many tumors characterized by hypervascular-ity: hemangioblastomas, meningiomas, schwannomas, oligodendrogliomas and ependymomas. Melanomas often bleed and the cells may contain both melanin and hemosiderin pigments.

Vascular Changes

In general, high-grade neoplasms are more vascular than low- grade ones, but there are many exceptions. Obviously, tumors of vascular origin, such as capillary hemangioblastomas, hemangioendotheliomas and hemangiopericy-tomas, are vascular by definition regardless of their degree of anaplasia. Meningiomas have long been known to be vascular, fed both intra-and extra-cranially, and their surgical resection was sometimes disastrous in the old days, but they are easily handled today with pre-operative selective embolization. Among metastatic neoplasms, melanomas, hepatomas and chorio-carcinomas are well known to be vascular and to bleed easily. Ependymomas are inherently vascular as part of their perivascular pseudo-rosettes, but an avascular cellular ependymoma without perivascular pseudorosettes can be readily mistaken for an astrocy-toma. Oligodendrogliomas can also be very vascular and have spontaneous intracerebral hemorrhages. Pilocytic astrocytomas and schwannomas often show focal areas of hyper-vascularity, usually markedly hyalinized, but occasionally even with capillary endothelial proliferation.

Capillary Endothelial Proliferation Excessive proliferation of capillary endothelium filling the lumen and/or extending externally to form glomerulus-like masses (Fig. 3.21) or chains is one of the common ancillary "proofs of malignancy" in glioblastomas. However, it is also common in oligodendrogliomas and the walls of cysts of any nature, and is occasionally found in pilocytic astrocytomas and schwannomas without affecting the grading or prognosis.

Telangiectasia and Angioma Formations Foci of telangiectasia and angioma formations are frequently found in high-grade glioblastomas and low-grade oligodendrogliomas, but are not useful as criteria for grading the neoplasm. Spontaneous hemorrhages most frequently occur in these tumors.

Capillary Networks Intersecting the Tumor into Lobules So-called "chicken-wire capillary networks" that intersect the tumor into multiple lobules are one of the characteristic architectural patterns of oligodendrogliomas (see Figs. 3.5b and 3.10b).

Sinusoidal Networks Intersecting the Tumor into Lobules The normal pituitary gland consists of small acini of cells of different types separated by connective tissue septa with sinusoidal vessels. The acini expand in adenomas to destroy the connective tissue septa but the basic pattern may remain, only with enlargement of the acini (see Fig. 3.10c).

Hyalinized Necrosis and Thrombosis of Blood Vessels Hyalinized necrosis of blood vessel walls and recent thrombosis, together with marked proliferation of abnormal blood vessels, including thin-walled and dilated veins and thick-walled fibrous blood vessels of indeterminate nature, may be at least as diagnostic of

Fig. 3.21. A chain of glomeruloid masses of endothelial proliferation is typical of glioblastoma. NIH-575, H&E.

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