Development of the laminar distribution of thalamocortical axons and corticothalamic cell bodies in the visual cortex of the wallaby

The distribution of afferents from the dorsal lateral geniculate nucleus (LGNd) and the lateral posterior nucleus (LP) and of cell bodies projecting to these nuclei has been studied in the visual cortex of the wallaby (Macropus eugenii) throughout development to determine how the characteristic laminar distribution of afferents and efferents of the mature cortex is achieved. Young are born after 26-28 days of gestation and do not open their eyes until around 140 days after birth. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected in the visual thalamus in adults and in pouch young aged from 22 days after birth, just after thalamic axons first reach the visual cortex, to 118 days, when cortical lamination resembles the adult. From 22 to 65 days, the developing visual cortex consists of a marginal zone (MZ), cortical plate (CP), and intermediate zone (IZ) including the superficial subplate (SP), subventricular zone, and ventricular zone. There is a thin compact cell zone (CCZ) at the top of the CP and below it a less densely packed region that increases in thickness with age. Retrogradely labelled cells in two bands were first seen at 40 days, one in the CCZ and the other at the base of the CP. Two bands of cells were seen at all subsequent times if the injection covered both LGNd and LP, and by 76 days, these cells were located within cytoarchitectonically recognizable layers V and VI. Anterograde label prior to 45 days was distributed densely and evenly throughout the IZ and the CP up to the CCZ. Label in MZ was first seen at 25 days and was substantial by 54 days. Anterograde label then became gradually reduced in the IZ, whereas in the CP it remained evenly and densely distributed until 82 days. At this age, coincident with the emergence of layer IV, label within the CP first showed variations in density and by 99 days was concentrated over layer IV and, to a lesser extent, over layer VI. By 118 days label resembled the adult after injections covering both LGNd and LP, with label concentrated in layer I, IV, and VI with a less dense projection to lower layer III and upper layer V. There is a relatively earlier initial ingrowth of axons into the visual cortex in the wallaby and throughout development thalamocortical axons appear to be more widely distributed in the depth of the visual cortex than has been demonstrated for placental mammals. They are densely distributed up to the thin CCZ of the CP long before lamination of the cortex emerges beneath this zone. Soon after retrogradely labelled cells are first seen, the two bands of these cells that project to the LGNd and LP and ultimately reside in layers V and VI are also located in the region below the CCZ occupied by thalamocortical afferents. The question arises as to whether there is a waiting period in the SP in the wallaby. If there is not, this may be a genuine difference with placental mammals or a result of the more sensitive technique used here to demonstrate thalamocortical afferents.