Thanks for your query,
For all our cells there is a time to die.
There are two ways in which our cells die:
They are killed by injurious agents (external or internal).
They are induced to commit suicide - following natures call, so to speak.
Biologically, Human Beings are built out of aproximately 100,000
billion cells. Humans are multicellular animals. Many of the basic
life processes of human cells are basically the same as in simple
unicellular eukaryotes such as yeast and even prokaryotes. What makes
us different from Yeast?
Yeast are microorganisms,found everywhere in nature, when a yeast cell
divides, it will producesa an independent organism.
A human being is initially just one cell, or zygote, but when that
zygote divides, it will form an embryo.
The early embryo is made up of totipotent stem cells, which go on to
Differentiation is a process in which an unspecialized cell becomes
specialized into one of the numerous cells that build the human body,
for instance the kidny, the liver, or heart. During this process of
differentiation, certain genes are activated, while other genes are or
inactivated. This process is intricately regulated. The resultis a
differentiated cell that develops specific structures and performs
specific functions. Differentiation can involve changes in numerous
aspects of cell physiology; size, shape, polarity, metabolic activity,
responsiveness to signals, and gene expression profiles can all change
All cells make up tissues or groups of cells dedicated to a certain
function - the muscle or the heart, for instance, in the human body.
Most human cells are frequently reproduced and replaced during the
life span of an individual. However, the process varies with the kind
of cell. Somatic (body cells), such as those that make up skin, hair,
and muscle, are duplicated by mitosis. The sex cells (sperm and ova)
are produced by meiosis:
A list of distinct cell types in the adult human body can be found here:
Programmed cell death (apoptosis) is the natural, necessary death of
individual cells for the good of the organism as a whole. It is a
"Much of what is known about programmed cell death comes from
pioneering studies by HHMI investigators Robert Horvitz at the
Massachusetts Institute of Technology (MIT) and Stanley Korsmeyer at
Washington University. Horvitz and his colleagues have explored the
genes that control this program in the tiny nematode worm
Caenorhabditis elegans. This worm is a marvelous animal for
developmental geneticists to work with because the lineage?and fate?of
every one of the worm's 1,090 cells is known. This has made it easier
to identify many of the genes that control the cells' developmental
history and function. Researchers have learned that 131 of the worm's
cells die or "commit suicide" during normal development. Horvitz and
his associates showed that two particular genes, ced-3 and ced-4, are
required for these deaths, while another gene, ced-9, prevents them.
They also found that, once a cell is committed to a programmed cell
death, it follows a reproducible pathway of events.
"That pathway can be said to consist of three sequential stages," says
Horvitz. "First, killing the cell. Next, getting rid of the body. And
third, destroying the evidence. Basically, you have a corpse and you
must do something with it, so a neighboring cell swallows the corpse
to remove it from the animal. But then the corpse must be degraded.
This sequence, which we have defined in the worm, looks as if it will
prove to be universal among organisms, including ourselves." Upstream
of these steps, Horvitz says, is the decision about cell death: "Do
it, or don't do it." "
What is the limit to the quantity of cell splits in the human body?
The "Hayflick limit" explains that:
"In 1961, Hayflick and a colleague, Dr. Paul Moorhead, demonstrated
that normal human cell populations cultured in the lab could only
double approximately 50 times before they stopped reproducing and
senesced (aged). His discovery flew in the face of the accepted
scientific dogma that normal cultured cells were immortal if properly
maintained. This cellular longevity cap would eventually be dubbed the
"Hayflick limit," and this discovery helped give birth to the new
field of cytogerontology, or the study of aging at the cellular
Hayflick insists that despite science's best efforts the outer limit
of human life will remain about 120. The most a human being has ever
What other contributors does the human body have to the 'programming
of cell suicide? TELOMERES, are the answer:
"Inside the nucleus of virtually all of our cells are chromosomes, 46
in all. At the tips of these chromosomes are telomeres, repeating
sequences of genetic material that shorten each time a cell divides.
Cell division is important because many cells in our body (e.g., those
that line our digestive tract) must be replaced over time. When a
cell's telomeres reach a critically short length, however, that cell
can no longer replicate. Its structure and function begin to fail as
it enters this state of growth arrest, called replicative senescence.
Some have likened the process of telomere shortening to a genetic
biological clock that winds down over time. Today, researchers
continue to probe the telomeric "timepiece," hoping to better
understand the aging process and fight diseases, particularly cancer."
Telomeres are defined as a fuse that becomes shorter and shorter,
until it sets off a kind of cellular time bomb that wreaks havoc on
the cell's internal workings.
Taking into account the "Hayflick limit" and apoptosis factor- some
calculatiosn were aproximated and claims were made that every year
about 98% of the atoms in your body are replaced. Maybe this
assumption was based upon the fact that oxygen, hydrogen, and nitrogen
gases and carbon make up 96% of the total weight of the human body.
Chiropractor Dr. Stew Bittman also claims that "The human body is
unfathomably intelligent. It builds itself from 1 cell into 100
trillion cells in 9 months. It rebuilds 98% of itself in 3 months!
That's right- 98% of the atoms that comprise your body right now will
not be in your body 3 - 6 months from now. They will have been
replaced by atoms that you accumulate from your environment, mostly in
the form of food."
Earlier this month the New York Times published an article titled
"Your body is younger than you think", stating that Dr. Frisen, a stem
cell biologist at the Karolinska Institute in Stockholm "has also
discovered a fact that explains why people behave their birth age, not
the physical age of their cells: a few of the body's cell types endure
from birth to death without renewal, and this special minority
includes some or all of the cells of the cerebral cortex."
Based on the facy that "all the carbon 14 in a cell's DNA is acquired
on the cell's birth date, the day its parent cell divided...Having
validated the method with various tests, he and his colleagues have
reported in the July 15 issue of Cell the results of their first tests
with a few body tissues. Cells from the muscles of the ribs, taken
from people in their late 30's, have an average age of 15.1 years,
The epithelial cells that line the surface of the gut have a rough
life and are known by other methods to last only five days. Ignoring
these surface cells, the average age of those in the main body of the
gut is 15.9 years, Dr. Frisen found."