Ephemeral cellular segmentation in the thalamus of the neonatal rat.

The distribution of thalamocortical relay cells in the rat ventrobasal complex was studied during the early postnatal period using the retrograde transport of horseradish peroxidase from the parietal cortex. It was found that the relay cells undergo marked changes in their distribution during the first two postnatal weeks. On postnatal days (PNDs) 0 and 1, the cells are rather homogeneously distributed throughout the ventrobasal complex. However, by PND 2, and more clearly by PND 3, the cells form a distinctly segmented pattern. This pattern consists of discrete curvilinear arrays of cells extending throughout most of the rostrocaudal extent of the nucleus. This distinct cellular pattern is present from PND 2 to about PND 8. In animals sacrificed on PND 15 or as adults, the pattern is no longer obvious. The cellular pattern seen at PND 8 was examined in the 3 standard planes of section and compared to both the somatotopic organization of the nucleus and to the organization of its major ascending and descending inputs. The developmental time course of the cellular segmentation was related to that of the lemniscal and corticothalamic afferents, which also show ephemeral segmentation patterns during the first two postnatal weeks.