Epithelia Flashcards
What is epithelia?
Epithelia is an important part of most organs in the body; Some epithelia line surfaces such as in the colon or lungs; others are the major cell types of organs such as the liver, thyroid and pancreas.
Many diseases involve epithelia, the most common of which include common cancers of the breast, prostate, colon, lung and skin.
Epithelium and its underlying connective tissue are given names in certain situations
What is the Mucosa/mucous membrane?
lines the body passage that communicates with the exterior, comprising of surface epithelium and underlying supportive connective tissue (lamina propria) and sometimes a layer of smooth muscle (muscularis mucosae); also associated with glands that secrete mucus or other secretions onto the epithelium.
What is the Serosa/Serous membrane?
lines pericardial, pleural, peritoneal cavities and tunica vaginalis and comprises surface mesothelium and underlying supportive connective tissue.
What are the two main types of epithelia?
- Surface Epithelia
2. Glandular Epithelia
Explain the difference between Surface and glandular epithelium?
SURFACE EPITHELIUM: Lines surfaces and lumina. Everything that enters or exits the body must pass across an epithelium. It is present in the skin, GIT, Respiratory system, kidney & reproductive tract. Surface epithelium may also be a component of glands such as in ducts.
GLANDULAR EPITHELIUM: predominantly involved in secretion and can nclude single cells, invaginations of multiple cells forming glands, and complete solid organs such as the pancreas and the thyroid.
What is the Function of the Epithelia?
Epithelia serves as a protective barrier with selective diffusion properties. It is also used in both absorption and secretion and is involved with specialized receptors for smell and taste.
Epithelial cells are also capable of great renewal. They have a finite life span and are either labile or stabile however there is a basal layer of stratified epithelial stem cells that replace them as they are lost. The cells move upward and shed at the surface.
What is the structure of Epithelium?
Epithelial cells exhibit polarity in their apical, lateral and basal domains. Epithelia cells are connected to one another via cell junctions. The layer of epithelial cells is supported by a basement membrane and is collectively avascular.
There are different types of SURFACE EPITHELIUM, how are they classified?
Variations of surface epithelial cells are classified based on:
- Number of cell layers (simple; stratified) - Shape of cells (of the top-most layer/squamous; cuboidal; columnar) - Surface specializations (cilia; keratinisation)
Where are simple squamous cells found?
Simple squamous cells are found in the mesothelium, endothelium, lining of the alveoli and the glomeruli.
Where are simple cucoidal cells found?
Simple cuboidal cells are found in the real tubules and the thyroid follicles.
Where are simple columnar cells found?
These simple columnar cells can be non ciliated such as those found in the stomach, small & large intestines, gall bladder, bile duct and endocervix.
They can also be ciliated such as those in the fallopian/uterine tubes and the bronchioles.
Where are pseudostratified ciliated columnar epithelia found?
These cells are referred to as pseudostratified because they have varying levels of nuclei but the cells are all attached to a single basement membrane so they are in fact simple.
Ciliated examples of these cells can be found in the respiratory tract; non ciliated examples include the epididymis and vas deferens.
Where are Stratified Squamous cells found?
Stratified squamous cells can be classified as either keratinising (produces keratin) or non-keratinising.
The skin is an example of a keratinising epithelial layer whereas non-keratinising examples exist in the oral cavity, oesophagus, anus, vagina and ectocervix.
Where are stratified cuboidal cells found?
This variety of cell is seen in some ducts.
Other stratified cells are often found in…?
There are some organs such as the breasts, sweat glands and salivary glands where the epithelium exists as a surface columnar layer
overlying a myoepithelial layer.
The prostate is an example where the surface columnar layer overlies a basal layer.
Where is transitional/urothelial cells found?
Found in the renal pelvis, ureters and bladder.
Explain the attachments of epithelia:
The outer/apical surface of the epithelial layer houses the microvilli, stereocilia and cilia as well as any biochemical modifications.
The inner/basal surface attaches to the basement membrane.
Lateral surfaces attach to adjacent epithelial cells via cell junctions.
Explain the apical surface elements of the epithelium?
MICROVILLI: 0.5-1 micron in length; most epithelia have only a few but some have numerous. They can be seen as either striated or a brush border on light microscopy. They function to increase surface area in small intestine and proximal renal tubules for absorption.
CILIA: Longer finger-like projections between 2 - 10 microns in length. There is an organized core of microtubules allowing movement in coordinated waves found in the respiratory tract and fallopian/uterine tubes.
What are the intercellular junctions between epithelial cells?
- zonular occludens [apical]
- zonular adherins
- Macula adherins (desmosome) [basement]
What is a tight junction?
Tight Junctions seal intracellular spaces to block passage of substances between cells; they form a continuous circumferential band around the apex of cells.
What is an adherens Junction?
Adhering junctions and desmosomes are mechanically strong attachments between cells that link the cytoskeletons of adjacent cells.
What is a gap junction?
Gap junctions also known as nexus or communicating junctions allow passage of small molecules, allowing for communication between cells.
What is a hemidesmosome?
These are modified desmosomes that link epithelial cells to the underlying basement membrane.
What are Cell Adhesion Molecules (CAMs)?
These are transmembrane proteins that link with the CAMs on neighboring cells or matrix. They are involved in cell adhesion, intracellular and extracellular communication, cell movement and differentiation.
There are different types of CAMs on different cells:
CADHERINS: component of adherens junctions
INTEGRINS: interact between actin and ECM molecules
SELECTINS: mediate white blood cell-endothelial cell interactions
IMMUNOGLOBULIN SUPERFAMILY: mediate homotypic cell-cell adhesions (ICAM, CCAM, PECAM)
