Preterm rabbit-derived Precision Cut Lung Slices as alternative model of bronchopulmonary dysplasia in preclinical study: a morphological fine-tuning approach

Bronchopulmonary dysplasia (BPD) is the most common complication of preterm delivery, with significant morbidity and mortality in a neonatal intensive care setting. Research in this field aims to identify the mechanisms of late lung development with possible therapeutic targets and the improvement of medical management. Rabbits represent a suitable lab preclinical tool for mimicking the clinical BPD phenotype. Rabbits are born at term in the alveolar phase as occurs in large animals and humans and in addition, they can be delivered prematurely in contrast to mice and rats. Continuous exposure to high oxygen concentration (95% O2) for 7 days induces functional and morphological lung changes in preterm rabbits that resemble those observed in BPD-affected babies. The preclinical research pays great attention to optimize the experimental procedures, reduce the number of animals used in experiments and, where possible, replace animal models with alternative assays, following the principle of the 3 Rs (Replace, Reduce and Refine). The use of in vitro assays based on the ex vivo culture of Precision Cut Lung Slices (PCLS) goes in this direction, representing a good compromise between controlled and flexible in vitro models and the more physiologically relevant in vivo ones. This work aims to set up morphological analyses to be applied in preclinical tests using preterm rabbits derived PCLS, cultured up to 7 days in different oxygen conditions, as a model. After a preliminary optimization of both lung preparation and histological processing methods of the lung slices of 300 µm, the morphological analysis was conducted evaluating a series of histomorphometric parameters derived from those widely used to follow the phases of lung development and its alterations in vivo. Our histomorphometric results demonstrated that the greatest differences from pseudo-normoxia and hyperoxia exposed samples at day 0, used as starting points to compare changes due to treatments and time, are detectable after 4 days of in vitro culture, representing the most suitable time point for analysis in preclinical screening. The combination of parameters suitable for evaluating PCLS morphology in vitro resulted to be Tissue Density and Septal Thickness. Shape Factor and Roughness, evaluated to highlight the increasing complexity of the airspaces, due to the formation of septal crests, gave useful information, however, without significant differences up to day 4. Other parameters like Mean Linear Intercept and Septal Density did not allow to highlight significant differences between different oxygen conditions and time points. Instead, Radial Alveolar Count, could not be applied to PCLS, due to the tissue changes following agar infusion and culture conditions.