Two types of nucleolus can be distinguished among eukaryotic cells: a tri-compartmentalized nucleolus in amniotes and a bi-compartmentalized nucleolus in all the others. However, though the nucleolus' ultrastructure is well characterized in mammals and birds, it has been so far much less studied in reptiles. In this work, we examined the ultrastructural organization of the nucleolus in various tissues from different reptilian species (three turtles, three lizards, two crocodiles, and three snakes). Using cytochemical and immunocytological methods, we showed that in reptiles both types of nucleolus are present: a bi-compartmentalized nucleolus in turtles and a tri-compartmentalized nucleolus in the other species examined in this study. Furthermore, in a given species, the same type of nucleolus is present in all the tissues, however, the importance and the repartition of those nucleolar components could vary from one tissue to another. We also reveal that, contrary to the mammalian nucleolus, the reptilian fibrillar centers contain small clumps of condensed chromatin and that their surrounding dense fibrillar component is thicker. Finally, we also report that Cajal bodies are detected in reptiles. Altogether, we believe that these results have profound evolutionarily implications since they indicate that the point of transition between bipartite and tripartite nucleoli lies at the emergence of the amniotes within the class Reptilia.

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The differentiation of fat cells in the regenerating tail of the lizards Lampropholis delicata and L. guichenoti was studied until the 55th day post amputation. To study the proliferation of and protein synthesis in fat cells, tritiated thymidine and proline were injected into lizards after 3 weeks of tail regeneration and the tracers were revealed by optical and electron microscopic autoradiography. After 3 weeks of tail regeneration, adipocytes are rarely seen in the apical regions of the regenerating tail and most of the preadipose cells are visible in the submuscular connective tissue in the proximal regions. Fat cells begin to proliferate and accumulate visible lipid droplets in the proximal submuscular connective tissue after about 3 weeks of tail regeneration. The kinetics of [3H]thymidine uptake after progressive postinjection periods suggest that fat tissue mass increases mostly as as result of fat cell hypertrophy, with only limited cell multiplication. During the initial stages of their differentiation, lipoblasts also take up [3H]proline, but the ergastoplasm and Golgi apparatus become reduced during the process of fat accumulation. The final size of regenerated fat cells was less than that of the original fat cells.

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