Abstract

Although the extraordinary size and idiosyncratic nature of Betz cells are well documented (Betz, 1874; Scheibel & Scheibel, 1978), little quantitative data exist with regard to their developing dendritic systems. The present investigation compares basilar/horizontal dendritic and spine systems in Betz cells and layer V pyramidal cells in infant human motor cortex (Brodmann's area 4). Subjects consisted of four neurologically normal infants (3, 9, 48, and 105 days of age). Tissue blocks were removed from the dorsolateral convexity of the left precentral gyrus near the vertex of the hemisphere. Ten Betz cells and ten layer V pyramidal cells per tissue block (N=80) were prepared by a modified rapid Golgi technique (Scheibel & Scheibel, 1978) and quantified on a Neurolucida computer/microscope interface system (Microbrightfield, Inc.). As expected, the somata of Betz cells (1004 µm2) were more than twice as large in cross-sectional surface area as those of layer V pyramidal cells (482 µm2). Although a section thickness of 120µm resulted in considerable attenuation of segment length, Betz cells remained substantially more complex than the layer V pyramidal cells. A nested analysis of variance revealed significantly greater dendritic and spine measures for Betz cells over layer V pyramidal cells. In particular, total dendritic length was 53% greater in Betz cells; dendritic spine number was 44% greater. These findings supplement qualitative observations (Conel, 1941) and indicate that, like other layer V cells in the human motor cortex (Koenderink & Uylings, 1995), Betz cells mature very rapidly, surpassing layer V pyramidal neurons in all dendritic and spine measures early in the perinatal period.

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