Fig. 5. A SMP-1 and PLX-1 based model of cell migration during vulva morphogenesis. (A) Lateral perspective; D, dorsal; P, posterior; unlabelled arrow indicates left-right depth. A series of short-range migrations is repeated for the formation of vulva rings. (B) Ventral perspective of the boxed area in A. Intermediate steps of the proposed model for vulva cell migration showing the PLX-1-expressing leading edge as a blue band on the future lumen side of cells poised to enter the forming vulva. The leading edge spreads ventrally under forming vulva rings expressing SMP-1 on their lumen surface (green spiked ring). SMP-1 expression increases dramatically in the newly formed ring, while the next external vulva half ring (not yet expressing SMP-1) initiates its migration by extending processes underneath it. (C) A genetically derived model of the molecular cascade regulating vulva morphogenesis. SMP-1, PLX-1 and CED-10 function in the same pathway when guiding vulva cells (attractive guidance to the vulva midline). UNC-73 and MIG-2 act in one or more parallel pathways whose function is similar to that of the SMP-1/PLX-1 signaling pathway. It is possible that MIG-2 and CED-10 also function downstream of PLX-1 (broken arrows from UNC-73 and MIG-2), as suggested by the literature, indicating that UNC-73 can activate MIG-2 and CED-10. Additional guidance mechanisms may function in parallel with PLX-1 and MIG-2 (broken arrows with question mark on the right).