Development and evolution of lateral line placodes in amphibians I. Development.

Lateral line placodes are specialized regions of the ectoderm that give rise to the receptor organs of the lateral line system as well as to the sensory neurons innervating them. The development of lateral line placodes has been studied in amphibians since the early 1900s. This paper reviews these older studies and tries to integrate them with more recent findings. Lateral line placodes are probably induced in a multistep process from a panplacodal area surrounding the neural plate. The time schedule of these inductive processes has begun to be unravelled, but little is known yet about their molecular basis. Subsequent pattern formation, morphogenesis and differentiation of lateral line placodes proceeds in most respects relatively autonomously: Onset and polarity of migration of lateral line primordia, the type, spacing, size and number of receptor organs formed, as well as the patterned differentiation of different cell types occur normally even in ectopic locations. Only the pathways for migration of lateral line primordia depend on external cues. Thus, lateral line placodes act as integrated and relatively context-insensitive developmental modules.