求翻译：Spontaneous neoplastic pulmonary lesions are rare in rats and arise mostly in the lung periphery whereas regenerative hyperplasia and squamous metaplasia occur mainly in the centroacinar region. Therefore tissue of the lung including parenchyma, bronchiolo-alveolar junctions and main bronchi should be investigated. In oral toxicity studies, at minimum, one longitudinal section of the left lobe should be examined. Additionally, transverse sections of the right cranial and caudal lobes may be examined. In these sections, the epithelium of the major bronchioles, which is one important site of lesions, can be examined at its widest diameter. In inhalation studies, sections of all five lobes should be examined according to the proposed scheme, which facilitates unambiguous identification of individual lung lobes. For histological identification of proliferative lesions in the lung, careful fixation by intratracheal instillation is recommended, even for oral studies.是什么意思？

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