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By help of a retrospective study the biological behaviour and the derivation of canine basal cell tumours were to be investigated. These epithelial neoplasms of the skin are considered to be semimaligne tumours, but according to several statements metastasizing is possible. Due to the fact that there is no histogenetic proof of this, there is every reason to believe that the observed metastases might have possibly descended from tumours with a different origin. In order to obtain clarification a special immunohistochemical staining was prepared. This staining method is reliable to identify basal cell tumours. With the immunohistochemical staining method certain epithelial elements (cytokeratins) are detected by specific antibodies and the tissue is defined precisely. Even cells with neoplastic transformation present themselves unequivocally and the derivation of the tissue can be determined. The results show that with routinehistological evaluation incorrect tumourdiagnosises have been established in all propability. As routine diagnostic is based on H.E.-staining another aspect was to determine whether the immunohistochemical method is superior to the H.E-staining method with regard to the detection of basal cell tumours.A total of 110 formalin-fixed, paraffin-embedded samples from the institute for veterinary pathology of the Freie Universität Berlin were used. The tissues originated from the years 1995 -1997 and according to the H.E.- based routine diagnose all of them had been classified as basal cell tumours. Having assessed various test methods of pre-treatment and dilution of the used antibodies in pre-tests, the patterns were prepared. They were stained by a modified BSA-method on the basis of the instruction guide for super sensitive detection systems of Biogenex, Hamburg. Deviating from the ecommended staining steps, the slides were predigested with Protease at 37ºC for 15 - 20 minutes, respectively heated in microwave oven for 5x2 minutes in citrate buffer, after they had been cleared of paraffin. The counterstaining was carried out with hämalaun instead of hämatoxylin. Furthermore the samples were rinsed with tris based saline buffer (TBS) instead of PBS whilst the staining took place. In addition, the dilution of antibodies was set up with TBS as well.The following cytokeratin antibodies were used: AE 1 (dilution 1:50) in order to express suprabasal cells; CK 14 (1:8) as an exclusive marker for basal cells; LP 34 (1:140) for panepithelial staining. The stained pattems were compared with H.E.stained slides and interpreted.The analysed samples derived from 58 male (52,7%) and 50 female dogs (45,5%). 3 of the male and 2 of the female dogs were castrated. The sex of 2 bitches was not accounted for.The average age of the patients was 7,0 years.Mongrels were most frequently affected (25,5%), followed by spaniels (12,7%), german shepards (11,8%) and poodles (7,3%).49,1 % of the neoplasms were located at the head, the remaining tumours were spread on the trunk, limbs, neck, tail and shoulder.6 of the tumours had already recurred. 2 of the tumours showed primary multiplicity.The evaluation of the immunohistochemical stainings demonstrated that the tissuestainings partially were not regular in reference to the specifity of the actual antibodies. It has been observed that in some slides purely solitary cells or focal cell formations (fractionally multiple) were stained. Some tissues had a diffuse stainingpattern, but in most of the cases all cells showed an equal intense reaetion. Occasionally antibody AE 1 merely reacted with hair follicles. Explanation für both phenomenons is probably a deficient antigenicity due to overtime fixation to formalin.Furthermore the investigation demonstrated that pigmented basal cell tumours occured to 6,4% in the collective. Considering the numerous cases of melanosis, a classification of the assessed samples as a melanotic variant seems appropriate.In face of the unequivocal immunohistochemical indications to histogenesis of the examined tumours, the basal cell could be defined as the degenerated tumour cell A positive diagnose of the tumour was possible and the result was that all investigated samples exclusively were basal cell tumours. Neoplasms of a different derivation could not be detected. Hence, the literature on the subject, metastases included, has to get reconditioned adequately to verify the histogenetical valuation made.With regard to the assessment of the biological behaviour of the basal cell tumour, especiaIly its metastasizing, the tests applied did not give any indication. With respect to deficient histogenetic verifications of observed metastases the specification of the basal cell tumour as a semimaligne neoplasm prevails.