Transient Axonal Branching in the Developing Corpus Callosum

During development, there is a transient overproduction of axonsin the corpus callosum; this overproduction of axons is due,in part, to a transient excess of neurons that send an axonthrough the corpus callosum. However, transient axonal branchingcould also contribute to the developmental overproduction ofcallosal axons. To investigate this possibility, we filled developingcal-losal axons in the Syrian hamster with the carbocyaninedye 1, 1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocy-anineperchlorate (Dil). Light microscopic analysis showed that, indeed,developing callosal axons branch transiently in the hamster:branching was robust on postnatal day 0 (P0) and P3 (P0 = thefirst 24 hr after birth), less prominent on P6 and P8, and absentby P11. Immature callosal axons branched before or after crossingthe midline and at all rostral-caudal and medial-lateral levelswithin the corpus callosum. The majority of callosal axon collateralsthat were contained within individual 100-µm-thick sectionswere relatively short (mean = 15.1 µm) but some collateralsextended up to