Anterior commissure of the wallaby (Macropus eugenii): adult morphology and development

In metatheria, all neo- and paleo-cortical commissural connections are made by the anterior commissure. We have examined the adult morphology of this commissure and its development in a diprotodontid metatherian, the wallaby (Macropus eugenii), at both the light and electron microscope level. The total number of axons in the adult anterior commissure was 21.7 million, of which 55-62% were myelinated. The dorsal two thirds of the commissure, containing neocortical commissural axons, showed a higher percentage of larger, myelinated axons than the ventral one third, which contains paleocortical commissural axons. The commissure also showed a topographical gradient, with cells in the dorsal cortex projecting through the dorsal region of the commissure, the fasciculus aberrans. In the rostrocaudal axis, axons from the frontal cortex tended to pass more anteriorly through the commissure and those from the occipital more posteriorly, but there was extensive overlap of projections from different areas. The gestation length of this wallaby is 28.3 days, and all commissural development occurs postnatally. The anterior commissure first appeared at P (postnatal day) 14, at which time commissural fibres were apparently derived from the external capsule exclusively. Commissural fibres passing through the internal capsule, and joining the anterior commissure via the fasciculus aberrans, were first noted at P18. By that age there were 94,000 to 161,000 axons. Peak axon counts of 50 to 63 million occurred between P100 and P150. The number of growth cones in a single midline section peaked at approximately P114 (480,000) and dropped to 0 by P170. The distribution of growth cones was analysed during the early stages of anterior commissure development (P18, P30, P82). At P18, growth cones were concentrated in the dorsal parts of the commissural bundle, suggesting a ventrodorsal sequence of addition of axons. There was no apparent preferential association of growth cones with the periphery of the commissure or glial structures at any of the three ages examined. The results show that axonal overproduction and regression in cortical commissural connections are features of development in diprotodontid metatheria, as in eutheria.