Could vertebrates once have relied on a different mechanism for developing neurons in the gut? From ScienceDaily:
Lamprey are slimy, parasitic eel-like fish, one of only two existing species of vertebrates that have no jaw. While many would be repulsed by these creatures, lamprey are exciting to biologists because they are so primitive, retaining many characteristics similar to their ancient ancestors and thus offering answers to some of life’s biggest evolutionary questions.
“We were interested in the origins of lamprey gut neurons because in other vertebrates they arise from a particular embryonic cell type, called neural crest cells,” says Stephen Green, postdoctoral scholar in biology and biological engineering and co-first author on the paper. “We knew that lamprey have many kinds of neural crest cells, but we knew little about which cells give rise to gut neurons.”
Neural crest cells are a type of stem cell; during vertebrate embryonic development, they eventually differentiate into specialized cells such as those that make facial skeleton cells or those that create pigment cells. In particular, a population called vagal neural crest cells are known to become the gut neurons. But Bronner and her team noticed that while mature lamprey have gut neurons like other vertebrates, lamprey embryos lack these vagal cells.
“Adult lamprey have gut neurons, but we were unable to find the vagal precursor cells,” says Bronner. “So, where do the gut neurons come from?” To find out, the team drew inspiration from studies of mice that, due to a mutation, lack vagal neural crest cells. The mice do, however, have a small number of gut neurons from an unexpected source — cells called Schwann cell precursors (SCPs). SCPs exist along nerves that run from the spine to various parts of the body. These cells are known to develop into Schwann cells, which form a protective barrier around the nerves.
Bronner and her team fluorescently tagged these cells in lamprey embryos and found that, during development, the cells migrated from the spine toward the gut. Sure enough, some of these SCPs developed into gut neurons.
“Our findings suggest that gut neurons in ancient vertebrates may have come predominantly from SCPs, and that these original gut neurons were later outnumbered by neurons that arose from vagal neural crest cells,” says Green. “Lamprey have relatively simple guts, with no looping and few total neurons. We speculate that vagal neural crest cells might be essential for the more complicated guts of higher vertebrates like mice and humans.” Paper. (paywall) – More.
An interesting approach but, absent a lot more information, it’s hard to be sure. For one thing, lampreys have been around for are least 360 million years. and we are told that they did not evolve during that period. But they might have devolved instead. ‘Tis the gift to be simple,” and all that. Incidentally, the vid below makes clear that simplicity has not harmed the lamprey at all in relation to other fish.
Too bad there are only a handful of types of jawless fish.
See also: Devolution: Getting back to the simple life
Stasis: Life goes on but evolution does not happen
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