Tumors of nervous system are various. They develop from different elements of the nervous system: 1) central; 2) vegetative; 3) peripheral; 4) mesenchymal elements, which are also a part of this system.

The etiology and prognosis of brain tumors are poorly studied.

According to many authors, there are six etiological groups of tumor and tumor-like diseases of human and animal nervous system: 1) genetically dependent (hereditary); 2) con-genital; 3) radiation; 4) chemically-induced; 5) meta-bolic (including dyshormonal); 6) viral.

Recklinghausen's neurofibromatosis is an example of a human hereditary tumor. Some authors believe that neurodermal melanosis belongs to this group. Congenital tumors and tumor-like diseases are medulloblastoma and astrocytoma in children. Radiation tumors of brain were described in experimental animals as well as in the people who were administered

intensive radiotherapy or exposed to high-dose ionizing radiation due to accidents (Hiroshima and Nagasaki). Brain tumors caused by exogenic administration of pure carcinogenic substances were observed in experimental animals. It is believed that some brain tumors occur as a result of disturbed metabolism, so-called hormone-depending tumors. Hormone-depending tumors are observed both in people and animals, e.g. arachnoidendothelioma. These tumors become active during pregnancy. In some cases, they disappear after delivery of a child. Virus-induced tumors are known only in animals. Thus, polyoma virus may cause hemorrhagic sarcomatosis of pia mater. Some adenoviruses may cause ependymoblastoma in hamsters. Some authors (L.I. Smirnov, A.P. Avtsin, B.S. Khominsky, A.N. Age-eva) introduced the idea about preblastoma (in particular focal proliferation with them).

According to the degree of maturity, brain tumors may be more or less mature (benign) or immature (malignant).

Special attention should be paid to the characteristic features of brain tumors.

1. The term «benign» is not suitable in this case as they are located in the brain and indeed are always malignant. Even slow growth affects vitally important centers and causes their dysfunction.

2. Neuroectodermal (neuroepithelial) tumors of brain originating from neuroectoderm derivatives are

dysontogenetic, i.e. develop from the cells which are known as precursors of mature CNS elements. Therefore, it may be difficult to determine their histological type. More often their cellular composition corresponds to definite stages of development of neuronal and glial elements.

3. Brain tumors produce metastases within the skull, that is with the help of liquor.

4. Their microscopic appearance is characterized by prolonged fascicular structures, lying either in wave-like or curl-like manner.

Neurodermal tumorsare subdivided into astrocyte, oligodendroglial, ependymal tumors and those of choroid epithelium, neuronal, poorly differentiated and embryonic.

Astrocyte tumors or gliomas are the most frequent brain tumors. They develop from astrocytes and can be found in all brain portions. The highest incidence is observed between the age 25—45. The diameter of the tumor is about 5—10 cm. They do not always have distinct boundaries with the surrounding tissue. It is homogeneous on incision. As a rule, considerable enlargement of the brain portions is observed. Astrocytoma is characterized by cyst formation (one or several). They contain colloid substance or yellowish fluid with large amount of protein. There are three histological types of astrocytoma: 1) fibrillar, 2) protoplasmatic, 3) fibrillar-protoplasmatic. Fibrillar tumor is rich in glial fibers looking like parallel bands,

it contains small amount of astrocytes. Protoplasmatic astroma consists of different in size cells with processes which resemble astrocytes, their processes form thick interlacing. A fibrillar-protoplasmatic tumor is characterized by even location of astrocytes and glial cells. Cerebellar astrocytoma and subependymal astrocytoma are separate subtypes. A malignant type is astroblastoma characterized by rapid growth, polymorphism and necroses in the tumor. This tumor is rare, it disseminates through the liquor routs.

Oligodendroglial tumors. In the majority of cases these are benign. The highest incidence is observed at the age of 30—40. In rare cases, they occur in children. They are mainly localized in the large hemispheres of the brain, more seldom in the region of visual tuber and in the trunk. Very seldom, they develop in the area of cerebellum and spinal cord. Primary multiple oligodendrogliomas of meninges and visual nerves were also described.

