Behavior of pigment cells closely correlates the manner of gastrulation in sea urchin embryos

To know whether behavior of pigment cells correlates the process of gastrulation or not, gastrulating embryos of several species of regular echinoids (Anthocidaris crassispina, Mespilia globulus and Toxopneustes pileolus) and irregular echinoids (Clypeaster japonicus and Astriclypeus manni) were examined. In M. globulus and A. crassispina, the archenteron elongated stepwise like in well-known sea urchins. In the embryos of both species, fluorescent pigment cells left the archenteron tip and migrated into the blastocoel during gastrulation. In T. pileolus, C. japonicus and A. manni, on the other hand, the archenteron elongated at a constant rate throughout gastrulation. In these species, no pigment cell was observed at the archenteron tip during invagination processes; pigment cells began to migrate in the ectoderm from the vegetal pole side toward the apical plate without entering the blastocoel. These results clearly indicate that the behavior of pigment cells closely correlated the manner of gastrulation. Further, it was examined whether the archenteron cells are rearranged during invagination, by comparing the number of cells observed on cross sections of the archenteron at the early and late gastrula stages. The rearrangement was not conspicuous in A. crassispina and M. globulus, in which archenteron elongated stepwise. In contrast, the archenteron cells were remarkably rearranged in C. japonicus, alothough the archenteron elongated continuously. Thus, neither the behavior of pigment cells nor the manner of gastrulation matches the current taxonomic classification of echinoids.     

Effect of growth rate on the amounts of ribosomal and transfer ribonucleic acids in yeast.

The steady-state growth rate of Saccharomyces cerevisiae was varied by growing the cells in different media. The total amount of ribonucleic acid (RNA) per cell was found to decrease as a nonlinear function of decreasing growh rate. The RNA from cells growing in different media was analyzed by polyacrylamide gel electrophoresis. Although the amounts of both ribosomal RNA and transfer RNA decreased with decreasing growth rate, the ratio of ribosomal to transfer RNA was not constant. As the growth rate was reduced the ribosomal RNA fraction decreased slightly, whereas the transfer RNA fraction increased slightly. Thus the levels of ribosomal and transfer RNA were regulated to similar yet different extents. The levels of the different ribosomal RNA species were more closely coordinated. At all growth rates the ribosomal RNAs (including 5S RNA) were present in equimolar amounts. The rate of protein synthesis in yeast cells also decreased with decreasing growth rate. The low rates of protein synthesis did not appear to be due to limiting numbers of ribosomes or transfer RNA molecules.     

东方蝾螈原肠形成期瓶状细胞的结构及其对胚胎发育的影响

以１００只东方蝾螈早期胚胎为材料,用ＳＥＭ和ＴＥＭ对瓶状细胞形态结构和除去瓶状细胞的胚胎进行观察,去除瓶状细胞后,胚胎的原肠形成不会中断,外包和内卷仍能正常进行,并可以发育成幼螈,但有部分胚胎原肠前端缺失,长期培养的蝾螈胚对针刺激有反应,幼虫阶段中自由活动,我们的结论是：１）东方蝾螈原肠胚的瓶状细胞除了原肠前端存在外,还有部分瓶状细胞与原肠长轴垂直,从力学角度推断瓶状细胞在原肠形成过程中不起主要作     

Studies on protein multimers. VII. Stabilization of E. coli beta-galactosidase by a 260-nm absorbing compound in crude preparations

5-Azacytidine, which has been shown to inhibit the maturation of ribosomal RNA from its precursors when added to the medium of cultured Novikoff hepatoma cells, alters the electrophoretic mobilities of the 45S and 32S ribosomal RNA precursors formed in Novikoff cells. Coelectrophoresis of total cell RNA samples shows that the 45S and 32S RNA precursors from 5-azacytidine-treated cells migrate slower than the corresponding precursors from control cells. 5-Azacytidine causes some reduction in the rate and degree of methylation of the 45S and 32S RNA precursors; however, the alteration in electrophoretic mobility and maturation of these two RNA species do not appear to be a consequence of undermethylation. Coelectrophoresis of undermethylated RNA produced by methionine starvation with normal RNA shows no differences in mobility of the ribosomal precursor RNA_s. 5-Fluorouridine and 8-azaguanine, also inhibitors of ribosomal RNA maturation, were not found to cause detectable differences in migration of the ribosomal precursor RNA_s.     

