Wilhelm bernhard workshop on the cell nucleus

THE NUCLEOLUS OF THE TELEOST FISH Barbus barbus (L): AN EXAMPLE OF NUCLEOLUS WITHOUT FIBRILLAR CENTER

Thiry M.

Laboratoire de biologie cellulaire, Département des Sciences de la vie, Université de Liège, Institut A. Swaen, Liège, Belgium
Nucleoli are ubiquitous structural elements of eucaryotic interphase cell nuclei. They are the morphological structures where rRNA genes are expressed. On the basis of their structural compartmentalization, two types of nucleoli can be distinguished. The first type of nucleoli contains three main nucleolar components: the fibrillar centers (FCs), the dense fibrillar component (DFC), and the granular component (GC). It is typically accounted in mammalian cells. In the second type of nucleoli, in addition to a GC it contains only one fibrillar component. This ultrastructural feature is characteristic of amphibian oocytes where it is well known, but also seems to occur in somatic and germinal cells of many invertebrate and anamniote vertebrate species. In the present study, we studied the evolution of the nucleoli during oogenesis in the teleost fish Barbus barbus (L) using light and electron microscopies. We distinguished eight successive stages in nucleolar structure : (I) a single, compact, central nucleolus in the lobed nucleus of the oogonium, (II) the nucleolus remained compact, it lay in the central area of the oval nucleus of the leptotene oocyte, (III) the nucleolus became peripheral in the zygotene oocyte, (IV) a few tiny nucleoli appearing in the DNA cap of the pachytene oocyte, (V) several enlarged and vacuolized nucleoli at the beginning of the diplotene stage, (VI) Fragmentation of nucleoli into multiple spherical nucleoli with a fibrillar core and several short granular arches at their surface during the end of previtellogenesis, (VII) multiple reticulated nucleoli with a fibrillar core surrounded by a well-developped network of granular and fibrillar ribbons in the early vitellogenic oocyte, (VIII) Nucleoli fragmented into fibrillar and granular remains in the late vitellogenic oocyte. By cytochemical and immunocytological methods, we showed further that the nucleoli of germ and follicle cells comprised a fibrillar, silver-stainable component and a granular component whatever the oogenetic stage. Using the TdT method, we detected DNA, a characteristic constituent of fibrillar centers in the mammalian cell nucleolus, in the fibrillar component of the fish nucleolus. The nucleolar protein, fibrillarin, a marker of the dense fibrillar component in the mammalian cell nucleolus, was also located in the single fibrillar part of the Barbus nucleolus. These data supports the view that the nucleolus of the teleost fish, Barbus barbus, is an excellent model of nucleolus without fibrillar center.