Prenatal development of calbindin immunoreactivity in the dorsal thalamus of the rat.

The distribution of calbindin immunoreactivity was studied in the developing rat dorsal thalamus at embryonic days 14, 16, 18 and 20. At early stages (days 14-16), calbindin is expressed throughout the dorsal thalamic cell mass. Most intense labeling occurs in cells adjacent to the ventricular surface, in a spatial gradient reflecting the well-known outside-in generation pattern. Between days 16 and 20, calbindin-positive periventricular cells are redistributed in the dorsal thalamus according to two different patterns. They first become oriented tangentially within the periventricular layer, and diminish in number at the central locus where midline thalamic fusion occurs at 18 days. Periventricular calbindin immunoreactivity becomes restricted to a ring of late-born cells surrounding the gray commissure. Recognizable portions of this ring-shaped primordium will mature forming n.paratenialis, n.reuniens, n.paraventricularis, and n.subparafascicularis magnocellularis. Simultaneously, a massive contingent of radially-oriented, fusiform, calbindin-positive young neurons extends from the periventricular ring-shaped aggregate to the lateral brain surface at the caudoventral pole of the dorsal thalamus at embryonic days 17/18. These cells surround the primordium of the medial geniculate body, participating in the constitution of its marginal zone, and invade the lateral posterior nucleus, accumulating within its caudomedial part. Other portions of this stream form the parvocellular subparafascicular nucleus and the peripeduncular nucleus. The observed patterns of calbindin expression suggest that dorsal thalamic postmitotic neurons transiently express the marker during initial phases of axogenesis, whereas a specific, late-born population expresses calbindin continuously into adulthood. This late subpopulation displays migratory behavior, and finally subdivides into several nuclei of the mature midline, superficial and posterior thalamus.