Cytological and quantitative characteristics of four cerebral commissures in the rhesus monkey

Abstract

The number, types, and distribution of distinct classes of axons and glia in four cerebral commissures of the adult rhesus monkey (Macaca mulatta) were determined using electron microscopic and immunocytochemical methods. The two neocortical commissures, the corpus callosum, and the anterior commissure contain small but cytologically distinct archicortical components: the hippocampal commissure, which lies ventral to the splenium of the corpus callosum, and the basal telencephalic commissure, which forms a small crescent at the anterior margin of the anterior commissure. Each archicortical pathway is delineated from the adjacent neocortical commissure by a glial capsule. The glia cells that form this border are immunoreactive with antisera directed against glial fibrillary acidic protein (GFAP) and issue long processes that form numerous desmosomal junctions with one another. Braids of these glial processes envelop axonal fascicles within the archicortical commissures. In contrast, the GFAP-positive cells of the corpus callosum and anterior commissure are randomly distributed cells with relatively short stellate processes that do not form boundaries around axon fascicles. Quantitative electron microscopic analysis reveals that approximately 60 million axons connect the two cerebral hemispheres: the corpus callosum contains 56.0 million +/- 3.8 million axons (n = 8), the anterior commissure contains 3.15 million +/- 0.24 million axons (n = 8), the hippocampal commissure has 237,000 axons +/- 31,000 (n = 6), and the basal telencephalic commissure has 193,000 axons +/- 28,000 (n = 5). The number of axons is not directly proportional to the cross-sectional area in any of the commissures because of variation in axonal composition. On the basis of an estimate of approximately 3 billion neurons in the monkey cortex (Shariff, '53), we estimate that between 2 and 3% of all cortical neurons project to the opposite cerebral hemisphere. Subregions of the corpus callosum as well as each of the other commissures consist of characteristic subsets of five classes of axons and contain different proportions of myelinated to unmyelinated fibers. The largest myelinated axons and the smallest proportion of unmyelinated axons (approximately 6%) are found in regions of the corpus callosum that carry projections from primary sensory cortices, whereas the smallest myelinated axons and largest proportion of unmyelinated axons (approximately 30%) are found in regions of the corpus callosum that carry projections from association cortices. Axon composition in the anterior commissure is uniform and resembles that of callosal sectors that contain association projections.(ABSTRACT TRUNCATED AT 400 WORDS)

A universal scaling law between gray matter and white matter of cerebral cortex

“…The diameters of axons, which may vary over an order of magnitude, have an average value of roughly 1 m in corpus callosum (37,38). A rough estimate a Ϸ 0.8 m 2 for the average cross-sectional area of a single fiber, corresponding to a diameter about 1 m, is loosely compatible with the distributions shown in ref.…”

Human brain myelination and amyloid beta deposition in Alzheimer's disease

“…These regions contain predominantly thinly myelinated fibers with smaller axons and thus contain an increased proportion of myelin [9,43]. The age-related loss of myelin from primarily these thinly myelinated small axons (estimated at 10% per decade [13]) would take a disproportionate toll in late-myelinating regions and make much higher iron levels available for interaction with Aβ and deposition in amyloid plaques in these regions [41,42].…”

“…Myelination produces a continuum of increasingly vulnerable oligodendrocytes as laterdifferentiating oligodendrocytes that populate later-myelinating association regions ensheath increasing numbers of axons with smaller axon diameters [9,10]. Thus, during development, the myelination process produces a roughly bilaterally symmetrical continuum of oligodendrocyte vulnerability ( Figure 1B, D) [3,7].…”

Structural changes of the corpus callosum in tinnitus

“…The surface area of the midsagittal CC may mirror the number of axons interconnecting the two hemispheres (Aboitiz et al, 1992a) or a decrease in fiber density resulting from an increase in the diameter of or the distance between fibers (LaMantia and Rakic, 1990). Be that as it may, in either case the callosal transmission of information, of functionally excitatory and inhibitory influences, would be facilitated.…”