The disposition of early-generated neurons in the rat embryo predicts the pattern of major axonal tracts.
During embryogenesis, the fiber tracts grow in a highly stereotyped pattern. A very small number of predetermined paths, preceding the growth of fasciculi, are present in the young neural tube (10-12, 15). What is the origin of these substrate pathways defined by Katz et al. (16) as "... a set of similar guidance cues which are aligned in a continuous discrete pathway..."? Could the first neurons play a role in the guidance of early nerve fibers? Observations in the brain stem revealed the presence of two longitudinal columns of early-generated neurons. These longitudinal columns were associated with well-differentiated marginal zones, characterized by cell-free spaces and representing the prospective site of the medial longitudinal (mlf) and lateral longitudinal (llt) tracts. Nerve fibers were also traced in the brain stem of young embryos. Axons were seen to travel in the early mlf and llt, in close proximity to the regions of early-generated neuronal columns. The data suggest that the precocious neurons that are organized in a definite pattern could somehow be involved in the guidance of some longitudinal axonal tracts, either by directly promoting the formation of an adequate terrain in the marginal layer, or by inducing other cells to do so.
Bélanger
M C
MC
Centre de Recherche en Neurobiologie, Hôpital de l'Enfant-Jésus, Québec, Canada.
Auclair
F
F
Bertrand
L
L
Marchand
R
R
D000818:Animals; D001369:Axons / Q000502:physiology*; D001933:Brain Stem / Q000166:cytology; D004622:Embryo, Mammalian / Q000502:physiology*; D005314:Embryonic and Fetal Development / Q000502:physiology; D005865:Gestational Age; D009412:Nerve Fibers / Q000502:physiology; D009474:Neurons / Q000648:ultrastructure*; D051381:Rats