Nucleolar necklace formation in response to hemolymph ecdysteroid peaks

Nucleolonema as a fundamental substructure of the nucleolus

The nucleolus is the most obvious structure in the eukaryotic nucleus. It is known to be a ribosome-producing apparatus where ribosomal (r) DNA is transcribed and the primary rRNA transcripts are processed to produce three of the four rRNA species. Electron microscopy has shown that the nucleolus consists of three major components, a dense fibrillar component (DFC), a granular component (GC) and a fibrillar center (FC). The DFC and FCs are integrated into a fundamental nucleolar substructure called the nucleolonema. The DFC corresponds to the matrix of the nucleolonema, and the FC is an electron microscopic counterpart of argyrophobic lacunae localized in the nucleolonema. The spherical FCs are intermittently arranged along the length of the nucleolonema in actively growing cells but are fused with each other to form tubular FCs when rDNA transcription is hampered. The RNase-gold complex does not bind to the FC but to the DFC and the GC, suggesting that rDNA transcription does not occur in the FC although both fluorescence in situ hybridization (FISH) and electron microscopic in situ hybridization reveal that the rDNA is specifically localized in the FCs. Immunogold-labeling after bromo-UTP (BrUTP) incorporation shows that rDNA transcription takes place in the boundary region between the FC and the DFC, and primary rRNA transcripts are expected to be processed outward within the DFC. Data have accumulated suggesting that the nucleolonema is a fundamental substructure of the nucleolus, and its skeleton is the tandem arrangement of the FCs, which are resting harbors or storages of rDNA. This paper proposes that the transversal structural organization of the nucleolonema is centrifugally built up by several structural and functional domains: condensed and/or loosened rDNA, rDNA transcription zone, and transcript processing and ribosome assembly zones.

Juvenile hormone controls previtellogenic proliferation of ribosomal RNA in the mosquito fat body

During the previtellogenic development of mosquito fat body cells, the nucleolus, the organelle responsible for producing ribosomes, enlarges threefold, reaching maximal size between 2 and 3 days after eclosion. The granular component of the nucleolus containing ribosomal precursors increases considerably as well. These signs of nucleolar activation correlate with the synthetic rate and accumulation of poly(A)- RNA (predominantly ribosomal RNA) in the fat body cells. The amount of poly(A)- RNA in fat body cells increases during the first 2 days after eclosion and then declines gradually. The rate of RNA synthesis exhibits similar kinetics, but both the rise and the decline are sharper than for the accumulation of RNA. All the characteristics of nucleolar activation, its enlargement, accumulation of poly(A)- RNA, and the increased rate of RNA synthesis, are blocked by removal of the corpora allata (CA) in newly eclosed adult females but could be restored by either implantation of CA or topical application of juvenile hormone III or its analog, 7-S-methoprene, to allatectomized females. Thus, previtellogenic activation of fat body nucleoli for ribosomal RNA production is controlled by juvenile hormone from the corpora allata.

Nucleolar cycles during the fifth stadium in Manduca epidermis

The changing pattern of nucleolar structure in the epidermal cells of Manduca sexta has been correlated with hormonal changes taking place during the fifth stadium. The epidermal nucleoli show three cycles of development, the first and third of which occur at the beginnings of the intermoult and moult phases respectively and are related to larval and pupal syntheses. The second phase occurs in the middle of the stadium but prior to the onset of wandering and commitment to pupation. A phase of mitosis separates the second and third cycles. The three cycles thus correspond in time to those found in Calpodes. The three cycles of nucleolar change are superimposed over nuclear changes relating to the degree of ploidy. Each phase begins with an expansion of the condensed nucleoli to form lobed rings and then necklaces. In the first phase (day 0-3), the rings and necklaces progress to form threaded networks. Both rings and networks have many ribosomal precursor granules that are lacking in condensed nucleoli. The rings and networks are therefore presumed to be more active in rRNA synthesis than the condensed state. The first and third phases of nucleolar change occur after elevated titres of haemolymph ecdysteroid. Post-thoracic ligation of animals at ecdysis blocks nucleolar changes as well as the appearance of polyploid nuclei. Nucleolar changes may be a primary response of the epidermis to stimulation by ecdysone.