Muscularis propria (Detrusor muscle) consists of thick bundles of smooth muscle in contrast to wispy fascicles of muscularis mucosae. This distinction may be difficult in small biopsies. The thickness and the pattern of muscularis propria varies in different parts of the bladder. In the region of bladder neck, the muscle bundles are thicker and better organized than other parts. In invasive urothelial cancers, the involvement of muscularis propria (Stage T2) warrants a definitive therapy such as cystectomy or radiation.