Week 5: Epithelium 1 - Classification of Epithelium

Question 1

What is surface epithelium? What is its role?

Answer 1

• Cells form sheets that cover all natural surfaces and lines all natural cavities of the body
• Protective, barrier layer
• Interface between two environments: selectively controls passage of substances (secretion, absorption, bidirectional transportation)

Question 2

What is glandular epithelium? Where is it found?

Answer 2

• Secretory epithelium: cells produce a secretion
• Occurs as single cells in surface epithelia, clusters of cells (acini) located deep to surface epithelium within CT, or discrete glandular organs (exocrine and endocrine glands)

Question 3

What does cell polarity refer to?

Answer 3

Asymmetrical internal organisation, defined surfaces and membrane domains, and specialised function to different intra cellular locations

Question 4

Give an example of how epithelial cells exhibit cell polarity.

Answer 4

Epithelial cells form layers through cell-to-cell adhesion, with adhering junctions, tight (sealing/ communicating junctions). They also have a basal lamina, the layer at the base/ boundary, which induces polarity. Apical, lateral and basal surfaces of epithelial cells have different structure and functions. Cytoplasm within a cell has several defined areas of different organelle composition where different particular functions dominate.

Question 5

Where is a basal lamina found?

Answer 5

A basal lamina always lies at the interface between epithelial cells and adjacent CT. It is also present between muscle cells and CT, and nervous tissue and CT.

Question 6

What is the structure of the basal lamina?

Answer 6

20-100nm thick and is comprised of fine granular and fibrous proteins and glycans, e.g., collagen type IV, laminin (large glycoprotein), nidogen (short, rod-like protein), perlecan (proteoglycan linker) and anchoring fibrils (type VII collagen).

Type IV collagen, laminin, nidogen and perlecan secreted by basal surfaces of epithelial cells. Underlying reticular lamina and anchoring fibrils synthesised and secreted by cells of CT.

Question 7

What is meant by ‘basement membrane’? What is its functions?

Answer 7

basement membrane = basal lamina + reticular lamina (or 2 fused basal laminae).
Functions include filtering, creating boundaries, supporting epithelium, and attaching it to underlying CT

Question 8

What is the appearance of surface epithelia related to?

Answer 8

Different appearances of surface epithelia reflect exposure to different environments and hence different functional demands. Characteristics of epithelial cells are correlated with the cell’s contribution to the overall function of the epithelium.

Question 9

In what ways can a surface epithelia vary?

Answer 9

• number of layers
• shape of cells in contact with free (apical) surface
• presence of surface ‘modifications’ to cells in the apical cell layer

Question 10

How can epithelium be classified according to number of layers?

Answer 10

Stratified (more than one) or simple (single)

Question 11

How does number of layers relate to classification of epithelia?

Answer 11

simple: 1 layer
stratified: more than 1 layer
pseudostratified: looks like more than 1 layer, actually is 1 layer (all cells in contact with basement membrane)

Question 12

How does shape of cells in contact with free (apical) surface relate to classification of epithelia?

Answer 12

squamous: flat and long cells
cuboidal: about as long as they are wide
columnar: tall cells

Question 13

What (apical) surface modifications can be present on epithelia?

Answer 13

O the apical cell layer of surface epithelium, there may be:
- cilia
- microvilli
- stereocilia
- keratinisation

Question 14

What is a typical function of simple surface epithelium?

Answer 14

Facilitates (selective) transfer between environments, i.e., absorption, secretion, transport

Question 15

What is a typical function of simple cuboidal or columnar surface epithelium?

Answer 15

Facilitate and support absorptive and secretory functions. The larger cells (compared to squamous) allows for room for organelles.

Question 16

What is a typical function of simple squamous surface epithelium?

Answer 16

Minimal distance to facilitate diffusion, resulting in rapid exchange.

Question 17

What is a typical function of stratified surface epithelium? What type of cell is usually seen for this?

Answer 17

Protective barrier function
Most stratified epithelia have squamous cells a they create a better barrier (more layers) for less overall epithelial thickness.

Question 18

What does multiple cell types within an epithelium indicate?

Answer 18

More than one cell type in an epithelium indicates that the epithelium has more than one function, e.g., it may have both absorptive and secretory functions.

Question 19

What is transitional epithelium? Where is it found?

Answer 19

Transitional epithelium is a layer of cells that forms the mucosal lining of your ureters, a portion of your urethra, and your urinary bladder. These cells are called transitional because they can undergo a change in their shape and structure.