8/6/2019 1.1 Cell Adaptations

1/3

1

1.1 Cell Adaptations

Types of Body Tissues

1. Labile tissues

Cells of these tissues undergo continuous

turnover, where the mature cells are lost

and replenished by maturation of stem

cells or proliferation of other mature

cells.

Examples of such tissues include the

bone marrow and epithelial surfaces

(GIT, renal tubules, skin, mucous

surfaces, lining of glands, etc.)

2. Stable tissues

Cells of these tissues are quiescent (i.e.

have limited proliferative activity). When

there is a growth stimulus (e.g. injury tothe tissue), however, they have the ability

to proliferate.

These tissues include the liver (e.g. when

part of it is resected), endothelial cells,

fibroblasts, and smooth muscle cells (thelatter three important in wound healing.)

3. Permanent tissues

Mature cells of these tissues cannot

proliferate and there are no sufficientstem cells to replenish dead cells.

Examples are striated muscle cells and

neurons. If these tissues are injured, they

will heal by scarring.

Cell Adaptations

When a cell encounters a new type of

stress or demand, it will try to readjust

itself in order to adapt to that stress while

also maintaining its normal function. This

process will work as long as the stress can

be endured by the cell. The changes

which a cell can undergo when there is

stress are called adaptation and they are

of four types:

1. Hypertrophy2. Hyperplasia3. Atrophy4. Metaplasia

1. Hypertrophy

Hypertrophy is the increase in the size of

cells of an organ leading to an increase in

the size of the organ itself. The cells

increase in size by increasing their

structural proteins and their organelles.

The stimulus for cells to undergo

hypertrophy can be considered

physiological (as in the case of increase in

muscle cell size in weight-lifters) or

pathological (as in the case of increase in

cardiac muscle cell size in hypertension

or when there is valve stenosis).

If the stress persists , hypertrophy may

not be able to keep compensating for it.

Taking the heart as an example, if the

hypertension persists (and worsens), the

myocardium will start undergoing

degenerative changes causing ventricular

dilation. This can be explained by

different causes such as:

inability of blood vessels tosupply the demands of this

enlarging heart

8/6/2019 1.1 Cell Adaptations

2/3

2

inability of mitochondria toprovide adequate ATP

no more increase in myofilamentsThus, long-term stress can cause a switch

from adaptation to actual cell injury.

2. Hyperplasia

Hyperplasia is an increase in the number

of cells of a tissue. This process can be

considered normal as in the case of:

1. proliferation of the duct system inthe female breast at puberty

2. regeneration of liver tissue after apart of it is resected

Hyperplasia can be considered abnormal

in cases like:

1. endometrial hyperplasia due toexcessive stimulation by estrogen

(when there is no enough

progesterone to oppose it)

2. skin warts caused by humanpapillomavirus (which allows the

cells to go through the cell cycle,

thus divide)

Hyperplasia thus needs a stimulus tokeep going. If the stimulus is removed,

hyperplasia will stop. This is the main

difference between hyperplasia and a

benign tumor (which doesnt need a

stimulus to keep growing).

3. Atrophy

Atrophy is a decrease in the size of a cell.

This is done in an effort by the cell to

cope with situations like inadequate

blood supply, undernutrition, disuse (in

fractures for example), denervation

(because this leads to disuse), loss of

hormonal stimulation , or aging (senile

atophy).

This decrease in the size of the cell at the

molecular level is due to a decrease in

protein synthesis (because metabolism is

slowed down) and an increase in protein

degradation.

How to destroy a protein (do not try this

at home):

1. Activate ubiquitin ligases bydecreasing your nutrition or by

disuse of a tissue

2. Allow ubiquitin ligases to gocleave and activate ubiquitin

monomers

3. Tag the protein you want todestroy with ubiquitins (so it is

poly-ubiquitinated)

4. Allow the hungry proteasome torecognize the ubiquitinated

protein and break it down topieces (i.e. amino acids) in its core

4. Metaplasia

Metaplasia refers to the replacement ofmature epithelial cells by another type of

mature epithelial cells.

Normally the respiratory epithelium is

composed of ciliated cells. Because

tobacco smoke contains a lot of injuriousagents, the ciliated cells cannot easily

8/6/2019 1.1 Cell Adaptations

3/3

3

tolerate this stress so they will be

replaced by stratified squamous cells

which are more resistant to stress.

This transformation occurs by

reprogramming the genes of the local

stem cells, which then divide and mature

into squamous cells to take the place of

the ciliated cells.

While better resistance is achieved with

squamous cells in this case, the squamous

cells cannot secrete mucin nor do they

have cilia to propel materials out of the

respiratory tract. Thus there is loss of

important function.

Another example is in gastroesophageal

reflux disease (GERD). When there is

persistent or excessive irritation of the

lower esophagus by acid reflux from the

stomach, the normal squamous lining of

the esophagus will be replaced by

intestinal columnar cells which can resist

the acid better. This in the long-term can

cause adenocarcinoma of the lower

esophagus.

Metaplasia can also occur in

mesenchymal tissues (like bone forming

at site of injury) but are less common and

are considered to be less important as an

adaptive response.