THE POSTNATAL-DEVELOPMENT OF GENICULOCORTICAL AXON ARBORS IN OWL MONKEYS

To characterize the postnatal development of geniculocortical axon arb or morphology in owl monkeys at a series of ages from birth to adultho od, individual arbors were bulk-filled with HRP in brain slice prepara tions and were reconstructed from serial sections. At all ages, cortic al layers and sublayers were obvious. Presumed M or magnocellular arbo rs were largely confined to layer IV alpha, but they also extended int o layer IIIc (IV beta of Brodmann, 1909); presumed P or parvocellular arbors were almost exclusively confined to layer IV beta. Other axons that may reflect feedback projections from MT terminated in layer IIIc . Overall, M axon arbors increased in size and complexity from birth t o adulthood with mean surface-view arbor areas ranging from 0.08 +/- 0 .01 mm(2) in newborns to 0.24 +/- 0.02 mm(2) in adults. The developing P arbor areas were, on average, as large or larger than adult (newbor n = 0.07 +/- 0.01 mm(2), adult = 0.047 +/- 0.01 mm(2); n.s.) but the a rbors were somewhat less complex. Since the brain and area 17 increase in size postnatally, the proportion of area 17 subserved by each P ar bor would decrease in postnatal development. Terminal boutons with imm ature features were evident in both M and P populations at all develop mental ages. The results indicate that, while both LGN axon types in m onkeys undergo morphological changes postnatally, M arbors appear to m ature by increasing arbor size and terminal branching complexity, wher eas P arbors increase in complexity but not in size. These distinct pr ograms of axon arbor development suggest that the periods of susceptib ility of geniculocortical axon arbors to postnatal influences of the e nvironment, and the types of plastic responses they potentially exhibi t, are class-specific.