Fig. 1. Major events and regulators in generation of the VPCs (A), vulval induction (B) and early stages of vulval morphogenesis (C). (A) During the L1/L2 stages, the fusion between P(3-8).p and hyp7 is blocked by the action of lin-39 Hox, which is expressed in these six cells (Clark et al., 1993; Wang et al., 1993). P3.p through P8.p (VPCs) have the potential to adopt the vulval fate, whereas other Pn.ps fuse with the hypodermis. (B) During vulval induction at the L3 stage, the RTK/Ras/MAPK pathway transduces a vulval inductive signal from the anchor cell and the synMuv genes prevent the vulval fate in the VPCs (Riddle et al., 1997). lin-39 is also required at this stage to maintain the competence of the VPCs and their progeny for vulval induction (Maloof and Kenyon, 1998; Clandinin et al., 1997). In wild-type animals, three VPCs are induced to become vulval cells and the other three divide once and then fuse with the hypodermis. (C) During the early stages of vulval morphogenesis, the induced VPCs first divide twice in a longitudinal orientation, each giving rise to four granddaughters. All but two of the granddaughter cells undergo one more round of division, and each has a specific division plane. L stands for longitudinal division, T for transverse division and N for no division. The dividing cells also migrate dorsally as well as towards the center of the vulva (where the inner cell P6.p is located).