Axons crossing in the ventral commissure express L1 and GAD65 in the developing rat spinal cord.

The neural cell adhesion molecule, L1, is thought to play a critical role in the formation and fasciculation of axon tracts during development. In the chick, the L1 cell adhesion molecule is expressed on both ipsi- and contralateral portions of commissural axons and perturbation studies produced a defasciculation of the ipsilateral commissural fibers. Yet in the rat, L1 is reported along commissural axons only after they have reached the contralateral marginal zone. When this species variation was reexamined, L1 was found to be expressed on rat commissural axons in a pattern similar to that observed in the chick. In addition, L1 is detected along commissural axons as early as embryonic day 12 in rats and maintained on both the ipsi- and contralateral surfaces during embryonic development. Other molecular markers that identify commissural axons in rats are TAG-1 (transiently expressed axonal glycoprotein) and DCC (deleted in colorectal cancer), and thus the pattern of L1 staining was compared with that of these other members of the immunoglobulin superfamily. Commissural axons emerging from dorsally located neurons are identified with TAG-1 and DCC, whereas L1 is detected only on ventrally located commissural axons. The pattern of L1 expression overlaps that of the more numerous laterally and ventromedially located GABAergic commissural axons. Furthermore, some of the GABAergic commissural axons express L1 on their surfaces. While commissural axons are often considered as a single population, differences in the combination of adhesion-type molecules on their surfaces and in their neurotransmitter phenotypes may signify distinctive neuronal subgroups.