Embryonic and Postnatal Development of the Layer I–Directed (“Matrix”) Thalamocortical System in the Rat

Abstract

Inputs to the layer I apical dendritic tufts of pyramidal cells are crucial in "top-down" interactions in the cerebral cortex. A large population of thalamocortical cells, the "matrix" (M-type) cells, provides a direct robust input to layer I that is anatomically and functionally different from the thalamocortical input to layer VI. The developmental timecourse of M-type axons is examined here in rats aged E (embryonic day) 16 to P (postnatal day) 30. Anterograde techniques were used to label axons arising from 2 thalamic nuclei mainly made up of M-type cells, the Posterior and the Ventromedial. The primary growth cones of M-type axons rapidly reached the subplate of dorsally situated cortical areas. After this, interstitial branches would sprout from these axons under more lateral cortical regions to invade the overlying cortical plate forming secondary arbors. Moreover, retrograde labeling of M-type cell somata in the thalamus after tracer deposits confined to layer I revealed that large numbers of axons from multiple thalamic nuclei had already converged in a given spot of layer I by P3. Because of early ingrowth in such large numbers, interactions of M-type axons may significantly influence the early development of cortical circuits.

Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats

“…This is based principally on the observations that barrel centers are discernible cytoarchitecturally earlier than septa (Rice and Van der Loos, 1977) and that VPM TCAs are present within infragranular layers at birth and rapidly (within ~1–2 days) form whisker-related clusters (Wise and Jones, 1978; Erzurumlu and Jhaveri, 1990; Senft and Woolsey, 1991; Agmon et al, 1993; Schlaggar and O’Leary, 1994; Catalano et al, 1996; Rebsam et al, 2002). Our data demonstrate that POm TCAs are also well-represented within infragranular layers by birth in both rats and mice, in agreement with a previous study of POm TCAs in developing rats (Galazo et al, 2007), and exhibit single-fiber morphology and a laminar innervation pattern very similar to those reported for VPM axons (Catalano et al, 1996; Rebsam et al, 2002; Lee et al, 2005). Thus, at birth, both sets of TCAs from POm and VPM nuclei are present and extend equally into the barrel cortex, which dispels the notion that there is a major temporal disparity in the arrival and positioning of the two sets of axons at the beginning of postnatal life.…”

Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats

“…This is based principally on the observations that barrel centers are discernible cytoarchitecturally earlier than septa (Rice and Van der Loos, 1977) and that VPM TCAs are present within infragranular layers at birth and rapidly (within ~1–2 days) form whisker-related clusters (Wise and Jones, 1978; Erzurumlu and Jhaveri, 1990; Senft and Woolsey, 1991; Agmon et al, 1993; Schlaggar and O’Leary, 1994; Catalano et al, 1996; Rebsam et al, 2002). Our data demonstrate that POm TCAs are also well-represented within infragranular layers by birth in both rats and mice, in agreement with a previous study of POm TCAs in developing rats (Galazo et al, 2007), and exhibit single-fiber morphology and a laminar innervation pattern very similar to those reported for VPM axons (Catalano et al, 1996; Rebsam et al, 2002; Lee et al, 2005). Thus, at birth, both sets of TCAs from POm and VPM nuclei are present and extend equally into the barrel cortex, which dispels the notion that there is a major temporal disparity in the arrival and positioning of the two sets of axons at the beginning of postnatal life.…”

“…Our second goal was then to describe the early postnatal development of POm innervation in both rats and mice. A recent study has described the development of POm TCAs in rats in the context of projections to layer I (Galazo et al, 2007); however, no studies have investigated the development of this circuitry in rats or mice in relationship to the development of the VPM projection to barrels.…”

Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats

“…This is based principally on the observations that barrel centers are discernible cytoarchitecturally earlier than septa (Rice and Van der Loos, 1977) and that VPM TCAs are present within infragranular layers at birth and rapidly (within ~1-2 days) form whisker-related clusters (Wise and Jones, 1978;Erzurumlu and Jhaveri, 1990;Senft and Woolsey, 1991;Agmon et al, 1993;Schlaggar and O'Leary, 1994;Catalano et al, 1996;Rebsam et al, 2002). Our data demonstrate that POm TCAs are also well-represented within infragranular layers by birth in both rats and mice, in agreement with a previous study of POm TCAs in developing rats (Galazo et al, 2007), and exhibit single-fiber morphology and a laminar innervation pattern very similar to those reported for VPM axons (Catalano et al, 1996;Rebsam et al, 2002;Lee et al, 2005). Thus, at birth, both sets of TCAs from POm and VPM nuclei are present and extend equally into the barrel cortex, which dispels the notion that there is a major temporal disparity in the arrival and positioning of the two sets of axons at the beginning of postnatal life.…”