Morphology and morphometry of the lungs of two East African mole rats, Tachyoryctes splendens and Heterocephalus glaber (Mammalia, Rodentia)

The lungs of two fossorial rodents, the mole rat Tachyoryctes splendens and the naked mole rat Heterocephalus glaber were investigated by transmission and scanning electron microscopy and a comparative morphometric analysis of the lungs carried out in an attempt to find out whether there are any possible structural adaptational features which may be associated with fossoriality. The data from these two ecologically disparate fossorial rodents were compared with those of surface dwelling rodents on which equivalent data are available. Morphologically, the lung of T. splendens is essentially similar to that of terrestrial mammals while that of H. glaber shows features of underdevelopment. In H. glaber, a cuboidal epithelium extends down the respiratory tree to line what appear to be alveolar spaces, the blood capillaries constitute a double capillary system and the type I pneumocytes have microvilli on their free surface. Morphometrically, H. glaber has notably lower values indicative of rather unspecialized lungs. While the volume density of the parenchyma is 88% in T. splendens, that in H. glaber is only 76%. The blood-gas (tissue) barrier in H. glaber is notably thicker than in T. splendens. When normalized with body weight, the surface area of the blood-gas (tissue) barrier, the pulmonary capillary blood volume, the diffusing capacities of the tissue barrier and of the whole lung are consistently appreciably lower in H. glaber. When compared with Mus musculus, Rattus rattus and Cavia porcellus, T. splendens has somewhat comparable values with the surface dwelling rodents but the values of H. glaber are the lowest in the group. It is suggested that T. splendens has not undergone full adaptation to fossoriality as is supported by its behavioural activities, particularly those of occasionally surfacing to feed and making overland excursions. The low values of H. glaber may be commensurate with its extreme physiological adaptations for fossoriality, features which culminate in low basal metabolism and may in part explain paedomorphic traits of its respiratory system.