Concept Formation

Current Russian psychology is understood in the sense of the Vygotskian tradition and Gal'perin's position. The central concepts in this tradition are the activity of a subject and his or her social-cultural interactions with the environment.     

Plastid Thylakoid Formation

Inclusion bodies found in proplastids and immature chloroplastswhich possibly play a role in thylakoid formation are described.These include membrane-bound bodies, the bounding membrane ofwhich is continuous with those of the grana and striated osmiophilicdroplets whose over-all morphology bears a resemblance to granastacks. It is suggested that both inclusions contain precursormolecules for membrane synthesis and that they may be actualsites of thylakoid formation.     

Morphogen Gradient Formation

How morphogen gradients are formed in target tissues is a key question for understanding the mechanisms of morphological patterning. Here, we review different mechanisms of morphogen gradient formation from theoretical and experimental points of view. First, a simple, comprehensive overview of the underlying biophysical principles of several mechanisms of gradient formation is provided. We then discuss the advantages and limitations of different experimental approaches to gradient formation analysis.How a multicellular organism develops from a single fertilized cell has fascinated people throughout history. By looking at chick embryos of different developmental stages, Aristotle first noted that development is characterized by growing complexity and organization of the embryo (Balme 2002). During the 19th century, two events were recognized as key in development: cell proliferation and differentiation. Driesch first noted that to form organisms with correct morphological pattern and size, these processes must be controlled at the level of the whole organism. When he separated two sea urchin blastomeres, they produced two half-sized blastula, showing that cells are potentially independent, but function together to form a whole organism (Driesch 1891, 1908). Morgan noted the polarity of organisms and that regeneration in worms occurs with different rates at different positions. This led him to postulate that regeneration phenomena are influenced by gradients of “formative substances” (Morgan 1901).The idea that organisms are patterned by gradients of form-providing substances was explored by Boveri and Hörstadius to explain the patterning of the sea urchin embryo (Boveri 1901; Hörstadius 1935). The discovery of the Spemann organizer, i.e., a group of dorsal cells that when grafted onto the opposite ventral pole of a host gastrula induce a secondary body axis (Spemann and Mangold 1924), suggested that morphogenesis results from the action of signals that are released from localized groups of cells (“organizing centers”) to induce the differentiation of the cells around them (De Robertis 2006). Child proposed that these patterning “signals” represent metabolic gradients (Child 1941), but the mechanisms of their formation, regulation, and translation into pattern remained elusive.In 1952, Turing showed that chemical substances, which he called morphogens (to convey the idea of “form producers”), could self-organize into spatial patterns, starting from homogenous distributions (Turing 1952). Turing’s reaction–diffusion model shows that two or more morphogens with slightly different diffusion properties that react by auto- and cross-catalyzing or inhibiting their production, can generate spatial patterns of morphogen concentration. The reaction–diffusion formalism was used to model regeneration in hydra (Turing 1952), pigmentation of fish (Kondo and Asai 1995; Kondo 2002), and snails (Meinhardt 2003).At the same time that Turing showed that pattern can self-organize from the production, diffusion, and reaction of morphogens in all cells, the idea that morphogens are released from localized sources (“organizers” à la Spemann) and form concentration gradients was still explored. This idea was formalized by Wolpert with the French flag model for generation of positional information (Wolpert 1969). According to this model, morphogen is secreted from a group of source cells and forms a gradient of concentration in the target tissue. Different target genes are expressed above distinct concentration thresholds, i.e., at different distances to the source, hence generating a spatial pattern of gene expression (Fig. 1C).Open in a separate windowFigure 1.Tissue geometry and simplifications. (A) Gradients in epithelia (left) and mesenchymal tissues (right). Because of symmetry considerations, one row of cells (red outline) is representative for the whole gradient. (B) Magnified view of the red row of cells shown in A. Cells with differently colored nuclei (brown, orange, and blue) express different target genes. (C) A continuum model in which individual cells are ignored and the concentration is a function of the positions x. The morphogen activates different target genes above different concentration thresholds (brown and orange).Experiments in the 1970s and later confirmed that tissues are patterned by morphogen gradients. Sander showed that a morphogen released from the posterior cytoplasm specifies anterioposterior position in the insect egg (Sander 1976). Chick wing bud development was explained by a morphogen gradient emanating from the zone of polarizing activity to specify digit positions (Saunders 1972; Tickle, et al. 1975; Tickle 1999). The most definitive example of a morphogen was provided with the identification of Bicoid function in the Drosophila embryo (Nüsslein-Volhard and Wieschaus 1980; Frohnhöfer and Nüsslein-Volhard 1986; Nüsslein-Volhard et al. 1987) and the visualization of its gradient by antibody staining (Driever and Nüsslein-Volhard 1988b, 1988a; reviewed in Ephrussi and St Johnston 2004). Since then, many examples of morphogen gradients acting in different organs and species have been found.In an attempt to understand pattern formation in more depth, quantitative models of gradient formation have been developed. An early model by Crick shows that freely diffusing morphogen produced in a source cell and destroyed in a “sink” cell at a distance would produce a linear gradient in developmentally relevant timescales (Crick 1970). Today, it is known that a localized “sink” is not necessary for gradient formation: Gradients can form if all cells act as sinks and degrade morphogen, or even if morphogen is not degraded at all. Here, we review different mechanisms of gradient formation, the properties of these gradients, and the implications for patterning. We discuss the theory behind these mechanisms and the supporting experimental data.     

