Lumbar motor neuron size and number is affected by age in male F344 rats

Motor neurons in the spinal cord of old rats appear similar in size but les s numerous compared with those in mature rats; they also contain a large am ount of lipofuscin, the lipid peroxidation by-product whose function is lar gely unknown. The object of this study was to morphometrically characterize motor neurons found in the L4/L5 lumbar spinal cord of mature (6-month) an d old (22-month) rats. Paraformaldehyde-fixed, lumbar spinal cords from six rats at each age were embedded in paraffin, sectioned at 6 mu m and staine d with 0.1% toluidine blue. The nucleolar diameter and area from a minimum of 34 motor neurons per spinal cord were measured. Motor neuron number was calculated using Abercrombie's (Abercrombie, 1946) formula after correcting for tissue shrinkage. Motor neuron number was decreased with age while the neuronal area increased with age. Nucleolar diameter also increased in old rats. Frequency distributions of motor neuron area revealed unimodal distr ibutions of motor neurons rats of both ages. We suggest that larger nucleol ar diameter reflects more metabolically active neurons in old rats while la rger neuron area is a reflection of the presence of lipofuscin in old motor neurons. (C) 1998 Elsevier Science ireland Ltd. All rights reserved.