Today I have lectured for three and a half hours on histology. We have gone through slides of all the major organs, scanned into imaging software that enabled us to go to different magnifications in any part of the specimens.
Going through slides for demonstrations like these is almost always an aesthetic treat. This time, the single most beautiful thing that struck me was this:
This is a section from a human duodenum, the first part of the small intestine. Lovely, isn’t it?
What we are looking at here is a mitosis at the bottom of a duodenal villus. At the base of the villi, there are progenitor cells, or stem cells if you want, that constantly regenerate and that give rise to all the cells of the villus epithelium. Cells on the apex of a villus will degenerate and die, and will be replaced by new cells moving from the base where they are regenerated.
In essence, the epithelial cells are in a constant but slow motion towards the top of the villus, where they will reach their ultimate demise.
Any cell that thinks differently, and wants to stay, faces a stiff current of upwardly-moving cells that will sweep it along in its never-ending tide.
The epithelial cells are constantly exposed to the slightly toxic cocktail of eaten stuff and microbes in the gut, and even during their short life-time they will be expected to acquire genetic changes. But the constant flux makes it very difficult for any of them to ever develop into a tumour. I wonder if the turnover rate is faster in the small intestine, where tumors are much rarer, and where the energy content of the dying cells is recycled through absorption, compared to the large intestine, where tumours are much more common and only water is absorbed?
The really cool thing, however, is that the villus escalator model has been known for such a long time. Histologists noted more than a hundred years ago that mitoses only occured at the base. This simple observation was sufficient to make the inference that the epithelium is constantly rejuvenated at the base and degenerates at the apex – a model that has been extensivly corroborated since.
UPDATE: This post seems to get lots of traffic from people who look for histological images. Feel free to use the ones in this post for any purpose you like! It’s polite to mention where they came from, though.