Necrosis vs. Apoptosis – An overview
Necrosis
Cell death takes place due to either of the two distinct phenomena - necrosis and apoptosis. These phenomena are collectively called cell death which may be accidental or programmed. Necrosis is death by accidental and unexpected cell damage and several physical damaging events can cause necrosis like exposure to toxins, excessive heat, trauma, radiation effects, lack of oxygen and a block in the normal flow of blood (Jacobson et al, 2002). Necrosis type of damage disrupts the cell organisation and contents and causes faster cell death than most other methods. As cell death proceeds, necrotic cells swell and holes and gaps begin to be seen on the plasma membrane. Intracellular materials begin spilling out of these holes when cell death occurs and there is a total disruption of the workings of the cell.
Normally the levels of Calcium and Ca2+ intracellular concentration are
less than 10-7m whereas outside the cell it is generally ~ 1mM. High
levels of calcium are found within the mitochondria and endoplasmic
reticulum. When cell death or damage occurs, there is a rapid and
sudden increase in the intracellular Ca2+ concentration. cell damage
seems to draw out calcium to a great extent. Calcium is considered as
an allosteric effector of proteins and changes protein organisation and
activity of the cell. The release of this excess Calcium in the
intracellular environment seems to generate many toxic chemicals within
the cell environment and also induces harmful enzymes to activate the
degradation and degeneration of cell structures and molecules. A
release of excess calcium seems to be the initiating phenomenon leading
to a host of changes within the cellular environment from activating
enzymes and chemicals too degrading cellular structures.
With the initiation of cell breakdown, there is a subsequent breakdown
of chemicals and new breakdown products are formed that are released
into the cell environment (Potten and Wilson, 2004). The disassembling
is accompanied by the break down of cellular substances to form certain
new products such as phospholipids, such as arachidonic acid, free
fatty acid or FFA. As son as these breakdown products are released into
the intracellular environment, the neighbouring tissues and cells pick
up signals of tissue damage and react to defend themselves from the
process. The damaged and dead cells spew out calcium and in the process
generate arachidonic acid and free fatty acids. arachidonic aicd acts
as a substrate for several generating enzymes like the cyclo-oxygenases
which form prostraglandins, and eicosanoids mediating inflammatory
responses. This series of cell responses from the excess of calcium to
generation of arachidonic acid and enzymes and eicosanoids leads to
chronic inflammation in the body tissue. Medical and mutation studies
have pointed to the fact that the formation of eicosanoids is major
reason for chronic inflammation so a way to stop the necrosis and
chronic inflammation can be by stopping or inhibiting the production of
enzymes generating these eicosanoids. With cell damage,
vasoconstriction occurs which tends to stop the overall normal blood
flow and the breakdown products that are released like the fatty acids
also cause the capillaries to dilate. As this happens, the local blood
flow within tissues seems to increase and this is accompanied by tissue
redness and the release of histamines leading to stimulation of pain
sensing neurons. As this happens, capillary permeability increases,
there is an increase in the number of leukocytes and macrophages, white
blood cells that come from the circulatory system to the damaged area
and water from blood enters the tissues causing a swelling,
inflammation or oedema (Gibson, 2004).
The capillaries within the tissues break and these are sealed from
excess blood loss by forming a clot. These blood clots are formed by
catalysed proteins that precipitates based on proteolysis. All
bacteria, cellular remains, debris and other foreign media are engulfed
and digested by the white blood cells. As a result of the presence of
white blood cells in large amounts, the immune system gets activated
and the area is properly sterilised. pus formation occurs when white
blood cells fail to be adequately active and die leading to the damage
of the tissues in the region. A drying up of pus and the tissue wound
in general suggests that the area has begun to heal and there is
general tissue re-growth and healing process under way.
Chronic inflammation has to be treated immediately otherwise there can
be complete and irreparable damage of the cells in the inflamed area
and tissues will fail to regenerate. One of the anti-inflammatory drugs
given has cyclooxygenase inhibitors and these are found in the market
popularly under the names of ibuprofen and aspirin.
Apoptosis
From a discussion of the general tissue damage as it happens in
necrosis and the body defences that come into play, we turn to
apoptosis another method of cell death and damage that is different
from necrosis in certain fundamental features. Apoptosis has been
originally described in plants and refers to the process of removing
unwanted cells. The falling of leaves can be considered a sort of
Apoptosis although it now means a wider range of cell deaths and
include all non-traumatic and natural cellular death and damage
(Wyllie, 1997). Thus whereas Necrosis refers to cell death and damage
due to unnatural means, accidents and exposure to harmful substances of
events, apoptosis is natural programmed cell death. Apoptosis is the
method by which unwanted cells are removed from an organism. During
this type of cell death, there is no release of calcium or such
substances into the intracellular or extra-cellular environments, there
is no rupture of cell membrane and no inflammation occurs. The
neighbouring cells engulf and destroy the apoptotic dead cells and they
are removed without causing much pain or botheration to the body.
