The nucleolus during epidermal development in an insect

A role for hypertrophic astrocytes and astrocyte precursors in a case of rapidly progressive multiple sclerosis

The purpose of this study was to examine the roles played by astrocytes in a case of rapidly progressive multiple sclerosis (MS). Within early-active and active lesions, hypertrophic astrocytes played an important role in lesion pathology through the phagocytosis of myelin and axonal debris and through the internalization of other glial cells, including astrocytes. In addition to this critical role, hypertrophic astrocytes, in areas that lack significant inflammation (within the adjacent normal appearing white matter and within late remyelinating lesions) were found to be active in myelin and axonal debris phagocytosis with no evidence of cellular internalization. Hypertrophic astrocytes therefore not only play an important role in the pathogenesis of MS lesions but also exert a continued deleterious effect upon tissue in the absence of significant inflammation. In addition, we found evidence for a significant population of vimentin-positive, glial fibrillary acidic protein (GFAP)-negative, bipolar, astrocyte precursors within the late remyelinating lesions. Their significance is not known but a possible role may include their participation in the successful remyelination of the lesion.

Surface membranes, Golgi complexes, and vacuolar systems

In the absence of fossils, the cells of vertebrates are often described in lieu of a general animal eukaryote model, neglecting work on insects. However, a common ancestor is nearly a billion years in the past, making some vertebrate generalizations inappropriate for insects. For example, insect cells are adept at the cell remodeling needed for molting and metamorphosis, they have plasma membrane reticular systems and vacuolar ferritin, and their Golgi complexes continue to work during mitosis. This review stresses the ways that insect cells differ from those of vertebrates, summarizing the structure of surface membranes and vacuolar systems, especially of the epidermis and fat body, as a prerequisite for the molecular studies needed to understand cell function. The objective is to provide a structural base from which molecular biology can emerge from biochemical description into a useful analysis of function.

Chromosomes exhibit preferential positioning in nuclei of quiescent human cells

The relative spatial positioning of chromosomes 7, 8, 16, X and Y was examined in nuclei of quiescent (noncycling) diploid and triploid human fibroblasts using fluorescence in situ hybridization (FISH) with chromosome-specific DNA probes and digital imaging. In quiescent diploid cells, interhomolog distances and chromosome homolog position maps revealed a nonrandom, preferential topology for chromosomes 7, 8 and 16, whereas chromosome X approximated a more random distribution. Variations in the orientation of nuclei on the culture substratum tended to hinder detection of an ordered chromosome topology at interphase by biasing homolog position maps towards random distributions. Using two chromosome X homologs as reference points in triploid cells (karyotype = 69, XXY), the intranuclear location of chromosome Y was found to be predictable within remarkably narrow spatial limits. Dual-FISH with various combinations of chromosome-specific DNA probes and contrasting fluorochromes was used to identify adjacent chromosomes in mitotic rosettes and test whether they are similarly positioned in interphase nuclei. From among the combinations tested, chromosomes 8 and 11 were found to be closely apposed in most mitotic rosettes and interphase nuclei. Overall, results suggest the existence of an ordered interphase chromosome topology in quiescent human cells in which at least some chromosome homologs exhibit a preferred relative intranuclear location that may correspond to the observed spatial order of chromosomes in rosettes of mitotic cells.

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.

Ultrastructural evidence for the accumulation of insulin in nuclei of intact 3T3-L1 adipocytes by an insulin-receptor mediated process

Monomeric ferritin-labeled insulin (Fm-Ins), a biologically active, electron-dense marker of occupied insulin receptors, was used to characterize the internalization of insulin in 3T3-L1 adipocytes. Fm-Ins bound specifically to insulin receptors and was internalized in a time- and temperature-dependent manner. Fm-Ins was found in cytoplasmic vesicles within 5-10 min at 37 degrees C and subsequently was observed in multivesicular bodies and lysosomes. In addition, small amounts of Fm-Ins were associated with nuclei after 30 min. The number of Fm-Ins particles observed in nuclei continued to increase in a time-dependent manner until at least 90 min. In the nucleus, several Fm-Ins particles usually were found in the same general location--near nuclear pores, associated with the periphery of the condensed chromatin. Addition of a 250-fold excess of unlabeled insulin or incubation at 15 degrees C reduced the number of Fm-Ins particles found in nuclei after 90 min by 99% or 92%, respectively. Nuclear accumulation of unlabeled ferritin was only 2% of that found with Fm-Ins after 90 min at 37 degrees C. Biochemical experiments utilizing 125I-labeled insulin and subcellular fractionation indicated that intact 3T3-L1 adipocytes internalized insulin rapidly and that approximately equal to 3% of the internalized ligand accumulated in nuclei after 1 hr. These data provide biochemical and high-resolution ultrastructural evidence that 3T3-L1 adipocytes accumulate potentially significant amounts of insulin in nuclei by an insulin receptor-mediated process. The transport of insulin or the insulin-receptor complex to nuclei in this cell or in others may be directly involved in the long-term biological effects of insulin--in particular, in the control of DNA and RNA synthesis.

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.

