Name the basic laws of carcinogenic action of an ionizing radiation.

Though for development of tumours the doze and a kind of radiation matter, there is no linear dependence between frequency of occurrence of tumoral growth and the absorbed doze. There is no minimal threshold doze so any radiation in any doze is potentially dangerous. During other equal conditions long and constant influence of low dozes, from the point of view of carcinogenesis, much more dangerous, than short-term influence by greater dozes.

16.23. What is "plastic" carcinogenesis? What are its features?

In experiment has been shown, that after implantation of plastic plate under a skin the malignant tumour often is developed in this site. The given phenomenon has received the name of "plastic" carcinogenesis. Its following laws have been established.

1. For occurrence of a tumour the material of which is made a plate, has no special value. This conclusion was served with results of research of many substances among which cellophane, polythene, teflon, glass, gold, platinum, etc. If a plate is made of such substance and implanted, tumour will be developed. If a plate is crushed and entered into a tissue as a powder, the tumour won?t be developed.

2. Crucial importance has the size of a plate. The tumour arises after implantation of plates of the sizes not less 0,5 × 0,5 cm.

3. The plate should be continuous, i.e. not have apertures (perforations).

According to one of hypotheses, around plate the connective tissue capsule is formed, among collagen fibres are individual cells. They appear torn off from other cells and the signals sent by them adjusting intensity of proliferative processes. In this connection conditions which promote regeneration of normal cells in tumoral are created.

16.24. Who proved a role of viruses in occurrence of tumours and in what experiments?

Experimental proofs of a virus origin of tumours consider their occurrence after injection to animal acellular filtrates of a tumoral tissue. Such filtrates prepare from suspension of tumoral cells, passing it through the porcelain filters detaining bacteria and cells of a tissue (fig. 55).

For the first time in 1908 V. Ellerman and O. Bang have in a similar way reproduced leucosis at hens, entering acellular filtrates of leucosis cells.

In 1910 F. Raus has caused a sarcoma in hens introduction of acellular filtrate of sarcomatous tissues. For these researches it in 1966he has been awarded the Nobel Prize.

In 1932 R. Shoup has informed on allocation of a virus from a good-quality tumour of rabbits - fibromas, and a little bit later - from a papilloma.

In 1934 B. Ljuke has found out in kernels of cancer cells of kidneys leopard frog of corpuscles - inclusion reminding those at virus infections. Then it has reproduced a tumour introduction by an animal of the dried up extract of tumoral cells.

In 1936 Dj. Bittner has opened ?the factor of milk? (factor of Dj. Bittner) which caused a cancer of mammary glands of mice. The further researches have shown that this factor is a virus transferred from mother cubs with milk.

In 1951 L. Gross for the first time has reproduced leucosis of mice introduction of acellular filtrates of tumoral cells to a newborn animal. These researches have proved an opportunity of virus etiology of malignant tumours and at representatives of mammals.

16.25. What DNA-containing viruses are oncogenic for animals and human?

To oncogenic DNA-containing viruses concern:

a) papova-viruses They cause development of three kinds of tumours: papillomas, poliomas and a tumours arising under action of vacuolisative virus SV-40;

b) adenoviruses. Oncogenic for animals are adenoviruses of 12, 18 and 31-th types;

c) herpes-viruses, in particular, virus of Epstein-Barr.

As to the human organism in one case it is proved, and in other there are bases to believe, that the reason of some tumours are:

a) Virus of Epstein-Barr (it causes the Berkit lymphome, nasopharingeal cancer);b) a virus of a hepatitis B (it can cause a cancer of a liver);c) a virus of a papilloma of the human (it causes benign tumours of a skin, female genitals, throats).

16.26. What RNA-containing viruses are oncogenic for animals and human?

All oncogenic RNA-containing viruses belong to retrovirus family. Their general property is presence of a gene coding structure of revertase enzyme in virus genome, known still as return transcriptase, or RNA-dependent DNA-polymerase. This enzyme provides synthesis of two-spiral DNA on a matrix one-spiral RNA. That is why DNA-copy of retrovirus which has received the name of DNA-provirus is formed.

Depending on oncogeneicity retroviruses are divided into two groups.

I. Acutetransformate retroviruses. They are very oncogenic, cause development of tumours after the short latent period. These viruses have oncogen in the genome, therefore in a basis of transformation of cells in tumoral lays an epygenome mechanism. Viruses, in particular, are concerned to the specified group of sharp leucosis of birds, mice and sarcoma of Raus of hens.

II. Slowtransfomate retroviruses. Cause development of tumours after the long latent period. These viruses do not include oncogen; therefore the basic mechanism of their transforming action is mutational. Viruses, belong to this group are lympholeucosis viruses.

The oncogenic retrovirus of the person is the virus of the T-cellular leucoma-leucaemia. It is transferred from the person to the person during long intimate contacts, blood transfusion. It is necessary to note, that this lymphotropic virus has a big similarity to the human immunodeficiency viruses (HIV) causing AIDS.

Date: 2016-06-12; view: 99