Development of commissural neurons in the wallaby (Macropus eugenii)

Abstract

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.

GAP-43 Immunoreactivity in the brain of the developing and adult wallaby ( Macropus eugenii )

“…From P25 on, intense GAP-43 ir appeared in the deep intermediate zone, presumably reflecting the progressive addition of association and commissural fibers to this region, which begins at P14 (Shang et al 1997). This is still notable as late as P73 and is consistent with the prolonged addition of ascending and descending connections found with studies of visual and somatosensory cortex (Sheng et al 1990(Sheng et al , 1991.…”

“…Note low GAP-43 ir in layers II and IV of V1at this age immunoreactivity. Immunoreactive fibers in the deeper intermediate layer correspond to the addition of long association (Sheng et al 1990) and commissural cortical connections (Ashwell et al 1996b;Shang et al 1997). By P73 GAP-43 ir was uniform throughout the loose plexiform zone, which at this age contains cortical layers IV to VI .…”

Development of body, head and brain features in the Australian fat-tailed dunnart (Sminthopsis crassicaudata; Marsupialia: Dasyuridae); A postnatal model of forebrain formation

“…In eutherians, most interhemispheric projections from the isocortex and cingulate cortex cross the midline through the corpus callosum, while in non-eutherians these projections course through the anterior commissure. In dunnarts, as in other marsupials, the anterior commissure forms at least one week before the hippocampal commissure, while in eutherians they arise closer in time, with the corpus callosum being the last telencephalic commissure to form [ 6 , 19 , 29 – 31 ]. Interestingly, dunnarts also lack the astrogliogenic remodelling of the septal midline, a process required for callosal formation in eutherians [ 32 , 33 ] that could relate to heterochronies in the formation of interhemispheric circuits in the brain of mammals.…”

Reduced Gyral Window and Corpus Callosum Size in Autism: Possible Macroscopic Correlates of a Minicolumnopathy

“…It is during the perinatal period that the different subregions of the corpus callosum are sculpted by selective elimination (Innocenti 1986; Shang et al 1997) and lengthened in proportion to corresponding sites of cortical development (Moses et al 2000). Abnormal postnatal retractive events happening during the maturation of the corpus callosum and other long cortico-cortical projections could account for the natural history of some autistic patients that manifest normal development followed by loss of skills and onset of symptomatology.…”