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 morpholog y of this commissure and its development in a diprotodontid metatheria n, the wallaby (Macropus eugenii), at both the light and electron micr oscope level. The total number of axons in the adult anterior commissu re was 21.7 million, of which 55-62% were myelinated. The dorsal two t hirds of the commissure, containing neocortical commissural axons, sho wed a higher percentage of larger, myelinated axons than the ventral o ne third, which contains paleocortical commissural axons. The commissu re also showed a topographical gradient, with cells in the dorsal cort ex projecting through the dorsal region of the commissure, the fascicu lus aberrans. In the rostrocaudal axis, axons from the frontal cortex tended to pass more anteriorly through the commissure and those from t he occipital more posteriorly, but there was extensive overlap of proj ections from different areas. The gestation length of this wallaby is 28.3 days, and all commissural development occurs postnatally. The ant erior 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. Pe ak axon counts of 50 to 63 million occurred between P100 and P150. The number of growth cones in a single midline section peaked at approxim ately P114 (480,000) and dropped to 0 by P170. The distribution of gro wth 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 ass ociation of growth cones with the periphery of the commissure or glial structures at any of the three ages examined. The results show that a xonal overproduction and regression in cortical commissural connection s are features of development in diprotodontid metatheria, as in euthe ria. (C) 1996 Wiley-Liss, Inc.