WHITE-MATTER OF THE CEREBELLUM OF THE CHICKEN (GALLUS-DOMESTICUS) - AQUANTITATIVE LIGHT AND ELECTRON-MICROSCOPIC ANALYSIS OF MYELINATED FIBERS AND FIBER COMPARTMENTS

Low magnification light microscopic examination of the white matter in appropriately stained avian and mammalian cerebellum reveals a mediol ateral succession in which areas of large, heavily myelinated fibers a lternate with areas containing nearly exclusively small fibers. A larg e fiber accumulation (LFA) and its medially adjoining small fiber area (SFA) form a fiber compartment, which, with related parts of cortex a nd central nuclei, constitutes a so-called cerebellar module. The comp osition and the apparent mediolateral heterogeneity of cerebellar fibe r compartments was quantified in the chicken by morphometrical analysi s of myelinated fiber profiles in light (LM) and electron (EM) microsc opic micrographs. In LM versus EM, approximately 37% of the myelinated fiber population is neglected. This deficit concerns profiles that ar e smaller than 1.2 mu m(2) (diameter < 1.2 mu m) EM analysis is theref ore considered a prerequisite and forms the main part of this study. T he myelinated fiber population has a left-skewed log normal size distr ibution. Ninety-nine percent of the myelinated fibers fall within the range of 0.1 to 20 mu m(2) (diameter = 0.4-5.0 mu m) and 90% are even smaller than 7 mu m(2) (diameter < 3.0 mu m) Small fibers are abundant in both parts of the compartment. Statistical comparisons provide qua ntitative confirmation of the LM distinction of LFAs and SFAs. It appe ars, moreover, that, apart from typical LFAs and SFAs, transitional zo nes rather than sharp borders can be distinguished between the two. Th e medial border of the LFA appears to be more sharply defined than its lateral border. Distinct mediolateral fluctuations were found with re spect to fiber density (166-243 fibers/1,000 mu m(2)), mean profile ar ea (2.4-4.0 mu m(2)), and interspace (31-47%). These differences refle ct the contrast between LFA (lower density, larger mean profile area) and SFA (higher density, smaller mean profile area). The interspace di scriminates less well between LFA and SFA but is often smaller in the LFA and larger in the SFA. The presented quantitative characteristics of mediolateral heterogeneity in the cerebellar fiber layer can be use d as reference for morphometric studies on the different fiber systems of the cerebellar white matter and the functional organization of the compartments. (C) 1996 Wiley-Liss, Inc.