2 - Cell Junctions and Epithelia

Question 1

How are cells organized into organs?

Answer 1

They differentiate into distinct types, which aggregate to form tissue, which forms an organ.

Paramount in this process are cell-cell and cell-matrix interactions.

Question 2

What are the four basic types of tissues?

Answer 2

Epithelial (60% of all body cells)

Connective
Muscle
Nerve

Question 3

Where is epithelial tissue found? What is it derived from?

Answer 3

The external and internal surface of organs and the external surface of organisms.

Derived from embryonic endoderm.

Question 4

What are the functions of epithelial tissue?

Answer 4

Protection, diffusion, absorption, secretion, excretion, and sensation.

Question 5

How does epithelial tissue differ from connective tissue?

Answer 5

Epithelial cells are considered polarized: apical, basal, and lateral surfaces.

Completely cellular: no intervening ECM or vasculature.

No vasculature: transportive materials used to diffuse materials across the cell.

All basal epithelium interacts with a layer of ECM called a basement membrane.

Question 6

Describe connective tissue?

Answer 6

Cells are not polarized.
Lots of ECM, sparsely populated with cells.
Cells rarely interact.

Question 7

What are the different types of epithelial cells?

Answer 7

Simple: single layer

Stratified: two of more layers in which only the basal layer interacts with the BM. (never ciliated)

Question 8

What is transitional epithelium?

Answer 8

Special case of stratified epithelium, apparent number of cells differs when the epithelium is stretched.

Found only in the bladder.

Relaxed bladder - appears to have more layers.

Question 9

What is pseudostratified epithelium?

Answer 9

Special type of simple epithelium: appears to have more than one layer, but all cells contact basement membrane.

Mainly in the lung.

Question 10

What are the main shapes of epithelial cells?

Answer 10

Squamous: flat and scale-like and well suited for diffusion

Cuboid: cube-shaped (int between squamous and columnar)

Columnar: rectangular shape, usually for high metabolic function/absorptive.

Question 11

How is stratified epithelium named?

Answer 11

By the shape of the cells in the most superficial (apical) layer.

Question 12

What is keratinized epithelium?

Answer 12

Outermost surface of stratified epithelium becomes covered in intermediate filament keratin rather than living cells.

They lose their nucleus and accumulate keratin during maturation.

Provides tough, water-resistant, non-living surface for protection.

Question 13

What is glycocalyx?

Answer 13

Refers to a carbohydrate-rich complex on the extracellular surface of epithelial cells, which allow for protection and recognition.

Question 14

Where are cilia typically found? What is their structure?

Answer 14

Respiratory tract where there’s a need to propel extracellular material through a tube.

Anchored to a basal body or centriole which have nine triplets and a dynein ATPase motor.

From that structure is the axoneme: which is a 9+2 structure with nine doublets with two single mts in the center.

Question 15

What are microvilli? How do these differ from cilia?

Answer 15

Small, finger-like projections of the PM, abundant in intestinal epithelium and renal epithelium.

They are elaborations of the apical surface, designed for increased SA and absorption.

Question 16

What is the structure of microvilli?

Answer 16

Actin microfilaments cross-linked by villin that connect to a terminal web made of actin, spectrin, and int filaments.

Question 17

What is the function of lateral cell membrane specialization?

Answer 17

Maintain coherence, communication, and distribution of stress.

Question 18

What is the first type of lateral cell membrane specialization? What can move through this type of junction?

Answer 18

Zona occludens or tight junctions.

Near apical surface, only water and small molecules can pass through.

Crucial for transcellular transport across polarized cells - regulates what can go between the cell.

Question 19

What do tight junctions look like under a microscope? What proteins are associated with tight junctions?

Answer 19

Dark-light-dark-light-dark appearance.

Occludin, claudins, and junctional adhesion molecules (JAM), which interact with join the PM together.

Claudins key to determine what can and can’t get between the cell.

Question 20

How do tight junction proteins interact with the plasma membrane?

Answer 20

Via plaques called zonula occludens (ZOs) aka tight junction proteins (TJP), which interact with actin.

Question 21

What is a second type of lateral cell membrane specialization? Where are these located?

Answer 21

Adheren junctions, located just beneath the zonula occludens and is an anchoring junction.

Question 22

How do adheren junctions link cells together?

Answer 22

By specialized transmembrane Ca-dependent proteins called cadherins.

Cadherins interact with plaques (adapter proteins) containing catenins, a-actinin, and vinculin which link to the actin cytoskeleton.

Question 23

What are some prominent members of the classical cadherin family found in adheren junctions? Where are each typically found?

Answer 23

E-cadherins: many epithelia

N-cadherins: neurons, heart, skeletal muscle, lens

P-cadherins: placenta, epidermis, breast

VE-cadherin: endothelial cells

Question 24

What is a third type of lateral cell membrane specialization?

Answer 24

Macula adherens or desmosomes.

These do not encircle the cell, but link cells via non-classical Ca++ dependent cadherin called desmogleins and desmocollins.

Question 25

What are the adhesion proteins (plaques) of desmosomes?

Answer 25

Desmoplakin, plakoglobin, and plakophilin and the intermediate filament protein called keratin.

Question 26

What is the fourth type of lateral cell membrane specialization? What are they made up of?

Answer 26

Gap junctions (nexus) which are communicating junctions.

Made of two connexons, one in each of the two adjoining cells. Each connexon has six subunits composed of identical connexin proteins. (therefore each connexon has 12 connexin molecules).

Question 27

What is the function gap junctions? What is the linker protein?

Answer 27

Function in cardiac and smooth muscle myocytes.

There is no plaque or cytoskeletal protein associated with connexons, connexins comprise the transmembrane linker protein.

Question 28

What is the function of basal membrane specializations?

Answer 28

Link the basal cell surface to the ECM of the basement membrane.

Question 29

What are the two types of basal membrane specializations?

Answer 29

Hemidesmosomes: half desmosomes that use integrins, which attach to laminin, collagen, and fibronectin of the ECM.

Adhesion plaques: also link to ECM via integrins; but, similar to zonula adherens in that they contain a-actinin and vinculin which attached to microfilaments.

Question 30

What does epithelial cell turnover rate depend on? Give two examples.

Answer 30

The location and function of the type of epithelial cell.

Skin cells turn over every 28 days, while small intestine cells turn over every 5 days.

Question 31

What is metaplasia? What is it often the first step in?

Answer 31

The change in epithelial cells to a biochemically and structurally different type.

Often first step in carcinogenesis.

Question 32

What are the names of malignant tumors of epithelial cells? What about epithelial glandular cells? How common are these cancers?

Answer 32

Epithelial cells: carcinomas

Glandular cells: adenocarcinomas

These cancers comprise 85% of all cancers.

Question 33

How can you tell where tumor cells originated? Describe the property of tumors that originated in the epithelium, glia, muscle, and connective tissue.

Answer 33

By staining the cells to detect the type of int filament proteins present.

Epithelial origin: keratin
Glial origin: glial fibrillary acidic protein
Muscle: desmin
Connective tissue: vimentin