Medial gastrocnemius motor nucleus in the rat: age-related changes in the number and size of motoneurons.
The age-related alterations in the number and size of alpha- and gamma-motoneurons were studied in the medial gastrocnemius (MG) motor nuclei in rats at four ages: young (5 months), middle aged (10-13 months), old (26 months), and very old (31 months). Small volumes (0.1-0.5 microliter) of 40% horseradish peroxidase (HRP) solution were injected into the cut MG nerve bilaterally by using glass micropipettes and a pressure injection system. The number, position, and soma size (average soma diameter) of MG motoneurons were determined by using photographic maps of each TMB-stained section. The total number of myelinated axons was counted in seven MG nerves from the same animals. The average soma diameters in each MG nucleus were distributed bimodally; cells with average diameter greater than 21.0-24.0 micron were presumed to be alpha-motoneurons and those with smaller diameters were presumed to be gamma. The mean number of presumed alpha-motoneurons was significantly less in the old and very old groups as compared with the young and middle-aged. In contrast, the number of presumed gamma-motoneurons was the same across age groups. The mean average soma diameter of both alpha- and gamma-motoneurons was smaller in the old animals. The apparent decrease in the total number of labeled motoneurons in old animals was also reflected in a decrease in myelinated axon counts. We conclude that there is a significant decrease in the absolute numbers of motoneurons in rats aged 26 months and older, with most of the decrease occurring among the larger alpha-motoneurons.
Hashizume
K
K
Department of Kinesiology, Tokyo Metropolitan Institute of Gerontology, Japan.
Kanda
K
K
Burke
R E
RE
D000375:Aging / Q000502:physiology*; D000818:Animals; D002452:Cell Count; D006735:Horseradish Peroxidase; D008297:Male; D009046:Motor Neurons / Q000145:classification / Q000166:cytology / Q000502:physiology*; D024510:Muscle Development; D009132:Muscles / Q000294:innervation; D009413:Nerve Fibers, Myelinated / Q000502:physiology; D051381:Rats; D011916:Rats, Inbred F344; D013116:Spinal Cord / Q000166:cytology / Q000254:growth & development* / Q000502:physiology
D006735:Horseradish Peroxidase