Programmed cell death or apoptosis is the normal process of cell death.
It is necessary in human body as dead cells are constantly replaced by
new ones. Death and replacement of cells is a normal and necessary
process in every organism and this process is programmed within the
body and happens naturally at regular intervals (Gray, 2003). There is
no eternal reason for this and cell death seems to be a natural body
mechanism for regeneration of the body cells. 50% of neurons are
regenerated and replaced during the development of the human vertebral
system. During the formation of the foetus or metamorphosis is many
insects and animals as in frogs or moth happen due to death of old
cells and formation of new ones. This cell death and subsequent cell
formation is caused by the natural process of cell death by apoptosis
programmed within every living system.
Usually signals are transmitted for cell death and normal cells die
after a period of time on receiving these signals. Entering a stage of
apoptosis may be a very normal routine for most cells which die to be
replaced by newer cells although cells can also enter apoptosis due to
viral infection, DNA damage or cellular stress. The cell death is
mediated by protein molecules that destroy all aberrant and deviant
cells such as the cancer cells. The cell death of cancer cells are
generally inactivated by mutations and by virtue of stopping cell death
of aberrant cells by mutations, cancer cells can multiply fast with
aberrant DNA (Lomo, 1998). This can be stopped with an apoptotic
pathway and leads to cell death. The apoptotically dying cells can
activate a group of potentially degradative enzymes also called the
caspases and these enzymes mediate controlled disassembly and
degeneration of the concerned cell.
Quite like the eukaryotic cells, the single celled bacteria are also
capable of apoptotic cell death Apoptotic mechanisms seems to have some
important functions for the bacterial cell. Bacterial cells generate an
addiction module with a stable toxin and an unstable anti-toxin. When
there is a block in the protein synthesis anti-toxin synthesis and
toxin synthesis are both stopped. The unstable anti-toxins disappear
slowly although the stable toxins are retained. With the gradual
disappearance of the unstable anti-toxins along with the persistence of
toxins, there is a complete shortage of anti-toxins. In this condition,
the toxin takes control, becomes active and kills the cell. Bacterial
programmed cell death thus seems to happen very much due to an
imbalance in toxins and anti-toxins in the cell.
Differences between Apoptosis and Necrosis
Necrosis is uncontrolled unnatural cell death; it is not
pre-programmed or programmed by the body in any way and has some
distinguishing features:
1. It shows swelling, sometimes chronic inflammation,
2. there is considerable damage to the mitochondria and endoplasmic reticulum of the cells
3. there is a breakdown of homeostasis in the body
4. there is a rupture of the cell membranes, lyses leading to release
of intercellular substances in the intracellular regions of the cell
5. formation of certain breakdown products and enzymes and subsequent
formation of eicosanoids lead to oedema, and damage and death of cells.
6. In most cases, the formation of certain enzymes and release of free
fatty acids send signals to surrounding cells and these are then able
to defend themselves from damage
7. in case of neuronal necrosis sometimes, neurotransmitters are
released and this can have excitatory properties that can cause
excito-toxic injury to neighbouring cells.
Apoptosis on the other hand is controlled, programmed cell death and
keeps all intracellular substances of the dying cell separate and
sequestered so that one malfunction or cessation of function does not
affect the other and unlike necrosis, in apoptosis one cell damage does
not cause damage of surrounding tissues. The distinguishing features of
apoptosis are:
1. There are many cellular changes that seem to happen in a internally regulated manner
2. the cell undergoing apoptosis shrinks in size, gets sequestered from
surrounding cells and loses its connections with the surrounding
intracellular matrix.
3. The cell displays intracellular proteins on its surface, the
chromatin of the nucleus begins condensing and the DNA breaks into
smaller fragments with several base pairs and lead to DNA laddering
4. the plasma membrane puffs up like bubbles and small bodies attached
to the membrane break off carrying intracellular material like nuclear
matter and cellular organelles. These organelles however remained
unaffected by the process
5. the fragments of cellular material and cells which break off and
disintegrate from the surrounding matrix are called apoptotic cells and
these are quickly removed and destroyed by phagocytosis
6. If the removal of these apoptotic cells do not occur fast, there can
be a process called secondary necrosis that results in the breakdown of
intracellular organelles and plasma membrane causing lyses of the
fragments.
