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 i n the formation and fasciculation of axon tracts during development. In the chick, the L1 cell adhesion molecule is expressed on both ipsi- and contra lateral portions of commissural axons and perturbation studies produced a d efasciculation of the ipsilateral commissural fibers. Yet in the rat, tl is reported along commissural axons only after they have reached the contrala teral marginal zone. When this species variation was reexamined, L1 was fou nd to be expressed on rat commissural axons in a pattern similar to that ob served 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 con tralateral surfaces during embryonic development. Other molecular markers t hat identify commissural axons in rats are TAG-1 (transiently expressed axo nal glycoprotein) and DCC (deleted in colorectal cancer), and thus the patt ern of L1 staining was compared with that of these other members of the imm unoglobulin superfamily. Commissural axons emerging from dorsally located n eurons are identified with TAG-1 and DCC, whereas L1 is detected only on ve ntrally located commissural axons, The pattern of L1 expression overlaps th at of the more numerous laterally and ventromedially located GABAergic comm issural axons. Furthermore, some of the GABAergic commissural axons express L1 on their surfaces. While commissural axons are often considered as a si ngle population, differences in the combination of adhesion-type molecules on their surfaces and in their neurotransmitter phenotypes may signify dist inctive neuronal subgroups. Copyright (C) 2000 S. Karger AG, Basel.