What is the 'leading edge' of a transition zone in Hirschsprung's disease, and why is it important?

Time to dip into the old stash for an interesting paper. This one was out of St Louis, MO and looked at the properties of the transition zone of ganglion cells in HD by taking sequential cross sections of surgical specimens and quantifying the ganglion cells in the submucosal and myenteric plexus.

The authors noted that the transition zone was not uniform, but instead had a leading edge (analogous to dripping paint). This leading edge of the transition zone was measured to be up to 2.1 cm (average 1.1 cm) and 2.4 cm (average 1.4 cm) long in the submucosal and myenteric plexuses, respectively. Additionally, the number of ganglion cells at the tip of the leading edge was normal.

The significance of this finding is that a frozen section biopsy performed at the transition zone may result in a pull-through that includes abnormal bowel and potentially poor functional results. The authors thus recommended that a pull-through be performed using bowel that is at least 2 cm proximal to the area of 'normal' ganglionated bowel identified intraoperatively by frozen section

Reference:
White et al. Circumferential distribution of ganglion cells in the transition zone of children with Hirschsprung's disease. Pediatric and Developemental Pathology (2000) 3, 216