Fine structure of the astrocyte-like neuroglial cell in the planarian Dugesia dorotocephala

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Astrocyte-like neuroglial cells in the planarian central nervous system were studied by electron microscopy. The neuroglial cell observed in Dugesia dorotocephala is multipolar, extending 3 or 4 major cytoplasmic processes, each with many smaller daughter branches. The nucleus is often irregular, which seems to be related to the terminals of the cytoplasmic processes. Many glycogen granules, mitochondria, granular endoplasmic reticulum, lysosomes and phagosomes are characteristically observed in the cytoplasm of the neuroglial cells. Within phagosomes, debris of nerve or neurosecretory fibers appear in various degrees of disintegration. These neuroglial cells are closely associated with bundles of nerve fibers, nerve cells, neurosecretory cells, muscle fibers and other neuroglial cells. They also form gap junctions with each other in a manner similar to astrocytes of higher animals. A comparison of the planarian neuroglial cell with the migratory, phagocytic reticular cell reveals several similarities but also some important differences. That is, both cells have functions of phagocytosis, nutrient transportation and junctional communication, but their shapes and locations as well as phagocytized material are different from each other. From an evolutionary point of view, these observations suggest that the planarian neuroglial cell is not merely a reticular cell that has migrated from the mesenchyme, but represents a distinct cell type; e.g. a primitive astrocyte that transports nutrients for neurons and phagocytizes degenerating neurons, in addition to its role as a supporting element of the central nervous system.