Size-related increase in motoneuron number: evidence for late differentiation.
The number of motoneurons in the lumbar lateral motor column (LMC) was compared in bullfrogs (Rana catesbeiana) ranging in body length from 2.5 to 19 cm. Large frogs had 36% more motoneurons than small frogs; however, within the caudal third of the LMC, large frogs had over 70% more motoneurons than small frogs. Injection of small frogs with [3H]thymidine every third day for 20-22 weeks gave no evidence of motoneuron birth. Instead, a pool of small, incompletely differentiated (type L) motoneurons appears to be converted into mature (type M) motoneurons as the animal grows. This hypothesis is supported by several lines of evidence: (1) the number of type-M motoneurons varies directly with body size while the number of type-L cells varies inversely; (2) the increase in type-M motoneurons and the decrease in type-L cells are restricted to the same regions of the LMC; and (3) type-L cells exhibited both immunoreactivity to neurofilament antibodies and histochemical evidence of acetylcholinesterase activity, a marker for spinal motoneurons.
Department of Physiology, School of Medicine, University of North Carolina, Chapel Hill 27599.
P B
PB
Farel
S E
SE
Wray
M L
ML
Meeker
D000110:Acetylcholinesterase / Q000032:analysis; D000818:Animals; D002452:Cell Count; D002454:Cell Differentiation / Q000502:physiology; D006651:Histocytochemistry; D008159:Lumbar Vertebrae / Q000294:innervation*; D009046:Motor Neurons / Q000166:cytology*; D011892:Rana catesbeiana / Q000033:anatomy & histology*; D015203:Reproducibility of Results; D013936:Thymidine / Q000378:metabolism; D013997:Time Factors
D000110:Acetylcholinesterase; D013936:Thymidine