Transient axonal branching in the developing corpus callosum

During development, there is a transient overproduction of axons in the corpus callosum; this overproduction of axons is due, in part, to a transient excess of neurons that send an axon through the corpus callosum. How-ever, transient axonal branching could also contribute to the developmental overproduction of callosal axons. To investigate this possibility, we filled developing cal-losal axons in the Syrian hamster with the carbocyanine dye 1,1-dioctadecyl-3,3,3',3'-tetramethylindocarbocy-anine perchlorate (Dil). Light microscopic analysis showed that, indeed, developing callosal axons branch transiently in the hamster: branching was robust on postnatal day 0 (PO) and P3 (PO = the first 24 hr after birth), less prominent on P6 and P8, and absent by P11. Immature callosal axons branched before or after cross