USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A neonate is born to a 40-year-old woman, gravida 1, para 0. The neonate has upslanting palpebral fissures and a flat facial profile. The child feeds well, but by the second day still has not passed any meconium. An abdominal X-ray is done, and the results are shown below. Which of the following is most likely to be found on rectal biopsy?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Hirschsprung disease is also known as congenital aganglionic megacolon, so Hirschsprung’s is a disease that’s present since birth, in which a ganglion, or cluster of nerves is missing, which ultimately leads to a blocked colon, causing it to enlarge.
Alright, so the intestines move waste through the bowels via peristalsis, which is this series of coordinated wave-like muscle contractions that helps move feces in one direction, and this is essentially automatic, happening without you even having to think about it.
The type of muscle that causes these contractions is smooth muscle, as opposed to skeletal muscle or cardiac muscle.
In the gut, there’s a layer of smooth muscle just under the submucosa, which sits under the mucosa, which is the innermost layer of the gut nearest to the lumen.
On the other side of the smooth muscle layer is the serosa.
Now if we look closer at the smooth muscle layer, it’s actually composed of the circular muscle layer, arranged in circular rings which contract and constrict the gut behind the feces, which keeps it from moving backward, while the longitudinal muscle layer, arranged along the length of the gut, relaxes which lengthens and therefore pulls things forward.
Also though, within these layers are two plexuses, or networks of nerves, which are made up of ganglia—which are clusters of individual nerves, which help coordinate muscle contraction and relaxation.
First there’s the myenteric plexus, also known as Auerbach’s plexus, which when activated, primarily causes smooth muscle relaxation.
The myenteric plexus connects with the second plexus—the submucous plexus, or also known as Meissner’s plexus, which is buried in the submucosa and is responsible for helping to control blood flow and epithelial cell absorption and secretion.
These groups of nerves are clearly super important for normal bowel function.
For people with Hirschsprung’s disease, both these plexuses are gone—they’re completely absent in some parts of the gut. Why is that?
Well during fetal development, there are cells called neural crest cells, which are basically a group of fetal cells that migrate away and differentiate into a variety of different cell types.
In this case, some of them become neuroblasts, and eventually the nerve fibers of the plexuses in the gut.