Morphological changes of the nucleolus during oogenesis in oviparous teleost fish, Barbus barbus (L.)

In fishes, like in amphibians, it is well established that variations in rRNA activity occur during oogenesis. Contrary to amphibians, however, little is known about the ultrastructural changes of the nucleolus during fish oogenesis. Evolution of the nucleolus has been followed during oogenesis in the teleost fish Barbus barbus (L.) using light and transmission electron microscopies. We show that the behaviour of the nucleolus during B. barbus oogenesis resembles that reported in amphibians but also presents several peculiarities. The most striking feature is the marked vacuolization of nucleoli occurs at the beginning of the growth during previtellogenesis. The results obtained by means of the in situ terminal deoxynucleotidyl transferase-immunogold method for detecting DNA seem further to indicate that the chromatin cap becomes integrated into developing nucleoli during previtellogenesis and then segregate at the periphery of nucleoli at the end of glycoproteinic vitellogenesis. Our study also shows that the nucleoli of germ cells, like that of follicle cells, are devoid of fibrillar centre but comprise a fibrillar and a granular component whatever the oogenetic stage. Ultrastructural detection of DNA and nucleolar proteins (AgNOR proteins, fibrillarin, and pp135) supports further the view that the Barbus nucleolus is a bipartite structure.