Download Week 7 - Skin Epithelia - PBL-J-2015

Week 7
Epithelia
1. Know the criteria used for the histological classification of epithelia.
2. In a table, summarise the types of epithelia and their key features, and describe
the cells found in the different epithelial types.
Simple Squamous
 One cell thick and flattened at the top
 Because thin and flat it is specialized for exchange
 Between squamous cells are leaky tight junction
 Founding in portions of kidney (fluid exchange) and lungs (gas exchange).
Simple Cuboidal
 One cell thick but roughly same width as height
 Specialized for secretion and absorption (do both in thyroid gland)
 They line salivary ducts and male + female reproductive system
Simple Columnar
 One cell thick but long-like columns.
 Most strikingly polarized of epithelial cells (apex looks very different to base)
 Have deep and tight junctions
 Found in large portions in digestive system, male + female reproductive system,
urinary tract and auditory system
Stratified Squamous
 Several cells thick but top of cells thin and flat
 In a constant cycle of cell division, migration and cell death. Epithelial cells at
bottom divide and move to the top where they become flat.
 Keratinocytes produce keratin (protects cells from dehydrating as protein
keratin is mechanically strong and highly water insoluble due to di-sulfide
bridges).
 But for stratified squamous in body, keratin is not produced due to presence of
moisture.
 Stratified Squamous is found in areas where there is “wear & tear.” So skin, anal
canal and oesophagus.
Pseudostratified
 Looks like it is stratified but isn’t. Because all cells attached to basil lamina are of
different heights, thus it appears stratified when it isn’t.
 Found in lining of trachea.
Transitional Epithelium (specialised epithelium)
 Only found in lining of bladder and upper area of urinary tract.
 It can change its appearance and shape. i.e. in empty bladder it appears to be 7-8
cells thick but as bladder fills, lumen stretches and epithelium appears to get
thinner with only 2-3 cells thick (but really it still is 7-8 cells thick).
 Has distinct bi-nucleus, mushroom shape
 Has refractive border on its outside.
Note: refractive border has plates of lipids and proteins which are impermeable to
water, which stops cells from losing water to urine (prevents osmotic shock).
 Between each dome are deep, tight junctions
Other Specialised Epithelia
 Endothelium – lines blood vessels and heart
 Mesothelium – lines body cavities and is able to secrete
• Both of above specialised epithelia are simple squamous
3. Know the different types of junctional complexes and their function.
• Firstly, should be aware that epithelia has polarity
 Apical domain – exposed to lumen or external environment. Responsible for;
protection, absorption, transport of material, secretion, gas exchange etc
 Lateral domain – faces neighbouring epithelial cells linked by adhesion molecule
and junctional complexes
 Basal domain – associated with epithelial basil lamina (or basement membrane)
• In the basolateral you have junctional complexes and adhesion molecules
1. Junctional complexes
• Are responsible for anchorage
• Structures between two epithelial cells that provide strong stability
• Movement of solutes, ions and water across and between them
• Types are:
a. Tight Junctions (Zonula occludins)
 Protein occludin encircles cells and binds to neighbouring cells
b. Zonula adherens
 Has special protein cadherin and links with cytoskeletal actin filament network
c. Desmosomes
 Link with intermediate filament network
d. Gap Junctions
 Connect through cotton wheel like structure called connexons
 Carry out lateral communication
e. Hemidesmisome
 Anchors cells to basal lamina
2. Adhesion molecules
 Responsible for intercellular communication
 Have calcium dependent: cadherins and selectins
 Have calcium independent: immunoglobulin superfamily and integrins
4. Describe the structure and function of the basement membrane.
Basement Membrane (Basil Lamina)




At basal surface of all epithelia
Epithelial cells are anchored to basement membrane
Provides epithelial tissue support and acts as molecular filter
Usually refer to basement membrane as basil lamina for electron microscopy
o EM shows three zones in basil lamina: Lamina rara, Lamina densa,
Lamina reticularis
 Laminin is major component of basil lamina. It has 4 arms that can bind to four
other molecules. Three shorter arms are particularly good for binding to other
laminin molecules and long arm is capable of binding to cells (anchor the actual
organs to the basement membrane)
5. Know the different types of gland structures and be able to draw, describe and
identify simple tubular, compound tubular and acinar glands.
Acinar Gland
Note: if thinking of saliva, the duct system above would bring the secretions to the oral
cavity. Also notice the change in epithelial tissue through each section. Finally,
remember myoepithelial cells are non-muscle contractile cells – here they help push
secretions into ducts.