Rosenthal fibres in astrocytoma is composed of

Well-differentiated astrocytomas are poorly defined, gray, infiltrative tumors that expand and disto the invaded brain without forming a discrete mass (Fig. 22-28, A). Infiltration beyond the grossly evident margins is always present. The cut surface of the tumor is either firm or soft and gelatinous; cystic degeneration may be seen. In glioblastoma, variation in the gross appearance of the tumor from region to region is characteristic (Fig. 22-28, B). Some areas are firm and white, others are soft and yellow (the result of tissue necrosis), and still others show regions of cystic degeneration and hemorrhage. Well-differentiated astrocytomas are characterized by a mild to moderate increase in the number of glial cell nuclei, somewhat variable nuclear pleomorphism, and an intervening feltwork of fine, glial fibrillary acidic protein (GFAP)-positive astrocytic cell processes that give the background a fibrillary appearance. The transition between neoplastic and normal tissue is indistinct, and tumor cells can be seen infiltrating normal tissue many centimeters from the main lesion. Ana- plastic astrocytomas show regions that are more densely cellular and have greater nuclear pleomorphism; mitotic figures are present. Glioblastoma has a histologic appearance similar to that of anaplastic astrocytoma, as well as either necrosis (often with pseudopalisading nuclei) or vascular proliferation ref Robbins 9/e p 842