THE DEVELOPMENT OF NORMAL AND PERIPHERALLY DEPRIVED SYMPATHETIC NEURONS IN THE CHICK

Sympathetic neuron numbers in four brachial paravertebral ganglia (seg ments 13-16) were quantified during normal development and following w ing bud removal. In ganglia with an intact peripheral field, different developmental profiles were observed. In particular, a period of incr easing neuron numbers was seen in some, but not all ganglia from embry onic day 10. Similarly, a period of declining neuron numbers was prese nt only in two ganglia, and this occurred at a time when sympathetic n erve fibres were detectable in the wing. Peak pyknotic activity occurr ed well before nerve fibres entered the wing. Thus, the bulk of the ce ll death is unrelated to peripheral innervation. Wing bud removal at e mbryonic day 3 reduced neuron number by as much as 69% at embryonic da y 20 compared to the number in the corresponding ganglia on the intact side. The most obvious effect was the failure to generate the full co mplement of neurons. This could be explained by a shift in pyknotic ac tivity to a period of 1-2 days earlier than seen on the intact side. W e therefore conclude that the developing wing influences the size of t he sympathetic population both before and during the period of periphe ral innervation.