Development of primate retinogeniculate axon arbors.

In this study we examine the postnatal development of retinogeniculate axons projecting to the magnocellular (M axons), parvocellular (P axons), and koniocellular (K axons) layers of the lateral geniculate nucleus (LGN) in the prosimian primate, Galago crassicaudatus, in order to: (1) understand how individual retinogeniculate axons in primates mature postnatally, and (2) determine whether differences exist in the development of separate classes of axons that are known to be presynaptic to physiologically distinct cells in adults. In galagos, magnocellular, parvocellular, and koniocellular LGN layers contain Y-, X-, and W-like physiological cell classes, respectively (Norton & Casagrande, 1982). In vitro and in vivo optic tract bulk injections of horseradish peroxidase (HRP) were made in animals ranging in age from the day of birth (P0) to adulthood. Two hundred and fifty axonal arbors were completely reconstructed from serial sections and examined qualitatively for general features of maturity and compared quantitatively for changes in shape, arbor width, area, volume, bouton number, and bouton density. Our results confirm that in adult galagos M arbors are large and radially symmetric; P arbors are medium sized and elongated perpendicular to layer borders; K arbors are small and generally oriented parallel to layer borders. At birth, M, P, and K arbors, although still distinct and confined to layers, are qualitatively and quantitatively immature. Both the pattern and pace of maturation differ between classes. Overall, M arbors mature before P arbors which in turn mature before K arbors. Within classes, arbors representing central vision appear to develop about a week ahead of those representing peripheral vision; no differences are evident between the development of crossed and uncrossed arbors. In no case do arbors exhibit a period of postnatal exuberance, wherein arbors are larger than those of the adult as reported for cat X retinal axons. However, in width and bouton density P arbors are mature at P0 and thus occupy relatively more space in the nucleus compared to adult. All arbors mature rapidly and appear adult-like by the 4th or 5th postnatal week.