Thus apoptosis and necrosis have different methods of cell death and
completely different mechanisms, end results and physical outcomes. The
differences are marked and we would take our discussion further on
research studies emphasizing these two modes of cell death as well the
distinct and different findings involved.
According to our preceding discussion, cellular mechanisms have two kinds of death responses
Necrosis - indicates an unnatural and pathological response of cell to
external causes leading to cell damage and injury where the chromatin
lumps up and rupture and lyses of the plasma membrane is followed by
the swelling and rupture of mitochondria and cell contents spill out of
the cell causing inflammation, oedema and enzyme reaction and a general
overall tissue damage is triggered.
In Apoptosis however the chromatin condenses and migrates towards outer
regions of the cell. The cytoplasm also sinks and the plasma membrane
bubbles out and separate nodules are formed containing cellular
organelles (Lavin and Watters, 1993). These separate nodules break off
at a certain point and are engulfed by neighbouring phagocytic cells.
There is no spillage, rupture of cell membrane or inflammation.
Apoptosis represents the body's natural way of facing cell death so no
negative signals or damage of whole tissue areas is seen. Here cell
death occurs not due to toxins or unnatural means but simply due to a
lack of receiving survival signals. Necrosis represents unnatural cell
death with inflammation. Apoptosis represents natural cell death
without any inflammatory response.
There are other processes by which cells are destroyed. One of them is
autophagy. This occurs when there are inadequate nutrients within the
cell and certain organelles within the cell body have to be used up for
reuse of the components within the organelle. Double membranes form
within the cell, the material marked is engulfed and an autophagosome
is formed which fuses with the lysosome and the hydrolytic enzymes then
degrade the materials.
Research Evidence on Cell Death
In a study on cell death, Majno and Joris (1995) reviewed the
historical development of cell death and traced the origin of terms
necrosis, coagulation necrosis, autolysis, physiological cell death and
programmed cell death as also chromatolysis, karyorhexis, karyolysis
and cell suicide. According to the authors there are three forms of
cell death, by lysosomes, free radicals and genetic mechanism as in
apoptosis. In contrast to blebbing and zeiosis typical features of
apoptosis include budding, and the typical feature of necrosis, i.e.
inflammation is also discussed. Cell death is categorised into two
major divisions in this paper, either programmed cell death as in
apoptosis or accidental cell death as in necrosis.
According to the authors, necrosis however is an incorrect term for
cell death as it can indicate changes secondary to cell death which
occurs only by apoptosis. So according to Majno and Joris, whereas
apoptosis indicates primary cell death, necrosis refers to associated
and secondary changes that follow apoptosis. Some suggest this as
secondary necrosis which sees necrosis not as a different kind of cell
death but a category only secondary to the main kind of cell death,
namely apoptosis. One type of accidental cell death highlighted by
Majno and Joris is Ischemic Cell death which is a category of its own
caused mainly by a failure of ionic pumps of the plasma membrane.
Ischemic cell death is accompanied by swelling and thus it is not
called apoptosis but oncosis, derived from onkos, means ‘to swell’.
Oncosis which is always accompanied by some form of swelling of tissue
structure leads further to necrosis with karyolysis and this is in
contrast to apoptosis leading to necrosis with karyohexis and cell
shrinkage.
Afford and Randhawa (2000) discuss Apoptosis in greater detail.
According to them apoptosis is a genetically related form of cell death
and permits the safe disposal of cells at a point of time when their
use to the body is over and the period for which they lived was long
enough for them to complete their complete intended biological
function. Apoptosis is seen in plant as well as animal tissues. It is
considered as one of the vitally important processes and a normal
process in development. Unlike necrosis which is pathological,
apoptosis is an important part of normal development and adult life of
many living organisms. Within the human body any dysregulation or
malfunction of the apoptotic mechanism can result in inflammatory,
malignant, autoimmune and neurodegenerative diseases and can completely
damage tissues and also the entire body. Along with these possible
damaging effects, viruses and other infectious agents can exploit
cellular apoptosis and host and invade the immune system. Afford and
Randhawa's study give a brief description of some landmark discoveries
in apoptosis research and covers the morphological and biochemical
aspects of apoptosis and also discuss the implications of therapeutic
intervention in treatments of diseases associated with apoptosis. The
period of cell death or at the stage of apoptosis the tissues are
vulnerable to certain infections and this can cause irreparable damage
to the tissue structures of the system.
