Structure of the kidney of Bufo arenarum: intermediate segment, distal tubule and collecting tubule

The ultrastructure of the intermediate segment (IS), distal tubule and collecting tubule (CT) of the south american toad Bufo arenarum, was studied by light and transmission electron microscopy. The IS is composed of cubical ciliated cells which propel the urine along the renal tubule. The distal tubule is divided into two portions: the early distal tubule (EDT) and the late distal tubule (LDT). The EDT is characterized by only one type of cells with well developed basolateral interdigitations and numerous elongated mitochondria, which are oriented normal to the basal surface. The "macula densa--like" is a specialized zone of the EDT in contact with the vascular pole, where cells are more tightly packed than in the rest of the tubule. The LDT shows two types of cells called dark and light cells according to the appearance of their cytoplasm. Dark cells have microplicae and few but long microvilli at their luminal surface, and abundant mitochondria in their cytoplasm. Light cells show basal and lateral infoldings and few mitochondria. The CT, which is composed of dark and light cells, exhibits an enlarged lumen with an undulated surface and dilated spaces between neighbouring cells. This work is a contribution to the knowledge of the kidney of B. arenarum; frequently used as an experimental model for physiological and biochemical studies.

Ultrastructural characteristics of the lung of Melanophryniscus stelzneri stelzneri (Weyenberg, 1875) (Anura, Bufonidae)

The lung of the toad, Melanophryniscus stelzneri stelzneri was studied using scanning and transmission electron microscopy. In M.s.stelzneri the parenchyma forms a polygonal network arrangement, therefore the parenchyma is edicular. These spaces are delimited by the interconnection of third order septa which are covered by respiratory epithelium. Small patches of ciliated epithelium without goblet cells appear irregularly distributed on the septa. The respiratory epithelium consists of one type of pneumocyte, which shows characteristics of both type I and type II alveolar cells of higher vertebrates. The pneumocytes are irregular in shape and possess attenuated cytoplasmic processes, which spread around the capillaries to form the outer layer of the air-blood barrier. These cells contain different types of cytoplasmic bodies: electron dense bodies, multivesicular bodies and lamellar bodies. Dense bodies are probably the precursors of lamellar bodies and the multivesicular bodies are incorporated into the latter. Neuroepithelial bodies appear randomly distributed over the septa. These bodies are separated from the lumen of the lung by thin cytoplasmic processes of neighbouring pneumocytes. The air-blood barrier consists of three layers: epithelium, interstitial space and endothelium. The relatively simple pulmonary structure of M.s.stelzneri is due to a lower degree of partitioning of the pulmonary lumen in comparison to the lung of other bufonid anurans, could be correlated with a well developed cutaneous and buccopharingeal respiration. The testing of this hypothesis awaits further studies.

Structure of the Bufo arenarum kidney: renal corpuscle, neck segment and proximal tubule

The structural organization of the renal corpuscle (RC), ciliated neck segment (NS) and the proximal tubule (PT) were studied in the toad, Bufo arenarum, by means of light and transmission electron microscopy. The ciliated neck segment and the proximal tubule are located in the dorsolateral zone of the kidney, while the distal tubules are located in a ventromedial zone. RC are found between these two zones. The glomerular filter apparatus consists of the podocyte epithelium, a basement membrane, a subendothelial space and an endothelium. The podocyte emits cytoplasmatic processes extending over the surface of the glomerular capillaries. These processes divide into further processes ending in expansions known as pediceles. The basement membrane consists of a lamina rara externa and a rather thin lamina densa, while the subendothelial space contains collagen fibers and slender cytoplasmic processes of the mesangial cells. NS are composed of ciliated cells with a characteristic location of the mitochondria. The PT consists of prismatic cells with a dense luminal brush border of long microvilli and numerous apical vesicles. The basal cell membrane is increased by small infoldings. One characteristic structure of the cytoplasm is the presence of lipid droplets. The cytological structure of PT cells can be considered as an adaptation for the reabsorption of organic materials

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, Bufo arenarum, as observed by light and electron microscopy. The lung wall 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 coated by ciliated epithelium containing numerous goblet cells. Second order folds present the same type of epithelium but devoid of goblet cells, while third order folds are only lined by respiratory epithelium. The 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 in shape and possess attenuated cytoplasmatic processes which spread over the pulmonary capillaries to form the outer layer of the air-blood barrier. These cells present microvilli in the apical extreme and contain different types of cytoplasmic bodies: electron dense, multivesicular and lamellar