Regulation of RNA synthesis in plant cell culture: delayed synthesis of ribosomal RNA during transition from the stationary phase to active growth

Ribosomal RNA synthesis was studied during the early phases of growth activation in a cell suspension culture derived from peanut (Arachis hypogaea, L.) cotyledon. Upon dilution from stationary phase, these cells show a characteristic lag of 3 days before the commencement of cell division. An analysis of the nature of RNA synthesized during this early period of growth showed that the cells obtained immediately upon dilution from stationary phase synthesize primarily messenger RNA and essentially no ribosomal RNA. The synthesis of ribosomal RNA is delayed for about 24 hr after which it rises sharply resulting in a 2- to 3-fold accumulation of ribosomal RNA per cell during the subsequent 24-hr period. Both the messenger RNA and the ribosomal RNA were characterized by their cellular localization; by sucrose and CsCl gradient analyses, and by the determination of their base ratios.It would appear that a major facet of the lag phase in the cell growth is the diversion of a significant part of the RNA biosynthetic apparatus from the synthesis of messenger RNA to that of ribosomal RNA.     

Cellular basis of gastrulation in the sand dollar Scaphechinus mirabilis

The processes of gastrulation in the sand dollar Scaphechinus mirabilis are quite different from those in regular echinoids. In this study, we explored the cellular basis of gastrulation in this species with several methods. Cell-tracing experiments revealed that the prospective endodermal cells were convoluted throughout the invagination processes. Histological observation showed that the ectodermal layer remained thickened, and the vegetal cells retained an elongated shape until the last step of invagination. Further, most of the vegetal ectodermal cells were skewed or distorted. Wedge-shaped cells were common in the vegetal ectoderm, especially at the subequatorial region. In these embryos, unlike the embryos of regular echinoids, secondary mesenchyme cells did not seem to exert the force to pull up the archenteron toward the inner surface of the apical plate. In fact, the archenteron cells were not stretched along the axis of elongation and were in close contact with each other. Here we found that gastrulation was completely blocked when the embryos were attached to a glass dish coated with poly-L-lysine, in which the movement of the ectodermal layer was inhibited. These results suggest that a force generated by the thickened ectoderm, rather than rearrangement of the archenteron cells, may play a key role in the archenteron elongation in S. mirabilis embryos.     

Behavior of pigment cells in gastrula-stage embryos of Hemicentrotus pulcherrimus and Scaphechinus mirabilis

The behavior of pigment cells in sea urchin embryos, especially at the gastrula stage, is not well understood, due to the lack of an appropriate method to detect pigment cells. We found that pigment cells emanated autofluorescence when they were fixed with formalin and irradiated with ultraviolet or green light. In Hemicentrotus pulcherrimus, fluorescent pigment cells became visible at the archenteron tip at the mid-gastrula stage. The cells detached from the archenteron slightly before the initiation of secondary invagination and migrated toward the apical plate. Most pigment cells entered the apical plate. This entry site seemed to be restricted, because pigment cells could not enter the ectoderm and remained in the blastocoele at the vegetal pole side when elongation of archenteron was blocked. Pigment cells that had entered the apical plate soon began to migrate in the aboral ectoderm toward the vegetal pole. In contrast, pigment cells of Scaphechinus mirabilis embryos were first detected in the vegetal plate before the onset of gastrulation. Without entering the blastocoele, these cells began to migrate preferentially in the aboral ectoderm toward the animal pole. When the archenteron tip reached the apical plate, pigment cells had already distributed throughout the aboral ectoderm. Thus, the behavior of pigment cells was quite different between H. pulcherrimus and S. mirabilis.     