Franko et al (2000) have worked on the mechanism of apoptosis in some
detail and noted future directions in apoptosis research. The rapidly
developing research paradigms on cell death and the associated
disorders has seen considerable progress in the last few decades and
especially in the last five years this are of study has reached new
horizons in medical research. According to studies by Franko and his
colleagues, identification of different kinds of morphological and
signalling aspects as well as the variances in requirement for energy
aided them to construct a theory of three kinds of cell death mainly -
apoptosis, necrosis and lysosomal cell death.
Oncoprotiens of the Bcl-2 family, mitochondria and certain catabolic
enzymes that participate in the process of cell death serve as targets
for pharmacological manipulation. In oncology, chronic inflammation and
in ischemic, neurodegenerative and autoimmune disorders, the up
regulation or down regulation of programmed cell death/ apoptosis is
usually implicated. The study by Franko and his colleagues is an
overview of genesis and development of theories on programmed cell
death and apoptosis and follows the basic theoretical approach of three
kinds of cell death as in apoptosis, necrosis and lysosomal cell death.
Along with summarising the basic facts of apoptotic mechanisms, they
also draw on the implications of their theoretical approach in medicine
and surgery. Such categorisation as specified by them seems to be
advantageous for medicinal, research and surgical purposes.
Laying emphasis on another aspect of apoptosis, Hashimoto (1997)
mentioned that apoptosis is a form of cell death which is not only
natural and necessary but also responsible for development and
homeostasis of living bodies. Hashimoto relates apoptosis to induction
factors and discusses the molecular mechanisms of induction and its
relation to various diseases like cancer. The relation of apoptosis and
necrosis to tumour growth and cancer is an important point which we
will be discussing in later section in some detail.
Fadeel et al (1999) have identified the role of apoptosis in the
genesis of diseases and emphasize that naturally occurring cell death
or apoptosis is necessary for the maintenance of tissue homeostasis and
helps in the removal of extraneous, unnecessary and dangerous cells in
a very swift and unobtrusive manner. They pointed out that apoptosis
has a significant role in a number of human diseases and pathological
conditions. Dysregulation and a malfunction of apoptosis have been
related to autoimmune diseases, acquired immune deficiency syndrome,
and many viral and bacterial infections, as also neurodegenerative
diseases and disorders like cancer.
When naturally damaged and unnecessary cells replicate with faulty DNA
and do not get removed naturally, they can lead to cancerous cell
growth implying defective apoptotic systems within the body and
possibly a faulty immune system. Also a dysregulated apoptotic process
can impinge on age related disorders like osteoporosis, atherosclerosis
and the process of aging itself. Fadeel and his colleagues give an
overview of human diseases associated with a defective or inadvertent
apoptosis and in their analysis they give examples of pathological
conditions in which putative apoptosis defects have been described
successfully at the molecular level. As a recommendation they suggest
some novel apoptosis modulating and regulating therapeutic strategies
which according to them can promote an apoptotic process free from
dysfunction or dysregulation.
Reiterating on the significant role of apoptosis in maintaining tissue
homeostasis and proper working of an organism in general, Hetts (1998)
indicate that the death of cells in tissues of humans and other
multi-cellular organisms in neither always abnormal nor always
detrimental ass in apoptosis cell death is completely normal,
necessary, and regulated by an internal mechanism. Hetts describes this
point further and writes that although necrosis ensues at sites of
massive cellular injury, most body cells die through a subtle, non-
inflammatory and energy-dependent form of cell death by the method of
apoptosis. So the more common form of cell death is apoptosis as it is
the most natural and painless and necessarily occurring phenomenon in
the bodies of living organisms. Hetts claims that the number of cells
in tissues of a living organism’s body is determined by the homeostatic
balance between proliferation of new cells and death of old cells and
the rates of proliferations and cell death by apoptosis vary widely
form one tissue to another. According to Hetts, recently conducted
research delving into the molecular mechanisms of apoptosis has
revealed that apoptosis is a genetically programmed cellular death and
removal process that can become dysfunctional and deranged when
components of the cellular apoptotic machinery are mutated or are
present in inappropriate quantities leading to a lack or insufficiency
in timely death signals for the cells. Although, cell death is in some
cases harmful as seen in necrosis, it is a necessary process of cell
growth and development of an organisms as seen in apoptosis and when
there is a dysfunction or dysregulation of apoptosis, a pathogenesis
occurs that may be associated with a wide variety of diseases such as
cancer, neuro-degeneration, autoimmunity, heart diseases and other such
disorders. There are many genes and gene products which are involved in
the regulation and execution of apoptosis and these genes are
potentially excellent targets and elements for diagnosis and
therapeutic intervention when diseases occur and identification of
these genes are helpful as targets as they offer renewed hopes for
cures and treatments of various types of diseases. Thus understanding
the mechanism of certain neuro-degenerative and cancerous diseases can
help identify the underlying gene processes involved and the treatments
that could be developed as a result of such identification of the
disease causing genes.
