In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of overlying smooth muscle (fibers running circularly within layer of longitudinal muscle).n the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of overlying smooth muscle (fibers running circularly within layer of longitudinal muscle).