Cranial sensory ganglia in all vertebrates, including humans, develop from two primordia, neurogenic placodes and the neural crest. Neurogenic placodes are thickenings of the surface ectoderm from which neuroblasts are formed. These cells subsequently become afferent neurons in the distal ganglia of nerves VII, IX, and X, the distal part of the trigeminal (V) and the vestibulo-acoustic complex (VIII). Neural crest cells arise from the marginal (neural) folds of the neurepithelium immediately prior to or following closure of the brain and spinal cord. In the head region crest cells give rise to neurons of the proximal sensory ganglia; they also form all accessory and Schwann cells, peripheral efferent neurons, and many craniofacial skeletal and connective tissues. The objective of this research is to define the mechanisms controlling development of these cranial sensory neurons, particularly those of placodal origin. Four developmental processes will be examined: (1) placode formation, (2) neuroblast production, (3) sensory ganglion development, which involves the participation of both neural crest and placode cells, and (4) specification of sensory neurons, by which is meant the establishment of appropriate central and peripheral projections. The method of analysis involves transplantation of the primordia of neurogenic placodes to heterotopic sites, either in the place of a different placode or to a non-placodal site. All donor tissues will contain a marker so that they can be distinguished from host cells. Examination of host embryos will involve cytological, immunohistochemical (using antibodies to neurofilaments) and neuroanatomical techniques. The latter include modified HRP tracing of afferent projections in host and control embryos. Ancillary transplants of the neural crest will be performed as a complementary series. This system provides an unique opportunity to investigate the interactions both between two distinct lines of neurogenic cells and also between these cells and other, non-neurogenic tissues in the head. In no other vertebrate embryonic neural system are the primordia identifiable and accessible both prior to and during the early stages of neurogenesis, at which times the cytological and biochemical properties and patterns of projections are determined.