Giving an account of nuclear apoptotic death, Martelli et al (2001)
suggests that apoptosis is a form of active cell death and is essential
for morphogenesis, development and cell differentiation of all
multi-cellular organisms. Taking a rather evolutionary approach in
their explanation of their study, Martelli and his colleagues claim
that the activation of genetically controlled pathways have been
conserved in evolutionary history in the characteristic morphological
features of all multicultural living organisms and these features are
usually evident in the nucleus of a cell body. Within the cell, certain
mechanisms are seen common in all living organisms associated with the
process of apoptosis. These include chromatin condensation, nuclear
shrinkage and formation of nodules, budding of the membrane and
formation of apoptotic bodies with their cell organelles. these
morphological changes within the cell structure that are seen in
apoptosis occur due to molecular alterations as in DNA and RNA
cleavage, post transitional modifications of nuclear proteins and
proteolysis of polypeptides which reside in the nucleus. Studies in the
last five years has shed considerable new evidence on the workings of
apoptosis and changed our understanding of this programmed nature of
cell death considerably. Martelli et al claim although the mechanism of
apoptosis itself has been made considerably clear in recent times due
to increased research and evidence, the mechanism that lead to
apoptotic changes within the nucleus is only particularly understood or
clarified. Their study tries to address this issue and tries to delve
deeper into why the nucleus shows the modifications that it dies during
apoptosis. The authors also discuss those apoptotic events that act as
a trigger for the generation of auto antibodies to the nuclear
components.
Apoptosis can not only occur in isolation within a human body but can
be associated with necrosis and there can be a cell death mode switch
from the unnatural death of cells as in necrosis to the natural
cellular deaths as in apoptosis. Ueda and Fujita (2004) report such a
case in their study pointing out that in brain ischemia cell
destructive necrosis occurs in the core of the brain tissue which in
turn links to cell death that tends to expand to the vicinity.
Thus necrosis occurs at the centre, the main region of cell damage but
cell damage expands and reaches the vicinity, the regions surrounding
the core also known as the penumbra and thus apoptosis starts off
several days after the initial necrosis. The authors further describe
the process where cells showing apoptosis disappear by microglial
phagocytosis in the brain, that is they are removed and digested by
neighbouring cells and as this happens the cell death which has been
induced by ischemia and brain ischemic stress is eventually terminated.
This according to the authors is a self protective mechanism in the
brain and their hypothesis is that a cell death mode switch in brain
ischemia when necrosis at the core is slowly replaced by apoptosis at
the periphery is an in vivo self protective mechanism. The authors
review the current understanding of the molecular mechanisms of both
necrosis and apoptosis in relation to the ASTP hypothesis and introduce
novel mechanisms by which they could explain the in vitro cell death
mode switch. A cell death mode switch from necrosis or unnatural death
of cells in the brain representing cell damage and inflammation to
apoptosis or natural cell death due to a change in signals received by
the brain represents a regulatory mechanism in the tissues that is
capable of changing a potentially destructive process into a
constructive and defensive body mechanism. This way the natural change
from necrosis to apoptosis is represented as a self protective
mechanism.
Stadelmann and Lassmann (2000) have studied extensively on the methods
involved in the detection of apoptosis. During the last five years or
so the detection of apoptosis has evolved from identifying the
predominantly morphological basis to using the more specific techniques
to understand its workings. The methods which are used widely to
visualize DNA fragmentation in tissue sections are now supplemented by
information on specific cell death pathways and the components
involved. According to the authors the essential requirements for
successful detection of apoptosis include detection techniques
considering high sensitivity of apoptotic cells, the ability to
differentiate between apoptotic and necrotic cell death and other
different forms of DNA damage as also the detection of different stages
of the cellular death process. The recent technical advance in
apoptosis detection covers improvement in DNA fragmentation techniques
and also the new tools that are available for detection of apoptotic
cells in the tissues. Sgonc and Wick (1994) detailed other methods of
detection of apoptosis and suggested that apoptosis is central to many
basic clinically oriented investigations and according to them the more
frequently utilized methods for detection of apoptotic cells include
the study of morphology, analysis of DNA degradation, DNA end labelling
techniques, flow cytometric analysis and nuclease assays.
