Epithelia I

Question 1

Where are epithelial layers found and what diseases are associated with them?

Answer 1

Epithelia are found on the surfaces of most critical organs and form the critical structures of the organs (liver, urogenital system, lungs, digestive tract, etc.) Cirrhosis of the liver, kidney failure, emphysema, most cancers are carcinomas- cancers of epithelia. They face all of the spaces in the body.

Question 2

What benefits do epithelial layers provide (6)?

Answer 2

Protection from damage and invasion from outside world.
Absorption of substances
Secretion/excretion of substances (Selective transport)
Biochemical modification -large scale (such as the liver)
Modified epithelia (such as the tongue) act in sensory perception
Communications (short range or long range) endocrine glands are epithelial cells that do not face towards spaces.

Question 3

What is the basic structure of most epithelial layers?

Answer 3

A series of cells that are tightly adherent to each other, with an apical side facing out to the lumen (or “space”) and a basolateral side facing away from the lumen. The cells re then attached to connective tissue, most commonly basal lamina.

Question 4

How vascular is epithelia and how does this affect nutrient and waste distribution?

Answer 4

Epithelia are avascular and rely on capillaries inside the basal lamina for nutrients. Nutrients from the blood and waste products or signaling molecules from the epithelia must cross the connective tissue to reach their targets.

Question 5

What are endothelia and where are they found?

Answer 5

Endothelial cells face the inside spaces of blood vessels (and possible others, not detailed). Endo = inside. Major exception is endocrine glands that do not face outside, but are still termed EPIthelium.

Question 6

What is the general arrangement of tissues near lumens in the body?

Answer 6

Where ever there is a space or lumen in the body, there is a layer of epithelial cells that attach through a basal lamina to connective tissue that has the other types of tissue - blood vessels, nerves, muscle.

Question 7

What is the EMT and what does it result in?

Answer 7

The Epithelial to Mesenchymal Transition occurs in the developing embryo when primordial epithelial cells differentiate and move out of epithelial layers. The mesenchymal cells can move and form new layers or invade other layers. Some of these mesenchymal cells form into the structures of the connective tissue underlying the epithelium.

Question 8

How is the EMT thought to be related to cancer?

Answer 8

A reactivation of the EMT pathways is thought to play a role in cancer development and metastasis.

Question 9

What is the lamina propria?

Answer 9

The lamina propria is the layer of connective tissue immediately deep to the outermost layer of epithelia, which together form the mucosa, which is superficial to the sub-mucosa. Common in the moist membranes that face the internal lumen (i.e., digestive tract)

Question 10

What roles do the mucosa and submucosa play?

Answer 10

Mucosa is cellularly rich, packed with immune cells and has a primary function in immune response and inflammation. The submucosa is a denser type of connective tissue and has a more supportive role, but contain epithelial glands.

Question 11

What is the general structure of the skin?

Answer 11

Epidermis = epithelium
Dermis = connective tissue (BV+epi+muscle+nerve)
Hypodermis = superficial fascia connective tissue (BV+epi+muscle+nerve)

Question 12

What sorts of layer structures do epithelial cells form?

Answer 12

Simple epithelia are a single sheet of cells
Stratified epithelia come in multiple layers (apical cells are detached from the basal lamina and attach to deeper epithelial cells instead, polarity is maintained on tissue-scale)
Pseudo-stratified appear to be multiple layers, but all attach to the same basal lamina and are a single layer
Transitional epithelia are stratified but stretched to appear to be a single layer (bladder is only example known by lecturer)

Question 13

What shapes to epithelial cells take?

Answer 13

Squamous - Flat (or at least wider than they are thick)
Cuboidal - Relatively equal height and width
Columnar - Tall and skinny

Question 14

How are epithelial cells categorized?

Answer 14

By the number of layers and by the shape of the cells in the outermost layer. Skin is stratified squamous, even though the inner cells may not be squamous.

Question 15

How do epithelial cells respond to environmental stress like smoking?

Answer 15

Normal linings of bronchi in the lungs are pseudo-stratified columnar. In smokers, these cells change to squamous, and this change affects their functions. Psudeo-stratified columnar are common in tissues that secrete lots of material such as mucus, while stratified squamous are designed for protection. Changes in morphology can help pathologists determine cancer types.

Question 16

What is cellular polarity?

Answer 16

Differences between the apical and basolateral faces of cells. These differences include membrane proteins found only in one side, such as proton pumps on the apical face in stomach lining. There is also polarity in the cytoplasm, including secretory pathways and cytoskeletal elements This polarity also means that transporters localize to one side or another, and disallow transport in the opposite direction simply through their absence on that membrane.

Question 17

What is transcytosis?

Answer 17

Membrane vesicle mediated movements through the cell. Allows for the endocytosis of a particle on the basal side, transport of that particle through the cell, and exocytosis on the apical face, or vis versa. The polarity of the cell assists in creating these transport mechanisms.

Question 18

What are tight junctions?

Answer 18

Tight junctions are important to polarity and orientation of cells. They are interlocking, transmembrane proteins that have cytosolic domains, transmembrane domains, and extracellular domains that interact with each other. (claudins) They force transcellular movement (through cells) and inhibit pericellular movement (between cells).

Question 19

How variable are tight junctions in different tissues or under different stimuli?

Answer 19

Different tight junctions have highly divergent permeabilities. Tight junctions in the bladder are impermeable to water, and are 10,000 times less permeable to Na+ than tight junctions in the small intestine. The presence of glucose in the lumen can also cause a relaxation of tight junctions that allows glucose (and other material) to flow down its concentration gradient pericellularly. This impacts the absorption of drugs with or without food.

Question 20

What creates specificity of transport in epithelia?

Answer 20

Tight junctions + cellular polarity = specificity of transport. Anything that loosens the tight junctions can increase non-specific transport of molecules pericellularly down their concentration gradients.