EXPRESSION OF THE CELL-ADHESION MOLECULE AXONIN-1 IN NEUROMERES OF THE CHICKEN DIENCEPHALON

Axonin-1/TAG-1, a member of the immunoglobulin (Ig) superfamily of adh esion molecules, has been shown to be selectively expressed by a subse t of neurons and fiber tracts in the developing nervous system of vert ebrates. Axonin-1/TAG-1 is thought to play a role in the outgrowth, gu idance, and fasciculation of neurites. In the present study, we map th e expression of axonin-1 in the diencephalon of the chicken brain at e arly and intermediate stages of development [2-8 days of incubation; e mbryonic day (E)2-E8] by immunohistochemical methods. Results show tha t axonin-1 is first expressed at about E2.5 by postmitotic neurons sca ttered throughout most of the diencephalon. During the neuromeric stag e of brain development (about E3-E5), axonin-1(+) nerve cell bodies ar e predominantly found in two neuromeric subdivisions: 1) in the alar p late of the precommissural pretectum and dorsal thalamus and 2) in the posterior preoptic region of the hypothalamus. The axonin-1(+) fiber bundles emerging from these areas grow across segmental boundaries. Fo r example, axonin-1(+) neurites originating in the dorsal thalamus cro ss the zona limitans intrathalamica at a right angle to project to the striatum. Later, the axonin-1(+) neuromere areas give rise to particu lar axonin-1(+) gray and white matter structures. Most of these struct ures correspond to the structures described to express TAG-1 in rodent s. In conclusion, axonin-1 can be used as a marker to study aspects of the transition from the early neuromeric structure to the mature anat omy of the chicken brain. (C) 1997 Wiley-Liss, Inc.