Tongues were removed from fetuses of mice on the 15th day of gestation (E15), from newborns (PO), from juveniles on the 7th day (P7) and on the 14th day (P14) after birth for examination by scanning electron microscopy. In the fetuses at E15, rudiments of fungiform papillae with a relatively regular, lattice-like pattern were visible on the anterior half of the dorsal surface of the tongue. The outline of the rudiment of a circumvallate papilla could be recognized on the median line between the lingual body and the lingual radix. No rudiments of filiform papillae could be seen. At PO, rudiments of filiform papillae were compactly distributed over the dorsal surface, as are the filiform papillae in the adult, their width was approximately one-third of that of fungiform papillae, and their tips were rounder than those of the filiform papillae in the adult. The fungiform papillae then increased in size and became somewhat irregular in shape. In juveniles at P7, the filiform papillae were long and slender, being relatively large on the intermolar eminence. A taste pore was visible in the center of each fungiform papilla at this stage. The shape of the circumvallate papilla was similar to that in the adult. In juveniles at P14, the shapes of all three types of papilla were almost the same as those in the adult. The rudiments of each of the three kinds of lingual papilla appeared at a different stage of development of mice; rudiments of the fungiform and circumvallate papillae, which are related to the sense of taste, were formed earlier than those of the filiform papillae, which are not involved in taste.
The structure of the nephron in the ranid frog, Rana cancrivora, was studied by light and electron microscopy. This frog is the only amphibian species to live in mangrove swamps of very high salinity. The nephron consists of the following parts: renal corpuscle, ciliated neck segment, proximal tubule, ciliated intermediate segment, distal tubule, connecting tubule, and collecting duct. The distal tubule is located in the ventromedial region of the kidney, and the other tubules are situated in the dorsolateral region. Renal corpuscles are found between the two regions. Some renal corpuscles have a wide Bowman's space because of the small glomerulus within them. The proximal tubules are composed of columnar cells with a dense luminal brush border of long microvilli and numerous apical vesicles and vacuoles. The initial part of the distal tubule consists of heavily interdigitated cells, characterized by a very regular palisade arrangement of mitochondria. In the terminal part of the distal tubule, shorter mitochondria of the infolding cells are situated irregularly around the nucleus. The connecting tubule consists of principal cells and canaliculus cells. The collecting duct consists of columnar or cuboidal cells; cytoplasmic organelles are relatively sparse. The canaliculus cells are intercalated between principal cells from the terminal distal tubule to the proximal part of the collecting duct. Our findings indicate that the kidney of R. cancrivora is structurally similar to kidneys of other amphibians. These findings are discussed with regard to probable correlations between ultrastructure and function in R. cancrivora.
Salinity tolerance and histology of gills were studied in Rana cancrivora larvae. The tadpoles at the external gill stages (W stages 21-22) were able to survive in media containing up to 40% seawater, but died in water of higher salinity. Their external gills appear to have no critical role in adaptation to seawater. However, advanced tadpoles with internal gills (T-K stages I-XVIII) were able to tolerate 50% or higher seawater. In the internal gills, there are numerous mitochondria-rich cells (MR cells) scattered on the ventral and lateral epithelia of the gill arches and the gill tufts in both freshwater- and seawater-acclimated tadpoles. In freshwater-acclimated tadpoles there are three types of MR cell: (1) microplicated, (2) microvillous, and (3) apically vacuolated. In tadpoles acclimated to dilute seawater, the ratio of type-1 to type-2 cells is lower, although all three types of MR cell are present. In 60%-seawater-acclimated tadpoles, a few MR cells with a lumen and concave cytoplasm at the apical membrane (type 4) are present. The changes in MR cell morphology under ambient conditions of low or high salinity may reflect alterations in the physiological roles of the gills with regard to transport of ions.
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