The dermis is displaced by a nodular unencapsulated hypocellular area containing mucin. These are not true cysts because there is no epithelial lining, however, often, there is a "collarette" of epidermal rete ridges clutching the cyst (Rapini).
Scattered stellate cells (likely fibroblasts) can be seen within the mucin. The overlying epidermis may be attenuated or hyperplastic .
A closer look at the central portion of the cyst reveals paucicellular myxoid material, although rare stellate fibroblasts may be seen within.
Digital mucous cysts are located at the dorsal aspect of the nail (at the proximal nail fold) and present as a translucent papule or nodule containing gelatinous material. It forms due to accumulation of mucin.
Note that focal mucinosis of the skin is an identical lesion except that it arises in other locations and is less like to form a large cystic space (Rapini).
A nodule or papule containing clear gelatinous material is located on the dorsal aspect of nail. These nodules may be painful and women are twice as likely to be affected(Busam).
While benign, it is not uncommon for these cysts to recur.
Busam KJ. Dermatopathology: Foundations in Diagnostic Pathology 1st Ed. Philadelphia, PA: Elsevier; 2010: 321.
Rapini RP. Practical Dermatopathology. Philadelphia, PA: Elsevier; 2005: 123.