Abstract
Mitotic activity in the lumbosacral spinal cord of Rana pipiens larvae reaches an initial, minor peak during larval stage III (limb-bud) and a major peak during larval stage VI (foot paddle) after which it tapers off sharply, concomitant with the sealing together, in a dorso-ventral direction, of the majority of the spinal cord lumen. Without exception, mitotic activity is concentrated in the dorsal ependymal region, immediately surrounding the lumen. Cell division appears to occur in a diurnally rhythmic pattern with peaks of mitotic activity occurring at approximately 3:00 am and 3:00 pm . Treatment by immersion in 50 μ g/per liter of thyroxine results in a significant increase in mitotic activity in all regions of the spinal cord, with the proportionately greatest increase occurring in both dorsal and ventral extraependymal portions. Exposure to 0.02% thiourea results in diminished mitotic activity, especially in the ventral extra-ependymal portion. In addition to the stimulus provided by peripheral musculature and the seemingly intrinsic mitotic ability of ependymal cells, it is apparent that cells in the spinal cord are competent to respond by dividing to a third agent, thyroid hormone.
Published Version
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