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RE: [HAPS-L] erythrocyte formaiton
It's strange to think about how the enucleation evolved. I don't have a
problem with the "it just happened that way" explanation. In fact, I am
not sure that the nucleus would necessarily have to be ejected to
achieve a biconcave shape. It would be possible to have a circular
nucleus. Some PMN's have such weird nuclei that they are virtually
circular anyway. Am I correct in remember that there is a granulocytic
pathology manifested as neutrophils with circular nuclei?
BTW: Nobody has mentioned the lowly mitochondria which also bid adios
to the RBC.
martini@xxxxxxxx 01/29/04 08:51PM >>>
I'm puzzled about why mammalian RBCs must differ from the
nucleated avian RBCs.
This brings us back to a question that's popped up from time to
time on this list: "does everything have to be adaptive?" The
answer is no, it doesn't - it just has to (1) not put you at a
disadvantage and (2) be associated in some way (appearing at the
same time and in the same individual/population, if not causally
linked) with something that DOES convey a selective advantage. I
would suggest that the enucleation of RBCs falls within this
category - it works OK but has no great adaptive significance.
I agree with Ric that it is a mistake to think everything is
adaptive. However, these anucleate biconcave RBCs are such a
primitive, class-specific, and nearly universal (within the class)
trait, and have such a broadly accepted functional significance,
that I highly doubt that this is an evolutionary spandrel.
I am not saying that the shape doesn't impact the functioning of the
RBC. Just that every lineage/clade solves the problem in their own
way, through juggling things like RBC shape, viscosity, blood volume,
hematocrit, and so forth.
The evolutionary bottom line is that you deliver adequate amounts of
oxygen at a rate that matches the needs of peripheral tissues. There
are obviously many different ways to do that. I don't think there
are any comparative studies that indicate that the biconcave shape
was selected for (as opposed to not selected against) nor that
mammalian RBCs or the oxygen delivery system as a whole (which is
what selection would target) are in some way "better than" those of
birds or their ilk. We are certainly not "better" in terms of
metabolic scope or oxygen delivery or even body size (if you consider
extinct and extant forms).
Perhaps a biconcave disk is the best solution, in terms of shape, for
an enucleated cell - but the only evidence is that almost all mammals
share that feature, and camels and llamas make that argument
unconvincing. I could just as easily suggest that the intracellular
changes associated with the ejection of the nucleus are what dictates
RBC shape, that camels and llamas are secondarily different, and that
the shape difference isn't functionally significant.
Anyway, the two things I think we can be sure of are that (1) the RBC
nucleus was lost in the stem lineage, which in itself explains the
distribution of the trait and (2) the biconcave shape works fine but
obviously isn't essential since some successful mammalian RBCs have a
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