THE LUNG OF THE COMMON TOAD, BUFO-ARENARUM (ANURA, BUFONIDAE) - A LIGHT AND ELECTRON-MICROSCOPY STUDY

The present study describes the lung morphology of the common toad, Bu fo arenarum, as observed by light and electron microscopy. The lung wa ll consists of four layers: mesothelium, dense connective tissue with thin elastic fibers, loose connective tissue containing smooth muscle fibers and an internal respiratory epithelium. The lung presents three types of folds defined by their epithelia. First order folds are coat ed by ciliated epithelium containing numerous goblet cells. Second ord er folds present the same type of epithelium but devoid of goblet cell s, while third order folds are only lined by respiratory epithelium. T he respiratory surface of the lung is lined by a single cell type, the pneumocyte, which presents characteristics of both type I and type II alveolar cells of higher vertebrates. The pneumocytes are prismatic i n shape and possess attenuated cytoplasmatic processes which spread ov er the pulmonary capillaries to form the outer layer of the air-blood barrier. These cells present microvilli in the apical extreme and cont ain different types of cytoplasmic bodies: electron dense, multivesicu lar and lamellar.