A cyto-embryological study of gastrulation in the sand dollar, Scaphechinus mirabilis

Processes of gastrulation in the sand dollar Scaphechinus mirabilis were compared with those in the sea urchin Hemicentrotus pulcherrimus , which seemed to show a typical pattern of gastrulation. Measurement of the archenteron length clearly demonstrated that invagination processes in H. pulcherrimus are divided into two phases, the primary and secondary invagination. On the other hand, invagination in S. mirabilis was revealed to continue at a constant rate. To see the movement of cells during gastrulation, embryos were labeled with Nile blue. In H. pulcherrimus embryos, labeled cells were observed along the full length of the archenteron, if the embryos had been labeled before and during the primary invagination. Labeled cells were never observed in the embryos stained after the primary invagination. In contrast, labeled cells were always discerned at the basal part of the archenteron in S. mirabilis , even if the embryos were stained after invagination had undergone considerable progress. The number of cells in the archenteron of S. mirabilis embryos increased with the advancement of gastrulation, while the numbers were almost constant in H. pulcherrimus . These results suggest that the cellular basis of gastrulation in S. mirabilis is quite different from that in well-known species of sea urchins.     

Intracellular transport of nuclear ribosomal RNA in Acetabularia mediterranea]

The ribosomal RNA transport from a nucleus to a perinuclear cytoplasm and its following distribution in the cytoplasm of Acetabularia mediterranea cells were studied using transplantation of RNA-labeled rhizoid into unlabeled stalk. In addition rifamycin treatment was used for inhibition of cytoplasmic RNA synthesis. Acetabularia nuclei contain the stable RNA fractions similar to those present in some other eukaryotes. Nuclear 25S and 17S ribosomal RNA rapidly enter the rhizoid cytoplasm whereas the following trasfer of them to other regions of the cell is a very slow process. Within two days only an insignificant part of 25S and 17S ribosomal RNA is transferred from the rhizoid to the stalk and is distributed there over the base-apical gradient. No preferential transfer of the nuclear ribosomal RNA to the apical region was observed.     

Local shifts in position and polarized motility drive cell rearrangement during sea urchin gastrulation

This study examines the mechanisms of epithelial cell rearrangement during archenteron elongation in the sea urchin embryo using scanning electron microscopy, differential interference contrast videomicroscopy, cell marking, and fluorescently labeled chimaeric clones. Archenteron elongation involves two major processes: local shifts in position of cells in the archenteron wall and polarized motility of the cells as they rearrange. Fluorescently labeled chimaeric clones introduced into the archenteron of Lytechinus pictus are initially 4-5 cells wide; by the end of gastrulation the clones elongate and narrow, so that they are one cell wide in the narrowest region of the archenteron. The extent of clonal mixing indicates that cells in the archenteron change their relative positions by only 1-2 cell diameters during cell rearrangement. Cells at the blastopore rearrange concomitantly with cells in the archenteron, resulting in a 35% decrease in blastopore diameter. Endoderm cells undergo polarized, stage-specific changes in shape and motility as they rearrange; (1) they flatten markedly along their apical-basal axis throughout archenteron elongation; (2) just prior to the onset of cell rearrangement, basal surfaces of all cells in the archenteron extend long, polarized lamellipodial protrusions along the axis of extension of the archenteron; (3) as cell rearrangement begins, basal surfaces round up and the cells become isodiametric; (4) by the 3/4 gastrula stage the cells become stretched along the animal-vegetal axis, apparently due to filopodial traction, and finally (5) they continue to rearrange, returning to a less elongated shape by the end of gastrulation. Direct observation of gastrulation in the cidaroid Eucidaris tribuloides indicates that in this species cell rearrangement is accomplished by progressive circumferential intercalation of cells without upwardly directed filopodia. This intercalation is accompanied by explosive, apparently stochastic, cortical blebbing activity at the boundaries between cells, suggesting that in addition to whatever cell rearrangement may be generated by filopodial tension, such activity is an important component of the active rearrangement process.     

Comparison of nucleotide sequences of large T1 ribonuclease fragments of 18S ribosomal RNA of rat and chicken.

Nucleotide sequences of large T1 ribonuclease fragments of 18S ribosomal RNA of Novikoff rat ascites hepatoma cells and chicken lymphoblastoid cells were determined and compared. Among the 19 large T1 ribonuclease fragments examined of rat 18S ribosomal RNA, 12 fragments were found to be the same in chicken 18S ribosomal RNA. Three fragments of rat 18S ribosomal RNA were not found among large T1 ribonuclease fragments of chicken 18S ribosomal RNA. Four fragments of rat 18S ribosomal RNA were found to be changed in chicken 18S ribosomal RNA. All the changes were point mutations except the change in the largest T1 ribonuclease fragment 1 which is 21 nucleotides long. 2'-0-methylation at the center of the fragment was lost in chicken 18S ribosomal RNA; all the other nucleotides were the same.     

