Serial cell differentiation: intricate system of design
by Shaun Doyle
Published: 4 June 2008(GMT+10)
Image Wikipedia
Figure 1. The process of hematopoiesis (the generation of blood cells) is an example
of the serial cell differentiation process.
Single celled organisms replicate as fully functional cells, and they maintain cellular
integrity through a system of direct epigenetic inheritance,1 or ‘cell memory’. Some tissues in multicellular
organisms proliferate in the same way. However, the majority of tissues in adult
multicellular organisms don’t.
Most tissues in mature multicellular organisms replicate via a method called serial
differentiation.2
Cells go through a series of differentiation stages as they duplicate, ending in
a fully differentiated cell, which eventually dies and passes out of the system,
or is recycled by apoptosis (programmed cell death). There are three different types
of cells in this system: stem cells, a class called ‘transient amplifying
cells’ (TACs) and fully differentiated cells.
Serial differentiation
Stem cells
The undifferentiated cells are the only ones in this differentiation process that
are self-renewing, i.e. they produce daughter cells that are exactly like the mother
cell. These cells have the capacity to divide and change into many different types
of cells. They are also very important during embryonic development, where new cell
types are constantly needed.3 These stem cells
are kept relatively few in number, and the cell lines proliferate through the differentiation
process.
Transient amplifying cells
The daughters of stem cells do more than just self-renew; they differentiate into
different kinds of cells. However, they don’t change into fully differentiated
cells immediately; they change into a class called ‘transient amplifying cells’
(TACs). While TACs divide; unlike stem cells, TACs do not self-renew. Rather, the
daughter cells of TACs are one stage further along the differentiation process than
the ‘mother’ cell. These cells amplify the number of cells that will
eventually become fully differentiated from the original stem cell that they started
from.
Fully differentiated cells
A particular stem cell goes through a number of cell division events and the differentiation
process of the TAC stage to produce fully differentiated cells. These are the mature
cells that carry out the different jobs of the tissues, such as blood cells (figure
1), reproductive cells and epithelial cells. These cells no longer divide or differentiate,
and once they have served their purpose, they are ‘deleted’ from the
system and their components are recycled.4
Designed for maintenance
This is a rather elaborate system to conjure up if you just want to maintain tissues!
It is also metabolically expensive because not only do the mature cells require
nutrients, but so do the stem cells and TACs. Therefore, you’re feeding cells
that don’t actually do anything in the body except replicate. So why bother
using so much energy?
As Pepper et al. point out, the aim of this process is to separate the
self-renewing and active proliferating properties of cells into different groups.2
This severely limits the number of duplications that any one cell line will undergo,
which limits the possibility of mutational damage taking hold in a particular tissue.
This system actively works against natural selection of individual cells in favour
of tissue integrity to suppress somatic evolution, which is the change that the
body is subjected to due to mutation and selection within the body’s cell
population. Pepper et al. comment:
‘We hypothesize that this is achieved in animals by compartmentalizing self-renewing
tissues such that one cell population (stem cells) undergoes self-renewal, while
another (TACs) undergoes active proliferation. If no cell population combines both
these necessary elements of somatic evolution, somatic evolution is thereby suppressed.’
The stem cells are maintained as a small and quiescent population through slow self-renewal.
The maintenance of the self-renewing population at low levels militates against
selection of highly proliferative strains of stem cells.
The later stages of the differentiation process are focussed on proliferation, but
they don’t self-renew. Each duplication event moves the daughter cells along
the next stage of differentiation, until the cells are shed after they have become
fully differentiated.
While it would cost less energy to just have self-renewing mature cells, it would
result in the quick death of the organism if something went wrong in comparison
to serial differentiation. Less energy would be used up because the body would not
have to support stem cells and TACs, but only fully differentiated cells. However,
there is a much higher chance a mutation that increases the reproductive success
of a particular cell would gain a hold in such a setup when compared to serial differentiation.
Therefore, the benefit of longevity far outweighs the energy cost incurred for maintaining
the system.
Evolution of multicellularity and serial differentiation
Pepper et al. also comment on the prospects of serial cellular differentiation
aiding the transition from unicellular to multicellular life:
‘It is believed that multicellular organisms could not have arisen or been
evolutionarily stable without possessing mechanisms to suppress somatic selection
among cells within organisms, which would otherwise disrupt organismal integrity.
