AXON OVERPRODUCTION AND ELIMINATION IN THE ANTERIOR COMMISSURE OF THEDEVELOPING RHESUS-MONKEY

We have analyzed axon overproduction and elimination in the anterior c ommissure (AC) of 16 fetal, neonatal, and juvenile rhesus monkeys. Axo ns are added to the AC at an average rate of 115,000/day during the la st two-thirds of gestation, and growth cones are present in a constant proportion to AC axons throughout this period. The peak number of app roximately 11 million axons in the AC is reached at birth. Thereafter, axons are eliminated at a net rate of approximately 1 axon/sec during the first 3 postnatal months until the adult number of approximately 3.3 +/- 0.5 million axons is reached. Although there is considerable v ariability in AC axon number during the period of axon loss, the adult number of AC axons is relatively invariant among the eight adult rhes us monkeys examined. Increase in axon diameter and myelination begins before the major phase of axon elimination and is completed long after the adult number of axons is reached. Apparently, myelinated axons ar e not eliminated from the AC. Quantitative differences in the magnitud e and timing of axon overproduction and elimination in the AC versus t hat in the corpus callosum (LaMantia and Rakic [1990] J. Neurosci. 10: 2156) indicate specific modulation of the development of each commissu re, perhaps reflecting differences in the developmental history and fu nctional identity of the distinct cortical regions that give rise to t hem. This process of overproduction and elimination of AC axons during postnatal development in primates might contribute to individual vari ations in AC size correlated with a wide range of physical and behavio ral differences. (C) 1994 Wiley-Liss, Inc.