ABNORMAL DEVELOPMENT
SSHD is a direct result of the failure of correct migration of the neural crest cells, the cause of which is unknown. Once they arrive in the foregut they have completed the first step of enteric nervous system development (3). The second step, as described in the previous section, “normal development of the gut”, is the final migration, proliferation, and differentiation of neural crest cells into enteric nervous system ganglia. Incorrect migration or proliferation tends to yield regular Hirschsprung’s Disease, which is an area of aganglionosis in the gut.
The more specific and rare “skip-segment” variety occurs when the neural crest cells fail to complete the shortcut they take to a more distal portion of the gut. This shortcut, mentioned in the description of the normal development of the gut, gives the migrating cells a more efficient path to their final destination. The convoluted nature of the developing gut makes a migratory journey along the entire gut unnecessary. This can be seen in figure 4, where the white arrows show the transmesenteric shortcut taken by neural crest cells. The “skip segment” in SSHD is a direct result of the migrating neural crest cells migrating to an area that is otherwise aganglionic because of premature arrest of other neural crest cells' migration. The inserted section of developing enteric nervous system creates the notable skip-segment appearance of SSHD (4). This specifically located shortcut is also the reason why SSHD always starts in the same place.
![Picture](/uploads/1/3/5/3/13535018/1433088141.png)
Figure 4. The green areas in the diagram exhibit normal ganglionosis, while the white areas illustrate aganglionosis. The signature “skip-segment” is obvious in the depiction of SSHD. The three white arrows show the transmesenteric shortcut taken by the migrating neural crest cells from the end of the midgut across to the hindgut (4).