Apoptosis and necrosis have also been reported in fetus cell death due
to intake of alcohol by mother during pregnancy. Maternal drinking
leads to severe damages in the fetus and the potential mechanisms
through which damage can occur are 'increased oxidative stress, damage
to the mitochondria, interference with the activity of growth factors,
effects on glial cells, impaired development and function of chemical
messenger systems involved in neuronal communication, changes in the
transport and uptake of the sugar glucose, effects on cell adhesion,
and changes in the regulation of gene activity during development'
(Goodlet and Horn, 2001).
As a result of the drinking, there are adverse effects in the offspring
including cognitive impairment, growth deficiency, and CNS disorders.
These associated changes happen because of inadequate development of
the fetus due to cell death and damage within the fetal structures.
This may be due to increase of toxins in the fetal blood.
Another factor that can be potentially dangerous to cells and cause
cell death and damage is exposure to magnetic fields. A study by Lai
and Singh (2004), found that acute magnetic field exposure increased
apoptosis and necrosis of brain cells in the rat. The authors
hypothesized that exposure to a 60-Hz magnetic field initiates an
iron-mediated process or a reaction that increases free radical
formation in brain cells, leading to DNA strand breaks and cell death.
The authors suggest that their hypothesis could have important
implications for health effects that are associated with exposure to
extremely low-frequency magnetic fields in the public as well as
occupational environments.
Exposure to metals, and toxins as well as magnetic fields lead to cell
death and damage and we have cited several research studies to support
this. Recent studies by Rahman et al (2002) provides further evidence
on this as they found that inhalation of ultrafine titanium dioxide
induces micronuclei and apoptosis in embryo fibroblasts as revealed by
electron microscopy.
Although necrosis results from exposure to toxins and damaging external
conditions, several recent studies have shown apoptosis is also induced
by such mechanisms leaving the debate on the differences between the
characteristics of necrosis and apoptosis open for further detailed
research.
Yamashima (2000) discuss at length the implications of necrosis,
apoptosis and cysteine proteases such as calpain, cathespin and caspase
in ischemic neuronal death of primates. According to Yamashima, recent
studies in ischemia and cell death have suggested that neuronal death
after brief global ischemia occurs by apoptosis which according to him
is an active and genetically controlled 'cell suicide' process.
Apoptosis is thus often seen as cell suicide when a cell dies because
of a genetically programmed and controlled process. Yamashima argues
that studies on monkeys and humans support necrosis which is a calpain
mediated release of lysosomal enzyme cathepsin. This follows from the
fact that ischemia contributes to cell degeneration of neurons.
Yamashima presents an overview of neuronal cell death in his paper and
presents the cascade of primate neuronal death keeping in mind the
roles of cysteine proteases and indicates that selective cathepsin
inhibitors is a novel neuro-protectant that works for this purpose. He
also suggests the significance of a possible interaction among calpain,
cathepsin and caspase within a cascade of ischemic neuronal cell death.
In another recent study on apoptosis and necrosis in the developing
cerebellum and brainstem which is induced after focal-cerebral hypoxic
ischemic injury, Peng et al (2005) discuss the associated cellular
death and changes following focal cerebral hypoxia and ischemia. The
authors highlight the fact that focal cerebral hypoxia-ischemia due to
isolated vascular insufficiency is generally known to cause ipsilateral
but not contra lateral cerebral apoptosis yet the hypoxic-ischemic
damage to the cerebellum and brainstem has not been studied
sufficiently. Peng et al's experimental study on rodents demonstrates
through DNA fragmentation and a labeling analysis (Peng et al, 2005)
that neuronal cells in certain infratentorial regions also suffer from
mild apoptosis and necrosis following a focal cerebral hypoxic ischemic
injury in a newborn rat. This study is similar to a previous study
where the damage of focal/ core brain cells due to necrosis is taken
over by apoptotic damage of the peripheral brain cells. Peng et al's
data definitely provide support to such studies and also provide
additional insights into the mechanisms of neuronal injuries in the
brain as a whole as also in specific areas as in brainstem and
cerebellum areas resulting from a focal cerebral hypoxic-ischemic
damage. Their study indicates and also experimentally demonstrates that
all future therapeutic interventions for hypoxic-ischemic
encephalopathic system must deal with the entire central system and
consider cells at the peripheral regions of the CNS as also cells in
the focus or core of the CNS that is the primary area subject to the
injury.
