Model of semaphorin functions in lung development. Sema3A opposes the effects of Sema3C and Sema3F, which activate branching morphogenesis in this tissue. The expression of the semaphorins and their receptors is tightly regulated, both spatially and temporally. The expression levels of semaphorins, neuropilins, and plexins are indicated as follows: ++, high; +, moderate; (see details in Kogashima et al, ref. 29).