Macroscopically, the tumor is pinkish-gray, resembles brain substance and is diagnosed by the enlargement of the brain portion. Its consistency may be paste-like, when calcifications are present it may be dense.

Microscopically it consists of homogeneous small cells with round nuclei and narrow outline of cytoplasm which is poorly colored. Sometimes it is characterized by the structure resembling honeycombs. The tumor is usually poor in vessels. Hyalinosis and calcification may also be observed.

The types of oligodendrogliomas are: 1) fusiform cell, 2) polymorphocellular.

A malignant type of the tumor is oligodendro-glioblastoma characterized by special cell location, marked polymorphism with giant cells. It is also characterized by numerous mitoses and necrosis foci. The metastases spread through the liquor routs, more often along the walls of the ventricles. Symmetrical location of the tumor nodes in the walls of the ventricles is typical.

Ependymal tumors and tumors of choroid epithelium. According to L.I. Smirnov, three types of ependymal tumors are distinguished.

Ependymoma (glioma connected with ventricular ependymoma) looks like intra- or extraventricular node. The foci of necrosis and cysts can be found in it. Clasters of uni- and bipolar cells around the vessels (so-called pseudorosettes) and cavities covered with epithelium (true rosettes) are typical. Most frequently they are located in caudal portions of rhomboid fossa. Ependymoma may go down the spinal canal (craniospinal tumors). Ependymoma is usually localized in the bed of the 4th ventricle and in the 3th ventricle. In the area of the spinal cord, they first grow intramedullary, then become extramedullar. Macroscopically they look like nodes of different size with tuberous (4th ventricle) or villous (lateral ventricle) surface. The color is pinkish-gray, the consistency is soft. Microscopic study reveals peri-

vascular structures of radially located cells. Their processes form a fibrous ring between the body of the cell and the wall of the vessel and over the body of the cell. In the rest of the tumor tissue, the cells are located in mosaic manner. Single and multiple clefts and tubes bedded with cylindrical epithelium are common.

Ependymoblastoma is a malignant type of ependymoma. This is characterized by marked cellular polymorphism. It grows quickly, metastases spread through the liquor system.

Dedifferentiated ependymoma is a transitory form between the two types.

Choroid papilloma is a tumor from the epithelium of vascular plexus, looking like a villous node in the cavity of the brain ventricle. It consists of numerous villous structures of cubic or prismatic epithelial cells. It is mainly observed in young people. It is located within the brain ventricles. Heterotopic types are rare (horse's tail).

Macroscopic study demonstrates well-outlined nodes of various size. The surface of the tumor is small or large-villous, has cauliflower- or mulberry-like appearance. Its consistency is either dense or soft, the color is pinkish-gray.

Microscopically it consists of villi, their connective tissue stroma is covered with cubic or cylindrical epithelium. Hyalinosis can be frequently observed.

Choroid carcinoma is a malignant type of choroidpapilloma. It is made of anaplastic cells covering the vascular plexus. Papillary cancer is a rare tumor.

Neuronal tumors

Ganglioneuroma is a rare mature tumor. Most frequently it is localized in the bed of the 3th ventricle, rarer in the hemispheres of the brain. It usually occurs in children and juveniles. The tumor consists of mature ganglionic cells divided with the bands of glial stroma. Macroscopically ganglioneuroma looks like a limited node. In the medulla oblongata it is diffuse, in the cerebellum it looks like hyperplastic folds.

Cerebellum ganglioma is characterized by proliferation of large nervous elements of Purkinjer's cell type.

Ganglioneuroblastoma is a malignant analogue of ganglioneuroma (malignant gangliocytoma). This is an extremely rare tumor of CNS. It is characterized by cellular polymorphism and similar to malignant glioma.

Neuroblastoma is a rare highly malignant brain tumor. It occurs mainly in children. The tumor is formed from large cells with bubble-like nucleus. Mitoses are numerous. The cells grow like sincitium. There are a lot of vessels.

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