The differences between Apoptosis and Necrosis at a more molecular
level will be taken up in our discussions. Catelas et al (2005) suggest
a qualitative analysis of macrophage apoptosis versus necrosis induced
by cobalt and chromium ions and report some differences. Their study is
based on the fact that the toxicity of metallic ions in tissues is a
matter of concern for researchers and several studies are being
conducted to know the exact effects of metal ions in cell mortality.
Some previous studies demonstrated in laboratories suggested that Co2+
and Cr3+ ions induce TNF secretion in, macrophages as also in cell
mortality. Catelas and his colleagues tried to quantify the rate of
macrophages mortality either by apoptosis or by necrosis induced either
by cobalt ions or by chromium ions. The researchers used electron
microscopy, flow symmetry and ELISA cell detection methods to
illustrate cell death differentiation between apoptotic and necrotic
cells. This study was performed experimentally on conventional cell
culture conditions as J774 mouse macrophages were incubated in a growth
medium for 24 to 48 hours. Cell culture indicated that cells which were
exposed to low concentrations of Co2+ revealed a low degree of
mortality whereas at highest concentrations of Co2+, late apoptosis
occurred within 24 hours of placing the cell in the concentration.
Although the initial 24 hours showed apoptotic cell death, after 48
hours there was clear evidence of an increased rate of necrosis and at
this time apoptosis occurred at a much slower rate. In contrast
macrophages which were exposed to Cr3+ demonstrated a predominance of
apoptosis after the 24 hour period and very low concentrations of Cr3+
ions early and late apoptosis both occurred at the same rate. With
higher concentrations the number of early apoptotic cells decreased and
the number of late apoptotic cells increased considerably. After 48
hours the concentration of chromium when low induced a higher degree of
early apoptosis and some necrosis also followed. At higher
concentrations the percentages of early apoptotic cells decreased
considerably and necrosis became the predominant process and replaced
late apoptosis. The study by Catelas and his colleagues demonstrates
that macrophage mortality which is induced by metal ions depends on the
types, concentrations of metal ions present and also the duration of
the exposure of the cells to the concentration being studied. According
to the authors, their study showed an overall predominance of apoptosis
after the first 24 hours for both Co2+ and Cr3+, cobalt and chromium
ions although higher concentration of these after 48 hours mainly
induced the necrotic cell death process. This finding definitely
suggests that when tissues have large concentrations of metal ions,
after 48 hours there may a tendency to show tissue damage and
inflammation as necrosis may start developing at this period. This
study has wider implications for research and understanding of tissue
and cell damage and necrosis and apoptosis in general. There are two
conclusions from this study. Apoptosis and necrosis are both possible
with metallic ions and whereas apoptosis is seen in the first 24 hours
of tissue exposure to metal ions, necrosis follows when high
concentrations of metal ions are present near the tissue and the
tissues are exposed for more than 48 hrs. The tissue culture results
are generally important as it has implications for the role of metal
ions in tissue death as well the differences of concentrations and
exposure times of metal ions in necrosis and apoptosis. This is one
further difference between the two types of cell death processes that
we have identified and discussed.
The role of metal ions in inducing apoptosis and necrosis and the
differential effects of Co2+ and Cr3+ ions have been studied further by
Huk et al (2004). They justify the renewed interests in the use of
metal-to metal (MOM) implants for total hip arthroplasty (THA) and
reiterates that MOM articulation generates both metal particles and
ions. However the exact physiological effects of these metallic ions
are not completely known and their potential toxicity is according to
many researchers a cause for concern as they are known to induce cell
death. In Huk et al's study, the researchers used murine J774
macrophages which were incubated with Co2+ and Cr3+ ions and the mode
of the cell death whether apoptosis or necrosis after a period of time
was evaluated in vitro by transmission fo electron microscopy and using
cell death ELISA procedures. As according to the study by Catelas
(2005), cell death was found to be dependent on ion concentration and
the incubation time. According to these results, at short incubation
times of 24 hours, the non inflammatory death and process of apoptosis
was predominant. At longer incubation times of 48 hours and more,
necrosis was found to be the predominant method of cell death when
metal ion concentrations were high.
