Mucoceles are a collection of mucus within the tissue. They are caused by trauma that cuts into or severs a duct allowing the mucus to escape into the tissue where it is perceived by the defenses as foreign and an inflammatory response ensues. Macrophages attempt to phagocytize the mucus and accumulate along with a surrounding granulation tissue which on this microscopy looks like a cyst wall, but note there is no epithelial lining. The glands associated with the accumulation are next to it. As long as the glands are actively producing mucus, the lesion expands and then retracts as the mucus is resorbed.
Treatment usually consists of excisional biopsy with the associated glands.
They may recur.