1.1 Cell Adaptations
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Transcript of 1.1 Cell Adaptations
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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
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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
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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.