White matter of the cerebellum of the chicken (Gallus domesticus): a quantitative 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 mediolateral succession in which areas of large, heavily myelinated fibers alternate with areas containing nearly exclusively small fibers. A large fiber accumulation (LFA) and its medially adjoining small fiber area (SFA) form a fiber compartment, which, with related parts of cortex and central nuclei, constitutes a so-called cerebellar module. The composition and the apparent mediolateral heterogeneity of cerebellar fiber compartments was quantified in the chicken by morphometrical analysis of myelinated fiber profiles in light (LM) and electron (EM) microscopic micrographs. In LM versus EM, approximately 37% of the myelinated fiber population is neglected. This deficit concerns profiles that are smaller than 1.2 micron2 (diameter < 1.2 microns). EM analysis is therefore considered a prerequisite and forms the main part of this study. The myelinated fiber population has a left-skewed log normal size distribution. Ninety-nine percent of the myelinated fibers fall within the range of 0.1 to 20 microns2 (diameter = 0.4-5.0 microns) and 90% are even smaller than 7 micron2 (diameter < 3.0 microns). Small fibers are abundant in both parts of the compartment. Statistical comparisons provide quantitative confirmation of the LM distinction of LFAs and SFAs. It appears, moreover, that, apart from typical LFAs and SFAs, transitional zones rather than sharp borders can be distinguished between the two. The medial border of the LFA appears to be more sharply defined than its lateral border. Distinct mediolateral fluctuations were found with respect to fiber density (166-243 fibers/1,000 microns2), mean profile area (2.4-4.0 microns2), and interspace (31-47%). These differences reflect the contrast between LFA (lower density, larger mean profile area) and SFA (higher density, smaller mean profile area). The interspace discriminates 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 used as reference for morphometric studies on the different fiber systems of the cerebellar white matter and the functional organization of the compartments.