Differentiation of sympathetic and enteric neurons of the fowl embryo in grafts to the chorio-allantoic membrane

Abstract

Sympathetic cells (adrenergic neurons, SIF cells and chromaffin cells) and enteric neurons differentiate from migratory cells derived from the neural crest. The development of these cell types was studied in chorio-allantoic membrane (CAM) grafts, using combinations of tissue from domestic fowl embryos. Neural anlagen (neural tube and crest) of the vagal, cervico-thoracic and lumbo-sacral axial levels were equally capable of sympathetic differentiation, but this required somitic tissue for its significant expression. However, the vagal somites possessed only slight sympathogenic activity, thereby accounting for the negligible contribution of the vagal neural crest to the sympathetic nervous system. The same three levels of the neural anlage could furnish enteric neurons when combined directly with the aneuronal colo-rectum. However, the scale of this line of differentiation varied with the level of origin of the neural anlage, in contrast to the apparent equivalence in the ability to diffentiate as sympathetic cells. The density of enteric neurons in combinations with the vagal neural anlage was estimated as 60 times greater than the neuron density in combinations with the cervico-thoracic neural anlage. The lumbo-sacral neural anlage gave results similar to those of the cervico-thoracic level. Moreover, neural crest-derived pigment cells, positioned ectopically in the wall of the colo-rectum, were rare in combinations with the vagal neural anlage, but common in grafts with the other levels. When tested physiologically, the colo-rectum grown with the vagal neural anlage showed non-adrenergic, non-cholinergic inhibitory nervous activity in addition to the expected cholinergic excitatory responses. The neurons derived directly from vagal neural anlagen were similar to those that had reached the colo-rectum via their normal migratory pathways, when studied in terms of histological appearance, density of distribution and physiological responses.