I know, it’s supposed to be a Friday thing to post stuff about science, but I’m disorganized and busy enough that you’ll just have to bear with me.  Today’s topic: Why don’t red blood cells have nuclei?  

As an extra bonus, there’s a tangential philosophical digression…  see you on the flip.
There are two ways to answer such a “Why…?” question–in terms of procedural reasons and in terms of motivation.  Aristotle actually described four types of causes, and I’ll come to this below in the philosophical digression I promised.  Anyway, the procedural reason is often called mechanistic or proximate, and the motivational reason is often called ultimate or teleological.

The proximate (mechanistic) answer to this question is that during the cellular differentiation process, the pre-red blood cell developmental program induces the enzymatic degradation of the nucleus.  Simple enough.  That explains why red blood cells don’t have a nucleus, but why do they destroy a perfectly good set of chromosomes?  The ultimate (teleological) reason is that red blood cells have one purpose, which is to ferry hemoglobin (the protein that carries oxygen) out from the lungs to muscles and other tissue, and then to ferry the depleted oxygen-less hemoglobin back to the lungs to get more.  Red blood cells do not reproduce–stem cells in the marrow divide asymmetrically and one progeny of each division becomes a red blood cell, in part by getting rid of its nucleus.  They do not express genes once differentiated (all their hemoglobin mRNA is made before they get rid of their nucleus, of course).  So there is no need for the nucleus–it would just be taking up space that is better used for additional
hemoglobin, which makes them more efficient at transporting oxygen.  

Notably, this is an innovation that is not present in all animals; birds for example have red blood cells with nuclei.  It is therefore likely that enucleation evolved after the mammalian line diverged from reptiles, and presumably was related to the appearance of endothermy since this results in a significantly higher metabolic rate and consequently a greater need for oxygen throughout the body.

Now, that digression–what were Aristotle’s other two kinds of causes?

It turns out that most modern folk don’t really think of these as causes at all.  They are usually labeled as the formal cause and the material cause.  Stealing from this web page, I think this makes things relatively clear:

If we ask “what makes something so-and-so?” we can give four very different sorts of answer – each appropriate to a different sense of “makes.” Consider the following sentences:

   1. The table is made of wood. (material)
   2. Having four legs and a flat top makes this a table. (formal)
   3. A carpenter makes a table. (efficient, or proximate)
   4. Having a surface suitable for eating or writing makes this a table. (final, or ultimate)

So, there you have it–four causes.  Sort of.

Have a nice Saturday, and go Red Sox!!!

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