The morphology of the lung of the black mamba Dendroaspis polylepis (Reptilia: Ophidia: Elapidae). A scanning and transmission electron microscopic study.

The lung of a snake, the black mamba (Dendroaspis polylepis), has been investigated by scanning and transmission electron microscopy. This species has only one lung, the right, which is long and occupies most of the pleuro-peritoneal cavity. Grossly, the lung could be divided into two discrete anatomical regions: an anterior respiratory area made up of a honeycomb network of capillary-bearing partitions, and a posterior membranous saccular region. The exchange region consisted of a central air duct, the bronchus, which was delineated both dorsally and laterally by morphologically and spatially distinct hierarchically arranged septa. The primary septa gave rise to the secondary septa from which the much deeper peripherally situated tertiary septa that formed the immediate openings to the faveoli arose. The faveoli were rather parallel elongated pockets separated by partitions, the interfaveolar septa, and terminated peripherally on the pleura. A double capillary disposition of the blood capillaries was observed on the relatively thick primary and secondary septa. These septa were lined by a heterogenous epithelium made up of ciliated cells, secretory cells, and smooth squamous cells. This epithelium was continued from the trachea and the bronchus. At the faveolar level the blood capillaries exhibited a single system where they formed a matrix on both sides of the partitions. The surface of the faveoli was covered by two types of cells: Type I cells were squamous and their remarkably attenuated cytoplasmic arborisations were notably extensive while the Type II cells were rather cuboidal, bore stubby microvilli and contained the characteristic osmiophilic lamellated bodies. On the basis of the clearly evident complete differentiation of the pneumocytes and the presence of both the double and single capillary systems, it was observed that this lung, and apparently the reptilian lung in general, manifests a transitional developmental and structural stage in the evolution of the lungs of the air-breathing vertebrates from lower through to higher vertebrates. The gross and ultrastructural heterogeneity of the organisation of the ophidian lung is illustrated and the dearth of pulmonary morphological data in this taxon is pointed out.