Ectodermal Placodes Contributions to the Development of the Vertebrate Head

Abstract

Neurogenic placodes are focal ectodermal thickenings that give rise to the sensory neurons, and in some cases, the receptor cells of vertebrate sensory systems. There are no markers for the identification of undifferentiated placodal epithelia, but derivatives of the nasal placode, for example, are characterized by unique production of GnRH and olfactory marker protein. Placode morphogenesis occurs by invagination and/or delamination to form sensory epithelia, sensory neuroblasts and in some cases, migratory receptor primordia (e g, lateral line receptors). Specification of neurogenic placodes and pattern formation of their derivatives has been a subject of study for over eighty years, and is still not well understood, but, several genes have been implicated in pattern formation in the derivatives of the otic placode. The lateral line system is unique among placode-derived sensory systems in vertebrates in that it is only present in anamniotes, it is derived from multiple placodes, has an extensive migratory component and gives rise to two classes of sensory receptor organs that mediate two distinct sensory modalities (mechanoreception and electroreception) which share nervous innervation, but project independently to the hindbrain. Nasal and otic placodes, like other epithelia are capable of inducing skeletogenesis in neural crest and mesodermal mesenchyme and thus via induction contribute to the morphogenesis of the vertebrate skull. The long-standing hypothesis that neuromast receptors induce the formation of the lateral line canals associated with the dermal bones on the heads of fishes remains untested, but it is evident that lateral line bones are composed of both dermal bone and lateral line canal bone and may be subject to two discrete and potentially conflicting sets of functional demands in the heads of fishes.