I ran into this article claiming that the number of hematopoietic stem cells is more or less identical between rats, mice, humans and elephants DESPITE their obvious differences in size and blood cell production needs (Abkovitz JL, Catlin SN, McCallie MT, Gutrop P: Evidence that the number of hematopoietic stem cells per animal is conserved in mammals. Blood 2002, 100(7):2665-2667.). Can this be true?
To extrapolate from this paper I tried to figure out how many intestinal crypts and villi and therefore how many stem cells mice, rats and humans have in their intestines. I assume the number goes up with body size contrary to the findings in the HSC area. I found a number for rats: about 130,000 villi per small intestine. Again, I assume humans have millions but do we have 13,000,000 (since we are about 100 times heavier; weight as an approximation of increase in intestinal surface from rat to human). Is the size and length of a villus different in different species?
Bottom line: Does the stem cell number correlate with body size, yes or no?
Before I get the chance to read the paper, I'm going to venture some requirements that the investigators had to address. Stem cells divide asymmetrically and they divide more often when needed so the total number of cells is actually unknown. 5-fluouracil is used to 'shake up' HSCs so you can expect larger number of HSCs during tissue replacement, healing or the like. However, stem cells reside in niches that allow them to remain relatively indifferentiated and quiescent. Expectedly, the number of niches have to correlate with tissue area since the niches are located, as you mention, in strategical positions for tissue regeneration and repair. So I would expect as you also believe, that stem cells (label retaining) keep in proportion with tissue surface, size and renewal. It's possible that smaller animals would have similar number of transiently amplifying cells due to their higher metabolic rate but the niches where the quiescent cells reside would be proportional to size and needs. A few thoughts before delving into the paper.
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