A technique for improved focused ion beam milling of cryo-prepared life science specimens.

The combination of focused ion beam and scanning electron microscopy with a cryo-preparation/transfer system allows specimens to be milled at low temperatures. However, for biological specimens in particular, the quality of results is strongly dependent on correct preparation of the specimen surface. We demonstrate a method for deposition of a protective, planarizing surface layer onto a cryo-sample, enabling high-quality cross-sectioning using the ion beam and investigation of structures at the nanoscale.

K-ras mutation in focal proliferative lesions of human pancreas.

The K-ras gene is mutated in > or =75% of human pancreatic adenocarcinomas and in a number of hyperplastic ductal lesions from noncarcinoma patients. In this study, the incidence of K-ras mutation was determined in a spectrum of focal proliferative pancreatic lesions to evaluate their preneoplastic significance. PCR-based mutation-enriched RFLP analysis was used to identify mutations in codon 12. Immunostaining for Ki67 and p53 was also performed. Forty-seven % of intraductal nonpapillary hyperplasias (8 of 17) contained codon 12 mutations, as did 55% of adenomatoid hyperplasias (6 of 11). This rate increased to 61% in papillary hyperplasias (27 of 44) and to 78% when there was severe dysplasia (7 of 9). The fraction of cells staining for the Ki67 proliferation marker showed a general correlation with the rate of K-ras mutation. Nuclear staining for p53 protein was seen only in two ductal lesions with severe dysplasia. No mutations were found in normal acinar tissue (n = 38), squamous metaplasia (n = 13), ductal complexes (n = 8), or focal acinar cell dysplasia (n = 5). There seemed to be a general correlation of proliferative potential with the presence of K-ras mutation in ductal lesions. However, because of the high prevalence of lesions with K-ras mutations, we conclude that this mutation alone cannot be taken as proof of significant risk for progression to carcinoma. Efforts to use the presence of K-ras mutations in DNA harvested from pancreatic juice or duodenal aspirates as an approach for diagnosis of occult pancreatic carcinoma seem vulnerable to a high false-positive rate.