Here, we propose that one such mechanism is a specific pattern of ongoing cell differentiation
commonly found in metazoans with cell turnover, which we call “serial differentiation.”’5
They believe that this transition from unicellularity to multicellularity is controlled
by epigenetic alterations:
‘Thus, our results support the suggestion … that epigenetic inheritance
played a central role in the transition from unicellular to multicellular life by
helping to control selection among the cells of the newly emergent multicellular
individual.’5
However, both serial differentiation and the multicellular organism have to be assumed
for this to work. At best it suggests how multicellularity persisted, but it does
not suggest its origin.
There is a fundamental evolutionary conflict in a multicellular organism: cellular
selection vs bodily integrity. Generally, natural selection at the cellular level
will favour those cells that are better at reproductive competition and survival.
However, if those cells are allowed to proliferate in an uncontrolled manner in
a multicellular organism, it will inevitably disrupt the organism’s bodily
integrity, and harm or kill the organism.6
This inevitably kills these ‘fitter’ cells too because they cause the
host to die.
Cancer is a prime example. A cancer is essentially a mess of excessively proliferating
cells within a multicellular organism. In an environment (the organism) with limited
resources, such cells will naturally out-compete normal cells because normal cells
generally don’t proliferate indefinitely. The cancer cells outstrip the normal
cells for resources and take over the system. However, this leads to malfunction
in the organism, and if left untreated, will inevitably kill the organism.
At the organismal level, selection will favour those traits that preserve bodily
integrity, which seeks to control proliferation of cells beyond what is necessary.
Pepper et al. confer:
‘Multicellular organisms could not emerge as functional entities before organism-level
selection had led to the evolution of mechanisms to suppress cell-level selection.’7
However, this leads to a conundrum for the evolutionist: how do multicellular creatures
evolve from single celled organisms when cellular selection is diametrically opposed
to organism-level selection? A single cell seeks to proliferate more than its competitors;
the multicellular organism seeks to control such proliferation to what is needed
at a higher level of organisation. This can be seen in the process of apoptosis
as well:
‘Even today, apoptosis serves an essential role in terms of “cellular
altruism”. It helps to ensure that an organism’s genetic integrity is
not compromised, by removing some somatic cells that have sustained irreparable,
genetic mutations. Crucially, apoptosis also helps to maintain a species’
genetic integrity, by eliminating aberrant germ cells that would otherwise carry
intact but faulty genes into the next generation.’8
The system has to be in place before it can be selected for, yet organism-level
selection cannot take over without measures such as serial differentiation in place.
The very existence of this system argues against the evolution of multicellularity.
The system of serial differentiation is designed to enhance bodily integrity,
not reduce it. The system has to be in place before it can be selected for, yet
organism-level selection cannot take over without measures such as serial differentiation
in place. The very existence of this system argues against the evolution of multicellularity.
Conclusion
Serial differentiation is an essential system for the maintenance of mature multicelled
organisms. It serves to separate the self-renewal and proliferative stages of cell
division, which limits the effect mutations have on tissues. Evolution cannot explain
the origin of the system, and neither can it explain the origin of multicellularity.
These features of life clearly speak of purposeful, intelligent creation consistent
with the Bible’s account of creation.
Related articles
Further reading
Recommended Resources
References
- Epigenetic inheritance refers to heritable characteristics
in a cell that do not involve a change in the DNA sequence. For more information
on the importance of epigenetic inheritance for life, see Alex Williams’ three-part
paper on biological inheritance:
Inheritance of biological information part I: the nature of inheritance
and of information, Journal of Creation 19(2):29–35,
2005; Inheritance
of biological information part II: redefining the information challenge , Journal of Creation 19(2):36–41, 2005; Inheritance
of biological information part III: control of information transfer and change,
Journal of Creation 19(3):21–28, 2005.
Return to text.
- Pepper, J.W., Sprouffske, K. and Maley, C.C.,
Animal cell differentiation patterns suppress somatic evolution, PLoS
Comput. Biol. 3(12):2532–2545, 2007. Much of the following
description of the process is based on this paper. Return
to text.
- Sarfati, J.,
Stem cells and Genesis, Journal of Creation 15(3):19–26,
2001. Return to text.
- Bell, P.B.,
Apoptosis: cell death reveals creation, Journal of Creation 16(1):90–102,
2002. Return to text.
- Pepper et al., ref. 2, p. 2532.
Return to text.
- Michod, R.E., Cooperation and conflict in the evolution of
individuality. I. Multilevel selection of the organism, The American Naturalist
149:607–645, 1997. Return to text.
- Pepper et al., ref. 2, p. 2533.
Return to text.
- Bell, ref. 4, p. 98. Return to text.
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