Abstract
A set of astrocytic process associated with altered myelinated axons is described in the forebrain of normal adult rodents with confocal, electron microscopy, and 3D reconstructions. Each process consists of a protuberance that contains secretory organelles including numerous lysosomes which polarize and open next to disrupted myelinated axons. Because of the distinctive asymmetric organelle distribution and ubiquity throughout the forebrain neuropil, this enlargement is named paraxial process (PAP). The myelin envelope contiguous to the PAP displays focal disruption or disintegration. In routine electron microscopy clusters of large, confluent, lysosomes proved to be an effective landmark for PAP identification. In 3D assemblies lysosomes organize a series of interconnected saccules that open up to the plasmalemma next to the disrupted myelin envelope(s). Activity for acid hydrolases was visualized in lysosomes, and extracellularly at the PAP-myelin interface and/or between the glial and neuronal outer aspects. Organelles in astrocytic processes involved in digesting pyknotic cells and debris resemble those encountered in PAPs supporting a likewise lytic function of the later. Conversely, processes entangling tripartite synapses and glomeruli were devoid of lysosomes. Both oligodendrocytic and microglial processes were not associated with altered myelin envelopes. The possible roles of the PAP in myelin remodeling in the context of the oligodendrocyte-astrocyte interactions and in the astrocyte's secretory pathways are discussed.
Highlights
Understanding cellular domains has been an essential preliminary step to define functional properties inherent to each cell type
Hildebrand (1977) concluded that: “if these bodies represent a morphological expression of a myelin turnover, the oligodendroglial cell should be the main element responsible for their breakdown,” adding: “in the rabbit and Guinea pig myelinoid bodies are present in the astrocytic cytoplasm.”
paraxial process (PAP) is defined as a differentiated collection of secretory organelles protruding from the AC process, which interacts with the disrupted myelin envelope of nearby axons (Figure 14)
Summary
Understanding cellular domains has been an essential preliminary step to define functional properties inherent to each cell type. An increasing amount of evidences for the existence of cytoplasmic domains in astrocytes (ACs) has recently been accumulated. Earlier descriptions of the AC established two broad subtypes as identified in silver-impregnated specimens. According to the cell location, size, and structure of its radial processes, two AC subtypes are identified: protoplasmic and fibrous ACs. Protoplasmic ACs are commonly associated with nuclei and neuron layers, whereas fibrous ACs lie between axonal fascicles in the white matter. While the overall branching and distribution of AC processes is radial, thereby occupying rounded to oval fields (Del Río Hortega, 1914; Butt and Ransom, 1993), regional variations of astrocytic processes (APs) that reside in elliptical and/or overlapping fields are frequent (Butt et al, 1994; López-Hidalgo et al, 2016). Primary APs generally follow a straight course, they each originate numerous
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