The appearance and development of somatostatin-like immunoreactivity (SLI) in the peripheral nervous system of quail embryos were studied using radioimmunoanalysis and immunocytochemistry. In vivo, no SLI is observed in neural crest cells before or during migration. SLI appears between days 3 and 4 of incubation in sympathetic ganglia, immediately following ganglion formation, and between days 4 and 5 of incubation in the adrenal gland, soon after the adrenal gland primordium first appears. The development of SLI in the adrenal gland differs from that in the sympathetic ganglia. While in the former the amount of SLI and the number of SLI-containing cells increase as the embryo ages, in the sympathetic ganglia the amount of SLI and the percentage of SLI-containing cells decrease. When migrating neural crest cells are obtained from the sclerotomal part of 3-day embryos and grown in culture, they first display SLI after 48 hr, and the amount of SLI increases thereafter. When the sympathoadrenal precursors are removed at 4 days of incubation and grown in vitro, SLI appears after 24 hr in culture and increases during the next few days. Our results demonstrate that SLI is present very early in the quail embryo and that its appearance parallels the differentiation of neural crest cells into autonomic sympathetic ganglionic cells. We also show that the differentiation of neural crest into SLI-containing cells can be reproduced in culture, thus permitting the study of peptide production and expression in vitro.

The developmental coexpression of galanin-like immunoreactivity with the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH) was studied in the avian embryo sympathoadrenal system using double-labeling immunocytochemistry. Galanin-like immunoreactivity is expressed by various catecholaminergic cell populations, namely sympathoblasts, chromaffin and small intensely fluorescent (SIF) cells, but not by principal neurons of the paravertebral sympathetic ganglia. Both galanin and somatostatin immunoreactivities are coexpressed in the adrenal and sympathetic ganglion primordia by the neural precursors, but the subsequent expression pattern of both peptides differs. Our results support the hypothesis that early sympathoblasts express a large repertoire of neuroactive substances and that the expression of these becomes restricted during further development as the sympathoblasts become principal neurons.