Early ingrowth of thalamocortical afferents to the neocortex of the prenatal rat.

TLDR

The study examined early thalamocortical ingrowth in fetal rat cortex by labeling thalamic fibers with the carbocyanine dye DiI. Thalamocortical afferents reach the neocortex by embryonic day 16, extend radially into layers VI and V by day 17, form a dense plexus at birth, and arborize within forming cortical layers without a waiting period.

Abstract

The initial ingrowth of thalamocortical afferents into the presumptive somatosensory cortex was examined in the fetal rat. Thalamic fibers were labeled in fixed brains with the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). On embryonic day 16, thalamocortical afferents arrive in the neocortex and course tangentially within the intermediate zone immediately underneath the cortical plate. By embryonic day 17, thalamocortical fibers have begun their radial growth into cortex and their arbors span the cell-sparse zone between layer VIb and the bottom of the cortical plate. By the day of birth (embryonic day 21), thalamocortical fibers from a dense plexus within layers VI and V below the dense cortical plate. Our observations indicate that in the rat thalamic afferents arrive in the cortex at a very early age and arborize within the forming cortical layers without an apparent "waiting" period.