The development of normal and peripherally deprived sympathetic neurons in the chick.

Sympathetic neuron numbers in four brachial paravertebral ganglia (segments 13-16) were quantified during normal development and following wing bud removal. In ganglia with an intact peripheral field, different developmental profiles were observed. In particular, a period of increasing neuron numbers was seen in some, but not all ganglia from embryonic day 10. Similarly, a period of declining neuron numbers was present only in two ganglia, and this occurred at a time when sympathetic nerve fibres were detectable in the wing. Peak pyknotic activity occurred well before nerve fibres entered the wing. Thus, the bulk of the cell death is unrelated to peripheral innervation. Wing bud removal at embryonic day 3 reduced neuron number by as much as 69% at embryonic day 20 compared to the number in the corresponding ganglia on the intact side. The most obvious effect was the failure to generate the full complement of neurons. This could be explained by a shift in pyknotic activity to a period of 1-2 days earlier than seen on the intact side. We therefore conclude that the developing wing influences the size of the sympathetic population both before and during the period of peripheral innervation.