of Na ions leaves the lumen negative with respect to the contraluminal side- couple the influx with other molecules such as glucose

• Basolateral membranes

  • Na-K ATPase

  • K leak channels = high permeability to potassium ions

  • Low permeability to Na

  • Ca ATPase

• Apical membranes

  • Have a high Pna, sodium channels

4. Tight and leaky epithelia

• It tight epithelia, tight junctions prevent significant movement between cells

• In leaky epithelia, tight junctions form imperfect seals, low resistance, leak pathway shunt for ions and water

5. Secretion –Glands

• Some epithelial cells- goblet cells secrete fluids

  • –basal region attached to the basement membrane, filled with membrane bound secretory droplets to lubricate the cell surface- respiratory tract and small intestine

-exocrine and endocrine glands are formed by downgrowths of epithelium into the connective tissue

• Exocrine glands retian their connection with surface epithelium

• Endocrine glands glands whose connection with the surface is lost during development- ductless, secretions are picked up and transported to their site of action by the blood stream

• Pancreas- Endocrine- islet of langerhans

• Secretory epithelia- lungs and pancreas- active transport of CL-

• Chloride ions enter the epithelial cells via the basolateral membrane – chloride ions accumulate within the cell and exite passively through chloride ion channels. The exit of chloride ions sets up a diffusion potential across the membrane= lumen negative

6. Movement

• Some epithelial cells have cilia aid in moving substances

Classify epithelia- according to their appearance under the light microscope

-number of cell layers – simple epithelium (single layer) or stratified epithelium (epithelium of several layers, pseudo stratified stratified- single layer of cells which has the appearance of stratified epithelium since the nuclei are arranged at different heights

-cell shape- squamous (cells are flat) , cuboidal (cells are square in section), columnar (cells are taller than they are wider) – shape of the sections that are taken at right angles to the epithelial surface

-surface specialisations (apical)- keratin, cilia or microvilli

Simple epithelium

-consist of a single layer of cells- found at the surfaces involved in selective diffusion, absorption, secretion

-little protection for mechanical abrasion

-shape of the cells determined by the function

-simple squamous epithelium- flattened, irregular shaped cells, supported by the basement membrane. Nuclei are also flattened Function: passive diffusion- lungs, walls of capillaries

-simple cuboidal epithelium: nuclei is spherical, found lining small ducts and tubules that have excretory/absorptive functions- collecting tubules of the kidney- high levels of active transports- cytoplasm rich in mitochondria

-simple columnar epithelium- absorption and secretion- found lining the small intestine/ stomach

Pseudostratified

• Single layer of cells that all rest on the basement membrane, nuclei of cells are at different levels- mostly in 2/3 rd of a cell- multi layered effect

• Found: airways of the respiratory system, lining of the bronchus- cilia propel a surface layer of mucus

Stratified Epithelium

-contains 2 or more layers of epithelium

-Function: protective and poorly suited for absorption or secretion due to thickness

-classified according to the surface cells since the basal cells are cuboidal

-stratified squamous epithelium- multiple layers of epithelia that are cuboidal at the base and flatten towards the surface. The basal cells are continuously proliferating and migrate to the top. FUNCTION: protection against abrasion as the loss of the surface cells doesn’t compromise the underlying tissue. FOUND: lining the oral cavity, oesophagus

Keratinsed stratified squamous epithelium: EPIDERMIS- forms the epithelial surface of the SKIN. As the epithelial cells migrate from the basal layer to the apical layer, they flatten and as they mature they accumulate cross linked cytokeratin intermediate filaments- keratinastion. Nuclei condense and eventually disappear- tough, non living surface layer of squames- protects against abrasion and water loss

-stratified cuboidal epiethlium – thin, stratified epithelium- 2/3 layers of cuboidal cells FUNCTION: Robust lining, FOUND: larger excretory ducts- salivary glands

-Transitional epithelium-CELL SHAPE: in a non distended state, transitional epithelium appears to be 4/5 cells layer thick- basal layers are cuboidal, intermediate cells are polygonal, surface cells are umbrella (large rounded, may contain 2 nuclei) stretched state- appears 2/3 cell layer, intermediate and surface layers are extremely flattened appear squamous FOUND: only in the urinary tract in mammals- accomadates fluctuation in volume great degree of stretch

Specialisations of the epithelial cell

Apical specialisations of the epithelia

-protection, absorption of nutrient/water, secretion

Cilia

-long cytoplasmic extensions (5-10micrometer, 0.25micrometre diameter)

-motile structures- cilia beat in wave like motion- moving fluid over cells. Each cilia bound by a membrane and consists of an axenome, made up 9 pairs of tubules with central pair of tubules, motor protein, cilary dyenin walks across the adjacent microtubules causing movement. Kartenger’s syndrome

-respiratory epithelia- mucus traps debris from inspired air, cili move mucus towards the throat where it is swallowed. Fallopian tube- cilary action propels ovum from the ovary to the uterus

Microvilli

-small cytoplasmic extensions- 1 micrometer long

-finger like projections in the luminal plasma membrane

-found in cells specialised for absorption as microvilli increase the surface area

lateral domain specialisations

-basolateral domain is in contact with adjacent cells and the underlying tissue

cell junctions

occluding/tight junctions-

-highly dynamic seals between adjacent epithelial cells, acts as a barrier and determines the degree of para-cellular transport- determines their leakiness

-Also maintain the polarity of the cell by sepearting the apical and basolateral membranes

-made up of membrane proteins- Occludins and claudins- adhere epithelial cells together near to the apical surfaces

Anchoring junctions

-provide the epithelial cells to resist shearing and tensile forces by linking cytoskeletal elements between the epithelial cells as wells as to the extracellular matrix

-compromise of cytoskeleton, cytoplasmic link protein, cell to cell adhesion molecule

-adherens junction- bind the actin networks of epithelial cells via catenins (cytoplasmic link proteins) and cadherins (cell- cell adhesion molecule) – found in columnar epithelial cells- adhesion belt- epithelial-mesenchymal transitions are mostly due to mutations in the anchor proteins of the adherens junctions

-desmosomes- connect cytoskeletal intermediate filaments between adjacent epithelial cells. Cytoplasmic link proteins are desmoplankins and cell to cell adhesion molecules are demogleins formation of antibodies to desmosomal proteins result in widespread blistering of skin- autoimmune disorder is known as pemphigus

-hemidesmosomes- connect intermediate filament networks to the extracellular matrix. The desmoplankings are linked to integrins- attach the epithelial cells to the basement membrane. Epidermolysis bullosa- mutation in fibres that anchor the basal epidermal layer to the dermis- if there is a mutation in the desmoglein the epidermis and the dermis fail to move together and friction develops between the two layers- loss of epidermal cell layer

Gap junctions

-cell to cell communication-...