Tissues 2- epithelial tissues

Question 1

Describe how epithelial cells are categorised

Answer 1

Epithelia were first described and classified by histologists based on their microscopic appearance.

The two main criteria of epithelial classification are:

their shape
squamous (flattened plate-shape)
cuboidal
columnar

their layering
single layer = simple epithelium
multi-layered = stratified epithelium

This classification continues to be useful, as it is related to types of epithelial function.

Question 2

Describe the simple squamous epithelium, where it is found and its function

Answer 2

Simple = single cell layer; squamous= flattened shapee.g. lung alveolar (air sac) epithelium, mesothelium (lining major body cavities), endothelium lining blood vessels and other blood spaces); form a thin epithelium that allows exchange to occur (e.g. gas exchange in the alveoli)

Question 3

Describe the simple cuboidal epithelium, where it is found and its function

Answer 3

Simple = single cell layer; cuboidal = approx. cube-shapede.g. lining kidney collecting duct, and many others ducts

Question 4

Describe the simple columnar epithelia

Answer 4

Simple = single cell layer; columnar = pillar-shapede.g. enterocytes (intestinal absorptive), many other absorptive and secretory epithelia.

Question 5

Describe the stratified squamous epithelia and the two classes of it

Answer 5

Stratified = multiple cell layers; squamous = flattened shapeIn the various layers, cell shapes vary. The squamous classification relates to the surface cells.Two main types:(a) keratinising: epidermis (skin epithelium) (nuclei not visible in surface layer cells)(b) non-keratinising: linings of mouth, oesophagus, anus, cervix and vagina (nuclei are visible in surface layer cells)

Question 6

Describe the difference between keratinising and non-keratinising

Answer 6

Non-keratinising- upper surface is wet- cells are alive

Keratinising- upper surface is dry- cells are dead there.

Question 7

Describe pseudostratified epithelia

Answer 7

Pseudostratified = falsely stratifiedLooks multi-layered, but surface cells have contact with basal lamina -but some also have contact with the apexe.g. airway (trachea and bronchi) epithelium, various ducts in the urinary and reproductive tracts
Pseudostratified epithelia are often ciliated

Question 8

Describe the importance of the polarisation of epithelial cells, and describe simply how this is achieved

Answer 8

Most epithelial functions must be directional, e.g. secretion, fluid and solute transport, absorption etc.

Epithelial polarity is required to give directionality to epithelial function, i.e. one surface of the epithelium is different from the other.

Plasma membrane polarity is key to epithelial polarity

Junctions separate many epithelial membranes into two biochemically and functionally distinct domains:
the apical domain
the basolateral domain

Question 9

What is a consequence of epithelial cells not having polarity

Answer 9

Pumping of substances in all directions- no net flow, waste of energy. In ion and fluid transporting epithelia, ion pumps and channels have apical-basolateral polarisation in the plasma membrane, i.e. its activity is restricted to only some parts of the plasma membrane.

Question 10

What is the importance of the belt-junction in achieving membrane polarity

Answer 10

Belt-junctions can segregate the epithelial plasma membrane into apical and basolateral domains.
These domains have different lipid and protein composition, and have distinct functions.

Question 11

Describe epithelial polarity

Answer 11

epithelial layers have a distinct polarity, with an apical surface at the lumenal (open) surface, and a basal surface in contact with the extracellular matrix

the membrane between these two surfaces, where adjacent membranes oppose each other, is the lateral membrane

basal and lateral membranes are usually grouped as one membrane domain, the basolateral membrane

Question 12

Describe the polarisation of the organelles that is required to secrete products out of the apical membrane

Answer 12

Basal membrane- nucleus
RER
Golgi- with trans face facing the apical membrane

Question 13

What are the clos contacts between the two plasma membranes in tight junctions called

Answer 13

Kiss points

Question 14

What is the basic role of the epithelia

Answer 14

They create a barrier, which has the same importance as the plasma membrane (selective permeability, receptors to respond to the environment, protects the interior of the organism from invading microorganisms and fluid loss.

Question 15

Describe the experiments that show how tight junctions seal the paraceullular pathway

Answer 15

A tracer substance added to the apical face of the epithelium cannot penetrate between the cells; it is restricted by the apical tight junction.
TEM shows the restriction of passage of the tracer (black) when added apically.
When added to the basal aspect of the epithelium, the tracer is able to pass into the narrow intercellular space until it reaches the apical TJ, when it is prevented from passing to the apical region.

