This image shows an unwound small intestine from a mouse embryo 15 days after conception. Blood vessels (blue) are responsible for carrying oxygen to the muscles of the gut and transporting dissolved nutrients from the intestine to the rest of the body. The intestinal nervous system (green) coordinates intricate digestive processes and is so complex that it has been nicknamed “the second brain.” Autonomic nerves (yellow) allow information to be passed back and forth between the brain and the intestine. During development, autonomic nerves grow along arteries to reach the gut before integrating with the intestinal nervous system, which forms several days earlier.
Contributed by John Hatch, a second year graduate student in the Biological and Biomedical Sciences program at Harvard Medical School.