A morphological and morphometric investigation into the morphogenesis of the crocodilian lung

Abstract

The reptilian lung has been well documented, morphologically and morphometrically, across the spectrum. Models on the different structural types and correlations to lifestyles have been made from these. Interestingly, very few studies have been conducted on the Nile crocodilian lung given its evolutionary significance. The previously conducted studies were limited to narrow scales of examination and used few techniques to develop hypotheses and reach conclusions. Furthermore, none of the studies conducted examined the Nile crocodile lung at various life stages, given their ecology. This study examined the lung morphology of the Nile crocodiles of different ages at various scales. The structural components that comprise the respective age groups were also quantified. This was done to conceptualise the full functionality of the lung and the effects on its lifestyle. It also served to examine and create new hypotheses on the actual mechanisms that the lung employs for direct and indirect functions, based on its structure. Latex casting, light microscopy, scanning electron microscopy, 3 dimensional serial section computer reconstruction, transmission electron microscopy and confocal microscopy were used to document the morphology of the Nile crocodile lung from macroscopic to ultrastructural scales respectively. Stereology was used to morphometrically quantify the structural components that comprise the lung. At a macroscopic scale the lungs were observed to be conically shaped with broad base decreasing to an apex cranially. Moving to microscopic scales, cranial regions of the lung were found to be more subdivided than the middle and caudal regions of the lung respectively. All of the respective ventral regions of the lung were discovered to be more subdivided than their dorsal counterparts. The presence of smooth muscle was also markedly observed in the middle to caudal regions of the lung. Upon examination of the ultrastructural composition of the lung, various septa were observed which facilitated the subdivision of the lung into favourable gas exchange units, mainly in the cranial regions of the lung. Interfaveolar pores, stereocilia and microvilli were also observed, the former two never documented in the scientific literature previously. It is hypothesised that these structures are integrally involved in ensuring efficient gas exchange occurs to suit particular behaviours. Stereological analyses confirmed the morphological observations. Interestingly it showed that the various regions of the young age groups had more highly subdivided lungs than the older age groups. Apart from degrees of subdivision, which has notable impacts on respiration, and allometric size scaling, the lungs of the various age groups were similar. All morphological and morphometric evidence illustrate that the ventral cranial region of the lungs is the most gas efficient region with the highest surface area to volume ratio. This is followed by the dorsal cranial region, ventral middle region, dorsal middle region, ventral caudal region and the dorsal caudal region of the lungs. Due to the arrangement and different levels of efficiency of the various regions of the lung, normal tidal ventilation doesn’t seem feasible. Instead of this, unidirectional airflow with a mechanism, made of the various structural components, rationing air usage for optimal gas exchange for particular activities is more plausible. This hypothesis strengthens the currently envisaged reptile – avian pulmonary evolutionary theory due to the similar nature of their respiratory systems.