Scanning and transmission electron microscopical evidence of the capacity of diatoms to penetrate the alveolo-capillary barrier in drowning

The diagnostic value of diatom analysis for drowning is considered to be one of the most controversial arguments in forensic medicine. However, the theoretical assumption of the method, i.e. the capacity of diatoms to penetrate the alveolo-capillary barrier during drowning, has never been addressed. Using scanning (SEM) and transmission electron microscopy (TEM), we have investigated the interaction of a natural population of diatoms and an unialgal culture of Phaeodactylum tricornutum (PT) with the alveolo-capillary barrier in an experimental model of drowning. The SEM analysis allowed the identification of several diatom species along the whole airways and their close interaction with the alveolar wall, but was poorly informative about the effective penetration of diatoms into pulmonary vessels. The TEM analysis was more informative and allowed a precise identification of the PT cells in alveolar spaces and to detect their phagocytosis by alveolar macrophages. PT penetrated into the pulmonary vessels through the thinnest portions of the alveolo-capillary barrier and through the interstitial spaces and were identified in pulmonary capillaries and venules. The morphological demonstration of the capacity of diatoms to penetrate the alveolo-capillary barrier is a step forward in assessing the potentiality, reliability and limitations of diatom analysis on a new basis as a tool for the diagnosis of drowning.