No abstract

BACKGROUND.Chaotic organization, abnormal leakiness, and structural instability are characteristics of tumor vessels. However, morphologic events of vascular remodeling in relation to tumor growth are not sufficiently studied yet.METHODS.By using the rat rhabdomyosarcoma tumor model vascular morphogenesis was studied by light and electron microscopy and immunohistochemistry in relation to tumor regions such as tumor surrounding (TSZ), marginal (TMZ), intermediate (TIZ), and center (TCZ) zones.RESULTS.The analyses revealed that blood vessels of TSZ display a regular ultrastructure, whereas blood vessels of TMZ showed a chaotic organization and unstable structure with a diffuse or even lacking basal lamina, and missing or irregular assembled periendothelial cells. In contrast, blood vessels of TIZ and TCZ exhibited a more or less stabilized vessel structure with increased diameter. Correspondingly, normal assembly of α‐smooth‐muscle‐actin (α‐SMA)‐positive cells into the vessel wall was observed in blood vessels of TSZ, TIZ, and TCZ. Also, Ang1 immunostaining was strongest in large vessels of TIZ and TCZ, whereas Ang2 staining was prominent in small vessels of TIZ. Tie2 staining was detectable in small and large vessels of all tumor zones. Immunostaining for αvβ3‐integrin was strongest in small vessels of TMZ, whereas large vessels of TIZ and TCZ were almost negative.CONCLUSIONS.The results indicate a zone‐specific remodeling of tumor blood vessels by stabilization of vessels in TIZ and TCZ, whereas small vessels of these zones obviously undergo regression leading to tumor necrosis. Thus, a better understanding of vascular remodeling and stabilization in tumors would enable new strategies in tumor therapy and imaging. Cancer 2007. © 2007 American Cancer Society.

Sequential morphological changes in tumor capillaries of isotransplanted R1H rat rhabdomyosarcoma were observed weekly by transmission electron microscopy during fractionated radiotherapy (75 Gy/25 fractions/5 weeks). During the first 2 weeks of irradiation (up to 30 Gy), edema of the tumor capillary wall was induced. Swollen endothelial cells bulged into the vascular lumen and were surrounded by a widened subendothelial space with increased amounts of collagen fibrils (subendothelial edema). The endothelial lining was preserved up to the 3rd week of irradiation (45 Gy). Prolonged irradiation was associated with progressive destruction of the vascular wall including shrinkage, gradual loss of cell contacts, disappearance of the normal chromatin pattern, and increase of cytoplasmic vacuoles in endothelial cells as well as disruption of basal laminae. One week after the end of radiotherapy (75 Gy), the tumor capillaries showed complete necrosis. Progressive damage to tumor capillaries in the course of fractionated radiotherapy might have adverse effects on blood supply and thus on tumor oxygenation.

The kinetics of depopulation and repopulation of the solid transplantable rhabdomyosarcoma R1H of the rat following local irradiation with single subcurative X-ray doses of 7.5, 15 and 30 Gy was studied. Several parameters were sequentially measured over a time interval of 4 weeks after irradiation: the ratio of the number of tumour to host cells, and the cellular DNA content of tumour and host cells, were determined by flow cytometry; the amount of DNA per gram of tumour tissue was determined biochemically; the clonogenic fraction of tumour cells was obtained from in vitro colony assay; and the tumour volume was assessed by in situ caliper measurements. From the amount of DNA per gram and the average DNA content per cell, the total number of cells per gram of tumour tissue was obtained. From this and the other parameters measured, the number of clonogenic tumour cells, non-clonogenic tumour cells and nucleated host cells per tumour, as well as their variation with time and dose, could be derived. The results showed that there was a lag period prior to depopulation amounting to 3.8 +/- 1.4, 1.4 +/- 0.8 or 0 +/- 0.7 days for 7.5, 15 or 30 Gy, respectively. The rate of depopulation of non-clonogenic tumour cells increased with dose; the halving times of non-clonogens were 4.7 +/- 1.8, 2.6 +/- 0.7 or 2.1 +/- 0.4 days for the three doses applied. There were no indications that proliferation of doomed cells contributed significantly to tumour growth after irradiation. After lag periods that were similar in length to those prior to depopulation, a massive immigration of host cells was observed. Under certain conditions more than 97 per cent of the cells present in irradiated tumours were found to be of host origin. There was a lag period before the onset of repopulation by clonogenic tumour cells, the length of which increased from 2.7 +/- 0.7 to 5.0 +/- 0.8 or 6.3 +/- 1.0 days for 7.5, 15 or 30 Gy, respectively. The initial rate of repopulation increased with radiation dose; after the end of the lag period the doubling time of clonogenic tumour cells (in controls amounting to 3.7 +/- 0.2 days) was 3.1 +/- 0.1, 2.1 +/- 0.1 and 1.1 +/- 0.1 days for the three doses applied.(ABSTRACT TRUNCATED AT 400 WORDS)