TRANSIENT AXONAL BRANCHING IN THE DEVELOPING CORPUS-CALLOSUM

During development, there is a transient overproduction of axons in th e corpus callosum; this overproduction of axons is due, in part, to a transient excess of neurons that send an axon through the corpus callo sum. However, transient axonal branching could also contribute to the developmental overproduction of callosal axons. To investigate this po ssibility, we filled developing callosal axons in the Syrian hamster w ith the carbocyanine dye -dioctadecyl-3,3,3',3'-tetramethylindocarbocy anine perchlorate (Dill. Light microscopic analysis showed that, indee d, developing callosal axons branch transiently in the hamster: branch ing was robust on postnatal day [1 (PO) and P3 (PO = the first 24 hr a fter birth), less prominent on P6 and P8, and absent by P11. Immature callosal axons branched before or after crossing the midline and at al l rostral-caudal and medial-lateral levels within the corpus callosum. The majority of callosal axon collaterals that were contained within individual 100-mu m-thick sections were relatively short (mean = 15.1 mu m) but some collaterals extended up to similar to 135 mu m from the main axon trunk before passing out of the section in which tNearly al l of the collaterals emanated from the main axon trunk; higher-order c ollaterals were rare. Some callosal axon trunks had multiple collatera ls. Branching callosal axons originated from multiple cortical areas, including area 17. Electron microscopic observations indicated that th e processes designated as axon collaterals by light microscopic criter ia would have been included in electron microscopic counts of developi ng callosal axons. Some callosal axon trunks and branches had ultrastr uctural features that suggested they were degenerating. In cats, devel oping callosal axons branch on embryonic day 57 (E57; the first 24 hr after conception = EO) and PO. Thus, it is likely that transient branc hing of immature callosal axons is a generalized feature of mammalian cortical development and that it contributes to the overproduction of callosal axons, albeit perhaps to varying degrees, in multiple species .