MadSci Network: Cell Biology |
Hi,
There are in fact plenty of examples that cells do have a form of memory
but sometimes these phenomena are given other names. I will concentrate on
selected examples from development, immunobiology, genetics, and
neurobiology.
A) Development
You might have heard about the so called HOX genes. HOX genes are
activated during certain stages of
develo
pment and play a role in determining the identity of cells (and
therefore
cell
groups and more complex structures) along any the 3 major axes of an
organism. HOX genes do not code for structural proteins but rather for DNA
regulatory proteins (=transcription factors) that determine where and when
other genes are active.
One of the most thoroughly researched organisms is the fruit fly Drosophila
melanogaster but there are plenty of other examples such as the vertebrate
neural tube or the wing bud of
birds.
Scientists use the catchy phrase of the HOX code to describe the
fact that certain groups of HOX genes are expressed only in
certain positions along an axis and thereby function to give cells unique combinations of active HOX
genes that encode a distinct position and/or identity. The gradient
itself is established gradually over time and refined through interactions
between different genes and their gene products (either mRNA or more
notably proteins, predominantly so called transcription factors that
regulate where and when and to what degree a gene is expressed). The
'memory' is in this case, very similar to the brain
and its component neurons, created by network-like interactions that fix
the expression pattern in any given individual cell. A cell for
itself doesn't really have a memory because it is to a degree dependent on
information provided by its neighbors. The whole system contributes to
maintain the information.
B) The immune system
Certain types of immune cells called memory (B or T) cells store
information on certain foreign structures (usually short snippets of
protein called peptides) they have encountered.
If they are challenged with this very specific piece of information later
on they jump-start a rapid and adaptive (meaning the answer to various
kinds of pathogens such as bacteria, viruses, etc. is not stereotypical
but has an ability to 'learn') immune response. But once again they can't
do it all by themselves because other cells have to provide information by
presenting antigen through specific receptors on their surfaces, or they
have to contribute specific activating factors such as soluble interleukins
or various kinds of surface molecules.
What does this system have in common with the brain? It is its ability to
learn which also requires 'memory' to compare a current input to a previous
one. This memory is stored in one specific cell or cell clone (and more
specifically in the pattern of active genes in either the immune cell or
the developing organism) but has to be activated (immune system) or
established/stabilized (development) by the interaction with other
cells.
This is a to a degree similar to what happens in the brain (here,
synaptic connections between cells are stabilized) although here
the network character of the whole system is much more prominent. This
becomes quite evident in pathological situations (stroke, tumors,
etc.) where immediate effects such as memory deficits or loss of other
brain functions occur due to the loss of neurons. Nonetheless, in many
cases the brain is able to compensate for these losses in a long process of
remodeling and rebuilding itself, i.e. different connections between
neurons are now being used and enhanced.
Finally, I'd like to mention changes in DNA where information seems to be
retained physically in the form of certain chemical modifications
(methylation) which constitute a sort of memory. Most notably these
specific patterns have been described while researching the phenomenon of
imprinting.
Here, the exclusive expression of one the alleles inherited from
father or mother makes the difference. Most of the genes where
imprinting plays a role have to do with early embryonic development but
also seem to be involved in cancer and other diseases.
I hope I could contribute my 5-cents-worth with this answer. The subject is
quite intricate and I hope this is a good starting point for further
research should it be necessary. In any case, feel free to email me for further explanations.
REFERENCES
Immunobiology
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