According to current dogma, there is little or no ongoing neurogenesis in the fully developed adult enteric nervous system.
This lack of neurogenesis leaves unanswered the question of how enteric neuronal populations are maintained in adult guts, given previous reports of ongoing neuronal death.
A specialized subset of intestinal macrophages, known as muscularis macrophages, are anatomically and functionally associated with the myenteric plexus (33, 34).
Because macrophages do not express the gene for choline acetyltransferase (Ch AT) (35, 36), which is expressed by a large number of myenteric neurons (35), we used the Ch AT-cre:td Tomato mouse to observe the phagocytosis of myenteric neurons by muscularis macrophages. Although we were able to observe engulfment of neurons by these macrophages through microscopy, we also studied the presence of td Tomato inclusions in intestinal macrophages using flow cytometry and found that although the mucosal and submucosal macrophages that are less associated with enteric neurons (33) showed a very small population of td Tomato-containing macrophages, significant numbers (∼7.5%) of myenteric plexus-associated muscularis macrophages contained td Tomato in both small and large bowel, further supporting our imaging data (Fig. Taking these data together, we observed that ENS-associated muscularis macrophages participate in phagocytosis of dying neurons and the active clearance of neuronal debris, and that this process is common for the both the small and large bowel.) Two-photon microscopy of myenteric plexus from the colon of a Ch AT-cre:td Tomato mouse shows td Tomato-expressing (red) cholinergic neurons along with MHC class II-labeled muscularis macrophages (green) shows an abundance of macrophages associated with the myenteric plexus.
(Scale bar, 50 μm.) ) A 2D projection image of a z-stack of confocal microscopy images of neurons within the small intestinal myenteric ganglia shows the colocalization of Ch AT-cre:td Tomato (red) signal along with that of CD11b-stained (green) muscularis macrophages.
Additional td Tomato-expression can be noticed (red arrow) which does not show corresponding Hu C/D colocalization. S3) form mature neurons that express Hu C/D (magenta; white arrow), showing that graft-derived cells give rise to neurons.) Mean ± SE of percentage of macrophages across small bowel (SB) and large bowel (LB) that contain td Tomato-inclusions compared with WT controls, suggesting that the robust phagocytosis of neuronal debris is similar across the length of the gut.:td Tomato progeny were administered tamoxifen (for 4 consecutive days) when adults. On the other hand, nearly a third (∼31%) of preexisting NOS1 neurons labeled with td Tomato were lost at day 7 (Fig.In mice that were killed immediately after induction (day 0), almost all NOS1-expressing neurons were labeled with td Tomato expression (Fig. 2) Representative images from myenteric ganglia that were stained with antisera against Hu C/D (green) and antibody against cleaved caspase-3 (red) shows a lack of cleaved caspase-3 immunoreactivity in mice that were given the pan-caspase inhibitor z VAD-FMK () Representative images from myenteric ganglia that were stained with antisera against Hu C/D (green) and antibody against cleaved caspase-3 (red) shows a lack of cleaved caspase-3 immunoreactivity in mice that were given the pan-caspase inhibitor z VAD-FMK ( neurons per myenteric ganglia in pan-caspase inhibitor treated mice at day 7 posttamoxifen: 5.20 ± 0.50).Although the ENS is constantly exposed to mechanical stress (3), as well as potential environmental threats from luminal contents (4), it is not known how the numbers of enteric neurons in the healthy adult small intestine remain remarkably constant for most of adult life (5).Although continuous production of new neurons in the gut would appear to be needed to offset the observed apoptosis-mediated neuronal loss (6), such neurogenesis has been remarkably difficult to demonstrate and in vivo enteric neurogenesis in adults remains highly controversial (7–10).