Vairetti et al (2004) discuss the role of apoptosis and necrosis in
glutathione-mediated cell death. They suggest that hypothermia induces
injury and lead to cell damage. Their study aimed to study the effects
that glutathione (GSH) depletion induces on cell death in the isolated
rat hepatocytes that was kept at 4°C for 20 hours. They modulated the
intracellular GSH concentration using diethylmaleate and buthionine
sulfoximine (DEM and BSO). The untreated hepatocytes showed Annexin V
stained cells (AnxV+), scarce propidium, iodide stained cells (PI+)
which were associated with LDH release within the incubation medium.
The addition of DEM and BSO during the re-warming phase caused a
radical increase in cell death by apoptosis. Production of reactive
oxygen species (ROS and the thiobarbituric species (TBARS) is
associated with a decrease in GSH concentrations which was higher when
DEM and BSO were added before cold storage. Cells which were treated
with DEM and BSO before the cold storage showed much lower TP energy
than the hepatocytes which were treated with DEM and BSO only during
rewarming. Hepatocytes when pre-treated with deferoxamine were
protected against apoptotic and necrotic morphological conditions and
GSH depletion. According to Vairetti and her colleagues the results of
their study suggests that pre-treatment of hepatocytes with DEM and BSO
before cold storage can induce necrosis while the treatment of
hepatocytes only during the re-warming period increases apoptosis. The
findings suggest that in both the conditions during pre-treatment and
during rewarming periods, iron represents the crucial mediator for cell
death and the presence of iron is important in causing cell death
either by apoptosis or by necrosis.
Borst and Rottenberg (2004) analyzed Zong et al's (2004) paper on the
effects of alkylating DNA damage stimulating a regulated form of
necrotic cell death. Zong and his colleges describe that alkylating
agents kill cells by the process of something called 'programmed
necrosis' which is induces by an excessive activation of PARP resulting
in degradation of cystolic NAD+ and inhibition of glycolysis. According
to Borst and Rottenberg, it cannot be sufficiently proved whether
chemotherapy in patients can induce a sufficient NAD+ loss to affect
glycolysis and it is also not evident in contrast to what argued by
Zong et al, whether necrosis can be really programmed and whether there
are mechanisms that make cancer cells hypersensitive to DNA damage
other than the fact that they have a high rate of aerobic glycolysis.
Discussions and Conclusion
In this essay we saw various approaches to the study of cell death in
accidental conditions as in necrosis or programmed cell death as in
apoptosis. Although there are general and controversial discussions
whether necrosis which is caused by toxins and other external agents
can ever be programmed internally. Necrosis results in tissue damage
irreparably at times whereas apoptosis is a natural and necessary way
to rid the body of damaged and unwanted cells.
We discussed several issues here, how necrosis and apoptosis could be
related and how they could be different going by the facts that
apoptosis is considered genetically and internally timed , regulated
and programmed and most cells die and are replaced by new ones.
Apoptosis happens in a natural and painless way as there is much
shrinking and budding off and finally breaking off of cell organelles
which are later engulfed and removed by neighboring cells. In necrosis,
accidental cell death leads to inflammation of cell tissues, painful
conditions, swelling and enzymatic reactions in the body. There have
been several studies though that an initial necrosis can be later taken
over by apoptosis leading to a natural removal of damaged cells and
curing of the diseased condition. However the opposite is dangerous as
natural cell damage when replaced by unnatural and repeated damage of
surrounding tissues can cause severe harm to the body. Some researchers
have claimed necrosis, apoptosis and lysosomal cell death related to
autophagy are the three main types of programmed cell death. However in
general apoptosis and necrosis remain largely the main types of
cellular death and damage.
The differences of apoptosis and necrosis are not only seen in their
structural and functional dissimilarities, they are also seen in the
manifestations of these cell death modes in reactions to certain
chemical concentrations of ions in the tissues. The presence of higher
concentrations of metal ions like Cr3+ and Co2+ can cause apoptosis at
lower incubation period of 24 hours and necrosis at higher exposure
periods at 48 hours. This suggests that apoptosis begins sooner than
necrosis in most cases of exposure to toxins or contaminants as well.
The reason for this is not completely known, however it may be
suggested that the metabolic and enzymatic reactions in apoptosis
begins much faster than in necrosis.
Knowledge of the molecular mechanisms of apoptosis has led to better
understanding of cell death processes in general and dysregulation of
apoptosis has been found to lead to cancer and certain pathological
conditions. A deeper understanding of apoptosis and necrosis will have
to be attained to find out the exact mechanisms of cancerous cell
growth and the means of counteracting such growth using further
research in medicine and biochemical studies.
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