Development of commissural neurons in the wallaby (Macropus eugenii)

We have examined the development of the laminar and areal distribution of cortical commissural neurons in a marsupial mammal, the wallaby Macropus eugenii. In this species, commissural axons approach the major cerebral commissure, the anterior commissure, via either the internal capsule or the external capsule and first cross the midline at postnatal day 14 (P14). By retrogradely labelling these axons with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI) at P15, we show here that the cell bodies of these neurons are restricted to a region of cortex adjacent to the rhinal fissure. Most of these labelled neurons are located in the compact cell zone of the cortical plate, with only a few labelled cells found in the zone of loosely packed cells deep to this layer. Over the subsequent 66 days, commissural neurons are found progressively more dorsally, rostrally, and caudally, so that, by P80, they are present throughout the extent of the neocortex. At this age, they are mainly pyramidal in morphology and form a single band within the deeper part of layer 5 of the developing cortex. From P80 to adulthood, the distribution of commissural neurons has been assessed in the visual cortex by using retrograde transport of horseradish peroxidase. At P80, labelled neurons with immature pyramidal morphology are present throughout the occipital cortex; as in DiI material, somata are located in deep layer 5. At P165, previously shown to be the age when commissural axon numbers peak, widespread labelling is present in the occipital region, with labelled cells now found in two bands corresponding to layers 3 and 5. After this age, neurons become more restricted in distribution, so that, by adulthood, commissural neurons are no longer apparent throughout area 17 but are restricted to a localised region around the area 17/18 boundary. Within this region, labelling is still present in layers 3 and 5 but is more dense in layer 3. The gradual restriction of commissural fields seen here in the wallaby is similar to that reported in the neocortex in many eutherians. These findings also support studies in eutheria, suggesting that subplate neurons do not appear to play a major role in commissural development.