Studies on chromatin organization in a nucleolus without fibrillar centres. Presence of a sub-nucleolar structure in KCo cells of Drosophila

In embryonic cell-line derivative KCo of Drosophila melanogaster, the nucleolus, like most nucleoli, contains a small proportion of ribosomal DNA (1-2% of the total nucleolar DNA). The ribosomal DNA is virtually the only active gene set in the nucleolus and is found among long stretches of inactive supercoiled heterochromatic segments. We have demonstrated by use of a Feulgen-like ammine-osmium staining procedure that, depending on the state of growth, more or less fibres of decondensed DNA emanating from the intra-nucleolar chromatin (which is in continuity with the nucleolus-associated chromatin) ramify and unravel within the central nucleolar core to be transcribed. The nucleolus expands or contracts with the variation of activity and could belong to a supramolecular matricial structure such as is shown after extraction of the nuclei. After a long period of exposure to high doses of actinomycin D, the central nucleolar core became an homogeneous fibrous structure that could be interpreted as an aggregate of protein skeletal elements. The mechanism of repression and derepression of the nucleolar chromatin could thus be explained by a mechanism involving in part a sub-nucleolar structure. We propose a schematic organization of the nucleolar chromatin in KCo cells of Drosophila and discuss it in relation with other nucleolar organizations.

Golgi complex beads in vertebrates and their relationship with clathrin coats

Addition of tannic acid to the primary glutaraldehyde fixative and the viewing of thin sections by stereo electron microscopy greatly simplifies the detection of vertebrate cell Golgi complex beads which are otherwise difficult to see since they do not stain with bismuth. These results confirm the generality of conclusions from experiments on arthropod beads which are easily observed because of their bismuth affinity. In vertebrate and arthropod cells, bead rings encircle the base of forming transition vesicles below the growing portion of the vesicle that is covered with a clathrin coat. Their unique position at such a sharp functional and structural boundary in intercompartmental transport suggests that the bead rings may specify a select region of rough endoplasmic reticulum devoid of ribosomes where clathrin coats can induce transition vesicle formation and prevent intermixing of the elements of a returning transition vesicle.

Bismuth chemistry and the glutaraldehyde insensitive staining reaction

The free metal ion, Bi3+, is the only chemical species of bismuth that stains a strongly bismuth-reactive molecule, polyarginine, in vitro. The bismuth solution specifically requires tartrate as a chelating agent for the reaction to occur between pH 7.4 and 8.0. Since Bi3+ reacts strongly with polyarginine, creatine and ATP fixed to cellulose acetate strips and DEAE-cellulose and P-cellulose, the free metal ion (Bi3+) may bind to phosphate or guanidyl groups, or both, after glutaraldehyde fixation.

The correlation between bismuth and uranyl staining and phosphorus content of intracellular structures as determined by electron spectroscopic imaging

Four groups of intracellular structures can be recognized according to bismuth and uranyl staining and phosphorus content. (1) Those which contain phosphorus and stain strongly with uranyl acetate but not with bismuth (ribosomes, heterochromatin and mature ribosomal precursor granules), presumably because of their nucleic acid content. (2) Those which contain phosphorus and stain with uranyl acetate and bismuth (interchromatin granules, immature ribosomal precursor granules and mitochondrial granules), presumably because at least some of their phosphate is available to react with bismuth. (3) Those which contain little phosphorus but which stain strongly with bismuth and weakly with uranyl acetate (Golgi complex beads), perhaps because some ligand in addition to phosphate reacts with bismuth, and (4) those which do not contain phosphorus and stain with neither uranyl acetate nor bismuth (portasomes). Uranyl staining correlates strongly with the phosphorus content of nucleic acids, proteins and inorganic deposits. Bismuth will stain some phosphorylated molecules but not all. Thus only some phosphates stain with bismuth.

High resolution microanalysis for phosphorus in Golgi complex beads of insect fat body tissue by electron spectroscopic imaging

Golgi complex beads are 10 nm particles arranged in rings on the smooth forming face of the Golgi complex that stain specifically with bismuth in arthropod cells. In vitro experiments with biological molecules spotted on to cellulose acetate strips indicated that bismuth bound to the beads through phosphate groups. We could detect a weak phosphorus signal from the beads using a new technique called electron spectroscopic imaging that is capable of very high spatial resolution (0.3-0.5 nm) and sensitivity (50 atoms of phosphorus). Detection was not obscured by tissue staining with bismuth or uranyl acetate of by using an inorganic buffer (Na cacodylate). Localization of phosphorus was greatly improved by using colour-enhanced computer pictures of the electron spectroscopic images and quantitating the images. The results indicate that the phosphorus content of the beads is large enough to account for their bismuth reactivity.

Epidermal feet in pupal segment morphogenesis

Epidermal cells in insect integumental epithelia develop branched cytoskeletal extensions or feet at their base that are similar in appearance to the processes put out by cells in tissue culture. We have developed a procedure to show the feet that gives an effect as if thousands of cells randomly arranged in the epithelium had each been injected with lead salt visualized as black lead sulphide. The procedure depends upon the fact that after brief glutaraldehyde fixation, tannic acid only penetrates some cells where it mordants lead ions and binds osmium. Individual cells visualized in this manner show their outlines as if they are separate in a tissue culture although they are part of a closely packed epithelium. The feet are metamorphic structures formed after pupal commitment and are necessary for metamorphic changes in segment shape. In Calpodes larvae the feet are orientated axially in the direction of the segmentally repeating gradient and may extend for several cell diameters. They extend under the influence of low titres of 20-hydroxyecdysone such as those occurring in the intermoult. When stimulated by high titres like those in pre-pupae, the feet contract at the same time as the segments shorten to pupal proportions. We believe that cell processes like the epidermal feet are ubiquitous but that they have often been overlooked because of the difficulty of demonstrating the outlines of single cells that are united in epithelia.