Temporal patterns of neurogenesis in avian cranial sensory and autonomic ganglia.

Avian cranial sensory ganglia are embryonically derived from neural crest and epidermal placodes. Cells from these two populations interact with each other and with other components of their environment to influence the complex structural and functional organization of the ganglia. To help understand these processes, the times of terminal mitosis of cranial sensory neuroblasts were established. Birthdate patterns within each ganglion are described with particular attention given to the structural organization of the VII-VIII ganglionic complex. Birthdate information on cranial autonomic ganglia is also included. Chick embryos ranging in age from 1 to 8 days of incubation were treated with 3H-thymidine and sacrificed on embryonic day 8, 10, or 18. Large, placode-derived neurons are generated between days 2 and 5 of incubation. Embryonically smaller, neural crest-derived cells leave the proliferative pool between days 4 and 7. Neurons of the acoustic ganglion cease their mitotic activity in an apical to basal fashion and are the only placodal neurons to form later than day 5. Of the cranial autonomic ganglia, the period of neuron production is best defined for the ciliary ganglion, where it is 2.5-5.5 days of incubation. Most later-dividing neuroblasts in the ciliary ganglion belong to the choroid cell population. Temporal patterns of neurogenesis are discussed in relationship to other aspects of sensory gangliogenesis including embryonic origin of neurons, condensation of ganglionic anlagen, cell degeneration, and cytological characteristics of mature ganglia.