Gastrulation in the sea urchin embryo: a model system for analyzing the morphogenesis of a monolayered epithelium

Processes of gastrulation in the sea urchin embryo have been intensively studied to reveal the mechanisms involved in the invagination of a monolayered epithelium. It is widely accepted that the invagination proceeds in two steps (primary and secondary invagination) until the archenteron reaches the apical plate, and that the constituent cells of the resulting archenteron are exclusively derived from the veg2 tier of blastomeres formed at the 60-cell stage. However, recent studies have shown that the recruitment of the archenteron cells lasts as late as the late prism stage, and some descendants of veg1 blastomeres are also recruited into the archenteron. In this review, we first illustrate the current outline of sea urchin gastrulation. Second, several factors, such as cytoskeletons, cell contact and extracellular matrix, will be discussed in relation to the cellular and mechanical basis of gastrulation. Third, differences in the manner of gastrulation among sea urchin species will be described; in some species, the archenteron does not elongate stepwise but continuously. In those embryos, bottle cells are scarcely observed, and the archenteron cells are not rearranged during invagination unlike in typical sea urchins. Attention will be also paid to some other factors, such as the turgor pressure of blastocoele and the force generated by blastocoele wall. These factors, in spite of their significance, have been neglected in the analysis of sea urchin gastrulation. Lastly, we will discuss how behavior of pigment cells defines the manner of gastrulation, because pigment cells recently turned out to be the bottle cells that trigger the initial inward bending of the vegetal plate.     

Nucleotide sequence study of mouse 5.8S ribosomal RNA.

The primary structure of 5.8S mouse ribosomal RNA has been studied and compared to the structures previously established for other animal species. The results obtained show that mouse 5.8S ribosomal RNA yields pancreatic oligonucleotides with the same nucleotide sequence as the homologous oligonucleotides from rat cells. Furthermore T1 oligonucleotides of 5.8S ribosomal RNA from rat, mouse and human cells behave identically on fingerprinting fractionation and have the same composition as judged by pancreatic digestion. These results strongly suggest that the primary structures of 5.8S ribosomal RNA from rat, mouse and human cells are identical. This identity of structure is also found when the presence of several modified bases (psi and methylated bases) is considered. The findings emphasize the remarkable evolutionary stability of ribosomal gene structure. Comparison of the terminal regional of 5.8S RNA with those of 18S RNA reveals differences which imply a more complex mechanism underlying the maturation of 45S precursor RNA than the finding of identical structure would have suggested.     

RNA Homologies Between Protozoa

SYNOPSIS. Comparison of RNA molecules between certain protozoa using the technic of nucleic acid hybridisation revealed that there are complementary sequences for ribosomal RNA molecules in the genomes of such cells. Furthermore the genes for ribosomal RNA have been conserved during evolution in this group of organisms. On the other hand, RNA molecules from these protozoa which can be considered to be "messengers" show little in the way of sequence relationships. By utilising the technic of hybridisation it was found that Oxytricha can compete effectively against Paramecium ribosomal RNA for Tetrahymena DNA but the ribosomal RNA sequences of the latter could not compete completely against Paramecium ribosomal RNA for Oxytricha DNA. The result is interpreted to show that different ribosomal sequences were hybridising with each of the DNA samples from Tetrahymena and Oxytricha. A general interpretation of this result in terms of ribosome evolution is presented.     

Transport of different RNA species from the nucleus to the cytoplasm in Xenopus laevis neurula cells

Isolated cells from Xenopus laevis neurulae were labeled, and the RNAs extracted from their nuclear and soluble cytoplasmic fractions were analyzed on polyacrylamide gels. In the soluble cytoplasm, 4S RNA emerged very rapidly, and this was immediately followed by the emergence of poly(A)-containing RNA and 18S ribosomal RNA. In contrast, the emergence of 28S ribosomal RNA was delayed by about 2 hr. The size distribution of cytoplasmic poly(A)-containing RNA was much smaller as compared to that of nuclear poly(A)-containing RNA. These results indicate that the newly synthesized RNAs in Xenopus neurula cells are transported from the nucleus to the cytoplasm in a characteristic sequence.