Question 16

How else can epithelial cells maintain polarity

Answer 16

Tight Junctions allow cells to establish and maintain apical- basolateral polarity by preventing the mixing of proteins and lipids between the different plasma membrane compartments.

Question 17

Describe the importance of tight junctions

Answer 17

This because the paracellular pathway (between cells) is sealed by tight junctions. This has two main outcomes:

Concentration differences across cell layers can be maintained

Any solutes crossing the cell layer need to pass through the cells, meaning that the cells control the passage. Strong barriers are particularly required when tissues need to transport against a large concentration gradient.

Question 18

Describe the difference between the gate function and fence function of tight junctions

Answer 18

act to seal paracellular pathways (i.e. limits the passage of fluids and solutes between cells) – gate function
segregates apical and basolateral membrane polarity – fence function

Question 19

Describe how cell junctions are dynamic

Answer 19

It should be noted that, although they provide coherence and mechanical stability to cells in tissues, cell-cell junctions are highly dynamic, and can be assembled and disassembled rapidly in response to extracellular and intracellular stimuli.

Question 20

Describe the characteristics of transport epithelia

Answer 20

In transporting epithelia, the plasma membranes contain high concentrations of ion transporters. Typically, mitochondria are closely associated with extensive basal membrane infoldings, providing energy for active transport across the abundant membranes. The infoldings increase the amount of basal membrane that can pump ions and water.

Question 21

Describe the three ways in which the SA of the epithelium lining for absorption can be increased

Answer 21

Make the tube longer
Fold the lining to form structures like villi in the small intestine.
Increase the surface area of the apical plasma membrane by creating tubular protrusions: microvilli
The more surface area of epithelium lining available for absorption, the more efficient it will be.

Question 22

Describe the characteristics of absorptive epithelia

Answer 22

Absorption of specific materials from the interior of a tissue tube, like the intestine or a kidney nephron, is a key epithelial function. Maximising the efficiency of absorption can involve increasing the surface areas of absorbing tissue, and increasing the surface area of the plasma membrane that carries the specific membrane transporters. A clear example of where this happens is in the small intestine, which absorbs digested nutrients.

The small intestine surface area is increased by the interior surface of the wall of the small intestine being folded into numerous finger-like processes that point into the interior: the villi (singular, villus). The villi are covered with intestinal epithelial cells.
The surface area for absorption is also increased by multiple plasma membrane projections: the microvilli. (TEM)

Question 23

Describe the histology of the intestinal villi

Answer 23

The intestinal epithelium is simple columnar, with both absorptive and secretory cells.
The darker epithelial cells are the enterocytes (absorptive) and the cells with pale apical cytoplasm are goblet cells (secrete mucus)
With this special stain, the goblet cells, which secrete mucus, stain dark purple.
Carriers transporting nutrients are found on the microvillous brush-border membranes, e.g. absorptive intestinal cells (enterocytes); kidney proximal tubule cells.

Question 24

Why do goblet cells appear pale under standard stain

Answer 24

They are empty cells, the mucus does not take up the stain.

Question 25

What are the two functions of the pancreas

Answer 25

The pancreas has both exocrine (into a duct or lumen) and endocrine (into the bloodstream) secretory functions

Question 26

What is zymogen in the acinar cells of the pancreas

Answer 26

Packets of enzyme.

Question 27

Describe the typical organisation of an epithelial cell that has an endocrine function

Answer 27

Secretes into capillary

Hence, nucleus is nearer to the apical membrane

Question 28

Describe how secretory epithelia may be organised into glands

Answer 28

In tissues whose main purpose is secretion, the epithelium is often arranged in tubules and glands of varying complexity.

Question 29

Describe stimulated secretion

Answer 29

Stimulated – secretory vesicles are stored in the cytoplasm and only fuse with the plasma membrane to release their contents, e.g. the release of adrenaline from cells of the adrenal medulla after a fight-or-flight stimulus (stimulated endocrine secretion); when stomach contents enter the duodenum, pancreatic acinar cells are stimulated to release their digestive enzymes into ducts (stimulated exocrine secretion)
Stored in vesicle until signal from external environment is received.

Question 30

What type of epithelia is used in protective epithelia and why is this important.

