Figure 8. A proposed model for the schistosome stem cell classes.

κ-cells are at the top of the hierarchy, and express nanos-2 and klf. Activation of several genes associated with somatic stem cell function in adults (fgfrA/B; p53; zfp-1) leads to the specification of δ-cells, which we propose serve to generate somatic tissues. Downregulation of nanos-2 and activation of hesl in δ-cells leads to the formation of φ-cells, which are associated with many transitory larval structures, including the sporocyst epidermis (tegument) and the cercarial tail. Only a small number of κ and δ-cells is transmitted from the asexual to the sexual life-cycle stages. After entry into the mammalian host, activation of hesl in δ-cells leads to δ’-cells, which appear to serve as the source of the adult somatic stem cells. κ-cells downregulate nanos-2 and activate expression of an intramammalian stage-specific transcript, eled, generating ε-cells, which are distributed in gonadal primordia and the posterior growth zone. During maturation, gonadal ε-cells subsequently activate germline-specific nanos-1, which commits ε-cells to germline fate.