I recently saw this bladder biopsy and it reminded me of the strange embryologic origin of transitional cells. This biopsy is an example of nephrogenic metaplasia. The adjacent urothelial mucosa shows non-specific mild chronic cystitis and then there is this papillary area.
On higher power, note the single layer of bland epithelial cells:
Nephrogenic metaplasia is a curious example of the reactive phenomenon associated with chronic irritation of the urothelium, like the more commonly seen squamous and intestinal metaplasia. The papillary component is usually identified at cystoscopy as "bladder mass" or "bladder cancer," as in this case.
What strikes me here is that, although interesting by being uncommon, is this finding necessarily unexpected? I would argue not at all. Recall that transitional epithelium has polyphylectic "mongrel" origins. The mesoderm (mesonephros, metanephros, paramesonephric and mesonephric ducts), and urogenital sinus (endoderm-ectoderm) all contribute to the development of urogenital epithelium. So somewhere in that mutt of an epithelium is the program to be nephrogenic-ish.