Answer 30

Protective epithelia are usually stratified-squamous (e.g. skin epidermis, lining of the oesophagus).

This type of epithelium can form thick layers that protect underlying tissues for various physical and chemical insults
e.g. heat, cold, solvents (alcohol), abrasion, etc.

Question 31

Describe the structure of the skin

Answer 31

The skin comprises three main layers: epidermis, dermis, and hypodermis.

The epidermis is the keratinising stratified squamous epithelial layer. Its main function is to act as a barrier to the environment.

However, some epidermal appendages have other functions (e.g. hair, nails).
Squamous surface cell layers
(protective surface)
Cuboidal/columnar basal cell layer
(stem cells for the renewal of upper layers)

Question 32

What is severe blistering caused by

Answer 32

This severe blistering, which also affected the mouth and anus, was the result of loss of function of desmosomes.
This person started making auto-antibodies to desmosomal cell-cell adhesion molecules, and these reduced desmosome adhesion to make the epidermis fragile.

Question 33

What does an abnormal pap smear show

Answer 33

Abnormality in shape and size of cells in keratinising layer

Question 34

Why is cell turnover important

Answer 34

A balance between cell proliferation and cell death is key to maintaining the architecture of tissues.
In most tissues, cells die and are replaced. The rate at which this happens varies considerably.
They need to be replaced as they are specialised cells and so cannot undergo mitosis- they are terminally differentiated.
In many epithelial tissues, cells turnover consistently, such that the whole cell population is replaced over relatively short periods.

Question 35

Describe self-renewal in the small intestine

Answer 35

The absorptive and secretory cells are single-layered- but the epithelium is continuous with the epithelium lining the crypts. the stem cells lie near the bottom of the crypts. The stem cells give rise to proliferating precursor cells which move upward in the plane of the epithelial sheet. As they move upward the precursor cells terminally differentiate into absorptive or secretory cells, which are shed into the gut lumen and die when they reach the tips of the villi.
If we mark a single new cell, we can follow its fate. As the marked cell migrates up the villus epithelium, new cells are constantly being produced by the crypt stem cells…..
to replace the cells constantly being lost from the villus tip.

Question 36

What happens when cell turnover becomes unbalanced

Answer 36

Disturbing the balance of cell proliferation/loss affects tissue dynamics: reducing cell proliferation.
Treating mice with the antimetabolite chemotherapy drug, 5-FU, has a profound effect on the architecture of the small intestinal villi. The drug slows down or stops cell division in the crypts. Cell loss from the villus tips continues as normal, but the failure to produce new cells to replace the cells results in a loss of tissue: the villi shorten.

This is the reason why chemotherapy can have strong gut side-effects.

Question 37

Describe how unbalanced cell turnover can increase cell proliferation

Answer 37

When there is too much cell proliferation, the rate of cell loss is not sufficient to maintain the normal tissue volume. In the colon, if a cell mutates such that its proliferation is dysregulated, too much tissue is formed. This is a benign tumour called an adenoma (polyp).

Although not malignant, these adenomas have a high risk of acquiring more mutations that switch it to cancer.

Question 38

Describe cell turnover in the epidermis

Answer 38

Surface cells are constantly being lost, but are replaced by new cells being formed in the basal layer which migrate up while undergoing a programme of differentiation that eventually leads to them flattening out and keratinising.

Each layer replaces the one above as the layers are lost from the surface.
Stem cells restricted to basal layer- travel outwards perpendicular to the plane of the sheet

Question 39

Describe how infectious agents can alter the rate of cell turnover in the epidermis

Answer 39

Most of us will have had a wart at some time. They are the result of infection of the epidermal keratinocytes by one of the family of human papilloma viruses. The virus hijacks the cell proliferation machinery and increases cell proliferation.

Question 40

Describe normal physiological responses that lead to a change in cell turnover

Answer 40

the cyclic production and loss of the endometrial epithelial lining of the uterus in the menstrual cycle.
the large increase, during pregnancy, in the number and size of the epithelial glands of the breast, and their loss after weaning (stopping breast feeding).

Question 41

Describe the importance of one stem cell giving rising to multiple differentiated progeny

Answer 41

Goblet cells
Absorptive cells
Secretory cells

Question 42

What regulates the proliferation of stem cells

Answer 42

Wnt proteins.