Pericyst: the outermost layer of hydatid cyst.

Hydatid disease, caused by the parasite Echinococcus granulosus, mostly affects the liver and the lungs with hydatid cysts which consist of three layers: (1) the outer pericyst; (2) the middle laminated membrane; and (3) the inner germinal layer. Pericyst, as the outermost layer of the hydatid cyst, is made by host cells encasing the hydatid cyst. An extremely close interaction exists between this host tissue and the parasite, and any degenerative changes of the pericyst would result in hydatid cyst degeneration or rupture. The pericyst plays an undeniably important role in the development and survival of the hydatid cyst.