Epithelia

Question 1

What is epithelia?

Answer 1

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

Question 2

What is the Mucosa/mucous membrane?

Answer 2

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.

Question 3

What is the Serosa/Serous membrane?

Answer 3

lines pericardial, pleural, peritoneal cavities and tunica vaginalis and comprises surface mesothelium and underlying supportive connective tissue.

Question 4

What are the two main types of epithelia?

Answer 4

1. Surface Epithelia

2. Glandular Epithelia

Question 5

Explain the difference between Surface and glandular epithelium?

Answer 5

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.

Question 6

What is the Function of the Epithelia?

Answer 6

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.

Question 7

What is the structure of Epithelium?

Answer 7

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.

Question 8

There are different types of SURFACE EPITHELIUM, how are they classified?

Answer 8

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)

Question 9

Where are simple squamous cells found?

Answer 9

Simple squamous cells are found in the mesothelium, endothelium, lining of the alveoli and the glomeruli.

Question 10

Where are simple cucoidal cells found?

Answer 10

Simple cuboidal cells are found in the real tubules and the thyroid follicles.

Question 11

Where are simple columnar cells found?

Answer 11

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.

Question 12

Where are pseudostratified ciliated columnar epithelia found?

Answer 12

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.

Question 13

Where are Stratified Squamous cells found?

Answer 13

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.

Question 14

Where are stratified cuboidal cells found?

Answer 14

This variety of cell is seen in some ducts.

Question 15

Other stratified cells are often found in…?

Answer 15

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.

Question 16

Where is transitional/urothelial cells found?

Answer 16

Found in the renal pelvis, ureters and bladder.

Question 17

Explain the attachments of epithelia:

Answer 17

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.

Question 18

Explain the apical surface elements of the epithelium?

Answer 18

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.

Question 19

What are the intercellular junctions between epithelial cells?

Answer 19

• zonular occludens [apical]
• zonular adherins
• Macula adherins (desmosome) [basement]

Question 20

What is a tight junction?

Answer 20

Tight Junctions seal intracellular spaces to block passage of substances between cells; they form a continuous circumferential band around the apex of cells.

Question 21

What is an adherens Junction?

Answer 21

Adhering junctions and desmosomes are mechanically strong attachments between cells that link the cytoskeletons of adjacent cells.

Question 22

What is a gap junction?

Answer 22

Gap junctions also known as nexus or communicating junctions allow passage of small molecules, allowing for communication between cells.

Question 23

What is a hemidesmosome?

Answer 23

These are modified desmosomes that link epithelial cells to the underlying basement membrane.

Question 24

What are Cell Adhesion Molecules (CAMs)?

Answer 24

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)

Question 25

Give the basic characteristics of the basement membrane?

Answer 25

The BM is the interface between support tissues and parenchymal cells (epithelia), mainly produced by the cells being supported.
There are three layers on EM, the components of which are the ECM (predominantly collagen IV, heparan sulphate & structural glycoproteins like laminins & fibronectin.
- Laminins and fibronectin are involved in linking integrins of epithelial cells to ECM.
It is also underlying collagen VII and reticulin; collagen VII involved in attachment of BM to underlying connective tissue.

Muscle cells, adipocytes and Schwann cells are surrounded by a similar layer of basal lamina which is usually too thin to see on the Light microscope. The function is to provide structural support, control epithelial growth, link epithelium to underlying tissue and act as a selective barrier to nutrients with underlying connective tissue.

Question 26

Explain the difference between Endocrine and Exocrine Glandular epithelium?

Answer 26

EXOCRINE
Secretes contents onto epithelial surface or duct as a single goblet cell, an invagination forming secretory structures or solid organs.

Exocrine organs secrete proteins, lipids or glycoproteins. There is serous secretion which is protein in aqueous medium such as in the pacreatic acini, salivary glands and lacrimal glands. There is also mucous secretion which is glycoprotein in aqueous medium such as in the respiratory tract, GIT and cervix.

ENDOCRINE
Secretes contents into blood as a single neuroendocrine cell or a solid organ.

Question 27

What are some of the varieties of exocrine gland structure?

Answer 27

Exocrine gland structures come in many varieties:

SIMPLE TUBULAR
Such as in the colon.

SIMPLE COILED
Such as in sweat glands.

SIMPLE BRANCHED TUBULAR
Such as in the stomach

Question 28

What are Goblet Cells?

Answer 28

These are mucous secreting cells / unicellular glands packed with membrane bound mucous droplets. The nucleus and organelles are displaced basally. Mucous is non-staining in H&E.

Question 29

What are serous acini?

Answer 29

Serious cells often form a secretory unit called an acinus. They secret into a common narrow lumen, which is then carried away by a duct. They are basophilic at base and acidophilic at the apex