198 Brain RescarciT. 509 11990 198-2t)4 Elsevicr BRES 15194 Calibers and microtubules of nerve fibers: differential effect of undernutrition in developing and adult rats Victor Fatindez z, Maria Elena Cordero 3, Pedro Rosso 2 and Jaime Alvarez ~ 1Unidad de Neurobiologia Molecular, Facultad the Ciencias Biol6gicas, 2 Escuela de Medicina, P. Universidad Cat61ica, Santiago (Chile) and 3Escuela de Medicina, Universidad de Chile, Santiago (Chile) (Accepted 18 July 1989) Key words: Axon; Peripheral nerve; Morphometry; Axonal growth; Maturation; Cytoskeleton Sural nerves of 9-week-old rats undernourished since birth, and of adult rats food-restricted for 27 and 48 days, were studied to explore the effect of severe undernutrition on the caliber and microtubules of axons in growing and non-growing animals. In 9-week-old undernourished rats, the number and caliber of myelinated fibers were normal while the cross-sectional area of non-medullated fibers was 29% smaller than controls. By contrast, in adult undernourished rats the cross-sectional area of myelinated fibers was affected sooner and to a greater extent (-28%) than that of non-meduUated fibers (-23%). Regardless of age, in both controls and in undernourished rats non-medullated fibers of equal caliber had similar microtubular content. The same was found in 3-/~m myelinated axons. These findings indicate that food restriction affects proportionately caliber and microtubules of axons. It is proposed that the anatomy of the axon is in a dynamic equilibrium and that microtubules participate in the specification of the axonal caliber. INTRODUCTION The microtubular density of axons is inversely related to the caliber of the fiber 15'2s. In the cat, 0.1 #m 2 non-medullated fibers have 150 microtubules//~m 2 while 10-~m (78 ~m 2) myelinated axons have 12 microtubules/ ~m z (ref. 12). In peripheral nerves, this correlation is not affected by the nature of the fiber (motor, sensory, sympathetic), nor by the length of the axon, or by the distance between the region examined and the soma L 2.11.12. In addition, the correlation between axonal caliber and microtubular density is maintained during normal development 13 and abnormal conditions such as dia- betes 21, nerve regeneration w and neuropathies 27. Chronic denervation of the superior cervical ganglion affects neither caliber nor microtubular density of post- ganglionic sympathetic axons 11, while a high firing rate increases both microtubules and caliber of axons ~. All these observations are consistent with the possibility that axonal caliber and microtubular content are causally related. Undernutrition affects growth and maturation of the nervous system. Studies carried out in growing animals have shown that undernutrition lowers brain weight, protein and nucleic acid content 35'36. Cortical neurons of undernourished pups have poorly developed dendritic trees 5-7. In tracts of the central nervous system, early undernutrition results in fewer and smaller myelinated fibers, with a reduced number of lamellae 9'25'32'33, a n d the growth of optic nerve fibers is retarded 4. The effects of early undernutrition on peripheral nerves is less clear. Axonal growth has been described to be either unaffected 17 or retarded 9'31. The number of myelin lamellae has been reported to be proportional to the axonal circumference 9 or to be fewer than normal 17. Alterations of the Schwann cell have also been reported, e.g., dense material in the Golgi apparatus 9. In adult animals, brain size is known to be unaffected by undernutrition, but little is known about the effect of undernutrition on peripheral nerves. We examined on a peripheral nerve the effects of undernutrition imposed during early postnatal growth and in adulthood to test two hypotheses: (i) a diminished availability of nutrients results in smaller calibers of peripheral axons and, if so, (ii) microtubular packing remains normal. MATERIALS AND METHODS Early postnatal undernutrition Wistar rats fed throughout pregnancy pellets containing 22.8% protein, 5.9% fat, 46.8% carbohydrate, 6% fibre and 6.5% ash were divided immediately after delivery into two groups: (a) Correspondence: J. Alvarez, Unidad de Neurobiologia Molecular, Facultad de Ciencias Biol6gicas, P. Universidad Cat61ica, Casilla 114-D, Santiago, Chile. 0006-8993/90/$03.50 (~ 1990 Elsevier Science Publishers B.V. (Biomedical Division)