Ultrastructure of a cloned astrocytoma in tissue culture

The ultrastructure of a cloned astrocytoma line which produces a protein unique to the nervous system (S‐100 protein) is described. The cells have features that are consistent with an origin from neoplastic astrocytes. They are characterized by a sparsity of organelles and the presence of many fine filaments in their cytoplasm. Two classes of filaments are present. One type is 90 to 100 Å in diameter filaments that frequently appear hollow, are gently curved, can be followed for long distances in thin sections, and are arranged in loose fascicles. These filaments are similar to the cytoplasmic filaments in normal and neoplastic astrocytes in vivo but are more loosely arrayed in TC‐158 cells than in normal astrocytes. The other type is 70 to 75 Å filaments that are uniformly electron dense, straight, can only be followed for short distances in sections, and are arranged in compact bundles. These filaments are without counterpart in normal or neoplastic astrocytes in vivo although they are frequently seen in cultured cells from diverse origins (so‐called “stress fibers”). In TC‐158 cells examined with the light microscope, PTAH‐stainable fibrils are present that mimic the distribution of the fine bundles of 70 to 75 Å filaments seen in the electron microscope. We suggest that the PTAH‐stainable fibrils and bundles of 70 to 75 Å filaments share a common identity in TC‐158 astrocytes. Although PTAH‐staining fibrils in cultured astrocytes have been called “gliofibrils” in the literature, our results indicate that they are analogous to the “stress fibers” which are a nonspecific finding when present in cultured cells.