Fig. 1: Examples of PPB types Ir and I.

Type Ir PPB is characterized by multicystic structures where the septa lack the characteristic small primitive cells underneath the epithelium (a, b). In contrast, type I PPB is characterized by the presence of these small primitive cells which can either present as a continuous or discontinuous growth pattern (c, d, e, f). In more than half of the cases the primitive cells underneath the epithelium are rhabdomyoblasts, and a classic association with immature island of cartilage is seen in PPB regardless of type (c, d). Occasionally the cystic septa are expanded by the immature cells, a possible sign of early progression to type II (e, f).