The levels of the neural axis from which parasympathetic and orthosympathetic neurons and adrenomedullary cells are derived under normal developmental conditions were determined in avian embryos by a biological labeling technique. The technique is based on nuclear differences between two species of birds, the chick and the quail. In quail interphase nuclei a part of the chromatin is condensed in large heterochromatic masses associated with nucleolus, while in the chick, DNA is evenly dispersed in the nucleoplasm. These characteristics provide a stable nuclear marker that can be used to study cell migrations and differentiation in chimeric embryos resulting from the association of quail and chick tissues. Isotopic and heterotopic transplantations of quail neural primordium into chick before the outset of neural crest cell migration show that the autonomic ortho- and parasympathetic neuroblasts are not determined to differentiate into cholinergic or adrenergic neurons when they begin to migrate. The neurotransmitter synthesized by crest autonomic neuroblasts depends on the microenvironment in which crest cells become localized at the term of their migration. The splanchnic mesoderm induces presumptive adrenergic cells to become fully differentiated cholinergic neurons.

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Latissimus dorsi muscles of the chick consist of a slow (ALD) and a fast (PLD) muscle. The influence of chronic spinal cord stimulation in the chick embryo upon the expression of myosin light chains and tropomyosin subunits was investigated. Early in development the two muscles exhibited the same ratio of alpha- and beta-tropomyosin subunits. Later, in the slow muscle the ratio beta:alpha decreased and in chicken the amounts of the two components were about the same. In the fast muscle, the alpha-subunit increased and reached 66% in young chicken. In the fast muscle, the alpha-subunit increased and reached 66% in young chicken. In the In the early stages of embryonic development, both muscles accumulated slow and fast light chains. However, in ALD the amount of slow light chains was greater than that of fast light chains and the reverse was observed in PLD muscle. Later during development, the slow components decreased in PLD while the fast components increased; the reverse was observed in ALD muscle. The fast myosin LC3f has been detected in 18-day-old embryonic PLD. Chronic spinal cord stimulation at a low rhythm was performed from day 10 of embryonic development to day 15 or 16. In both muscles from spinal cord-stimulated embryos, the beta-tropomyosin subunit was lower than in control embryos. In ALD, the pattern of light chains was unaffected by chronic stimulation while in PLD muscle the slow and fast components were modified. In particular the ratio LCs:LCf was increased in spinal cord-stimulated embryos with regard to controls.

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