POSTNATAL DEVELOPMENT OF HORIZONTAL AXONS IN THE INFERIOR TEMPORAL AND PRIMARY VISUAL CORTICES IN THE MONKEY.
O.X.Wang, H.Tanigawa, and I.Fujita
Patches formed by horizontal axon collateral of pyramidal cells in layer 2/3 represent a basic feature shared by many cerebral cortices. The size of the patches, and the periodicity and lateral extent of their distribution, however, differ among them. In the ventral visual pathway of the monkey, the values of these parameters increase from V1 through V2,V4, and are TEO to area TE. Here we analyzed the postnatal development of these patches in TE and V1. We made extracellular injections of an axon tracer, biotinylated dextran amine, into the TE and V1 of Macaca fuscata of different postnatal ages. At postnatal days(PND) 28-38, the earliest age examined, adult-like patches of horizontal axons were observed in both cortices. The two areas already differed from each other in respect to the size of the patches, and anisotropy, periodicity and lateral extent of the pathches as they did in adults. In both areas, the bouton density within the patches increased by 35-40% in 2- to 8-month-old monkeys compared to PND 28-38 monkeys, and thereafter returned to the PND 28-38 level. The number of retrogradely labeled cells within individual patches did not change systematically with age, suggesting that the change in bouton density cannot be attributed solely to a change in recurrent collaterals of the retrogradely labeled cells, and that it occurred in horizontal axons as well. Also the dynamic change in the bouton density was not accompanied by a change in the interbouton distance, and could most probably be attributed to a change in the branching frequency of horizontal axons. The results show that area-specific morphological characteristics of horizontal axons can be observed as early as PND 28, and that horizontal axons in areas V1 and TE undergo a concurrent developmental change in their bouton density after this age. (Supported by CREST)