(A) A simple regulatory network that controls stem cell self-renewal, pluripotency, and differentiation. Stem cells have the capacity to self-renew and give rise to specific cell types (differentiation). This network is tightly regulated by diverse extrinsic signaling pathways (e.g., Ras-ERK/MAPK) and intrinsic factors (e.g., transcriptional regulators). In particular, Ras-ERK/MAPK signaling pathway can either promote or inhibit stem cell differentiation, depending cellular context. (B) Conserved Ras-ERK/MAPK signaling pathway in mammals and C. elegans. In C. elegans germline, Ras-ERK/MAPK signaling pathway is critical for meiotic germ cell progression, oocyte maturation, and sperm fate specification.