The rostral and caudal boundaries of the diencephalon

Knowledge of nature and features of the boundaries between the main neural regions seems to be essential to understand the rules of brain regionalization. On the light of several current and classical criteria used to define cerebral boundaries, we examine the features of the places recognized as rostral and caudal boundaries in the developing diencephalon and provide new images about the glial features of these boundaries. One demonstrated property of some embryonic boundaries is the prevention of the crossing cells in the early ventricular zone (clonal restriction), while the intermediate zone seems to lack it. Data available so far indicate that the early boundary between diencephalon and mesencephalon (d/m) is a clonal restriction limit, but not between diencephalon and telencephalon (d/t). Later, while diencephalic nuclei form, cellular dispersion does not occur through the alar part of d/m, but it achieves in the corresponding d/t alar portion. The relationship between origin, migration, and cell-type specification of neural cells is being the object of special attention in the telencephalon, where specific cellular fenotipes can migrate to distant regions following non-radial routes. Such is the case of most GABAergic interneurons of avian and mammalian pallium and oligodendrocytes of the forebrain. In this regard, little attention has been devoted to the diencephalon, where this type of migration, specially those through the rostral boundary, has been reported by different authors. We introduce increasing evidence about non-conventional neuronal migration in the developing diencephalon and compare the reported migratory behavior with respect to both boundaries.