Ultrastructure of the Respiratory Epithelium in the Lungs of the Newt Triturus cristatus

Abstract

The respiratory epithelium in the lungs of the newt Triturus cristatus has been studied by electron microscopy. T h e entire pulmonary gas-exchange area is covered by a continuous epithelium, the cells of which are all of the same type and are termed "pneumonocytes". Typically each pneumonocyte is squamous and has attenuated sheets of cytoplasm which cover the pulmonary capillaries. Its free surface bears microvilli while mitochondria, multivesticular bodies and small inclusions are prominent in its cytoplasm. Many pneuomonocytes send cytoplasmic processes deep into the substance of the lung wall. I t is postulated that these processes may help to anchor the epithelium.

A scanning and transmission electron microscopic study of the lung of a caecilian Boulengerula taitanus

“…The average alveolar diameter in Boulengerulu (300 pm) was on the extreme upper end of the range 150-300 pm, estimated for Rana pipiens by Bils & Hughes (1978), fell within the range 250 pm-1000 pm for Zchthyophis (Pattle et al, 1977) and was smaller than that reported on five Anuran amphibian species (850 pm-2300 pm) by Tenney & Tenney (1970). The thickness of the blood-gas barrier in Boulengerula (2.35 pm) is comparable to that in Rana temporuria and Sulumandra sulamandra examined by Meban (1980) and Goniakowska-Witalihska (1978). A very thin barrier (0.6 pm) has been reported in the tree frog HyIu arborea by Goniakowska-Witalinska (1 986), while a relatively thick barrier (2-81 pm) was found in the crested newt (Triturus cristatus) by Meban (1980).…”

“…Note the numerous intracellular vesicular organelles (presumed to be lysosomes), and pseudopodia (arrow) that characterize the cell. ;Pattle et al, 1977) and basically to that of some other air-breathing vertebrates like the Dipnoi (Grigg, 1965;Hughes & Weibel, 1976Maina & Maloiy, 1985) and Urodela (Okada et al, 1962;Meban, 1977;Goniakowska-Witalinska, 1982;Stark-Vancs et al, 1984). The caecilian lung appears, however, to be more elaborate than the non-septate lungs of the urodeles like Proteus, Cryptobranchus and Salamandra (Noble, 193 1;Marcus, 1937;Hughes, 1970;Goniakowska-Witalinska, 1978) which are smooth and poorly vascularized.…”

“…Increased septation of the lung appears to characterize the more metabolically active amphibians (Goniakowska-Witalinska, 1985). Structural similarities in pulmonary design, such as a double capillary system, preponderance of connective tissue elements in the interalveolar septa, and a single population of the respiratory alveolar pneumocytes have, despite the gross differences, been noted in Amphibia (Meban, 1973(Meban, , 1977Dierichs, 1975;Hightower et al, 1975;Goniakowska-Witalinska, 1978, 1986Starck-Vancs et al, 1984;Pohunkova & Hughes, 1985). On closer examination, fine structural differences between the amphibians have, however, been observed.…”

“…The amphibians in general appear to have undifferentiated alveolar pneumocytes. In Anura (Meban, 1973;Dierichs, 1973Dierichs, , 1975Pohunkova & Hughes, 1985), in Urodela (Meban, 1977;Goniakowska-Witalinska, 1978, 1980a and Apoda (Pattle et al, 1977;Welsch, 1981), the alveolar pneumocytes were observed to bear morphological characteristics common to the mammalian type 1 and 2 cells. We have observed this unclear differentiation of the pneumocytes in Boulengerula and it has also been noted in the Dipnoi lung (Hughes & Weibel, 1976Maina & Maloiy, 1985;Maina, 1987).…”

“…As suggested by Maina (1988), the morphological differentiation of these cells in the lungs of the higher vertebrates may reduce the total alveolar pneumocyte density and hence bring about a thinner blood-gas barrier and a concomitant reduction of the oxygen consumption by the lung tissue, factors which enhance the efficiency of the lung. That indeed the barrier gets thinner from amphibians, reptiles to mammals has been established by Meban (1980); the birds as a group apparently have the thinnest barrier among the air-breathing vertebrates (Maina & King, 1982). PLATE 111.…”