心脏血管的形成

心脏的血 管 形成 是 血管 发生 (vasculogenesis)、血 管 生成 (angiogenesis)及 动 脉生 成 (arteriogenesis)三种 机制 共同 作 用的 结 果 .血管 发 生是 指在 胚 胎期 ,来 源 于中 胚 层的 干细 胞增 殖 和分 化 ,形 成 内皮 细胞 ,进而 与其 他细 胞形 成 原始 的 心血 管系 统 .血 管生 成 出现 在血 管 发生 之后 ,是指 通过 内 皮细 胞的 增 殖由 原始 血 管丛 或已 存在 的血 管 形成 无 完好 血管 的 膜中 的毛 细 血管 .而 动 脉生 成是 指 具有 完好 的 动脉 中膜 的 小动 脉 的生 成,也包 括原 有的 侧 支循 环 的改 建及 成 熟 .总结 了 出生 前后 心 脏脉 管系 统 形成 的细 胞 及分 子机 理 ,并 从生 物 学及 临床 治疗 上就 一 些内 皮 前体 细胞 及 其它 脉管 起 源相 关问 题 进行 简单 的 介绍 .     

Formation of Peyer's patches

Formation of Peyer's patches requires complex interactions between the gut epithelium, the mesenchyme, and bone-marrow-derived hematopoietic progenitors. The first Peyer's patches anlage appear around embryonic day 15.5, when the endoderm has undergone transition to a simple epithelium, the lymphatic vessels have reached the intestinal mucosa, and mesenchymal cells have started to form clusters. Recent data using knockout mice provide insight into the molecular nature of the signals that mediate Peyer's patch ontogeny. These include members of the tumor-necrosis factor family and homeostatic chemokines.     

Tumor Formation in Plants

The data on genetic tumors in plant species and interspecific hybrids, as well as the problems of Agrobacterium-induced tumors are reviewed. The role of the horizontal gene transfer in the induction of genetic tumors is discussed.     

Rhizomorph Formation in Fungi

The effect on growth and rhizomorph formation of 12 alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butyl alcohol, tert-butyl alcohol, 1-pentanol, iso-amyl alcohol, ethylene glycol and glycerol) at different concentrations has been examined for 2 isolates of Armillaria mellea (Vahl ex Fr.) Quél. and 1 of Clitocybe geotropa (Bull. ex Fr.) Quél. The fungi were cultivated for 28 days on a synthetic, liquid glucose medium with the alcohols as supplement. The following alcohols strongly stimulated growth and rhizomorph formation: ethanol, 1-propanol and 1-butanol. A great variation was demonstrated between the isolates in relation to rhizomorph production, morphology, and ability to be stimulated by different alcohols.     

Nef-mediated Clathrin-coated Pit Formation

The sequence of events leading to clathrin-coated pit (CCP) nucleation on the cell surface and to the incorporation of receptors into these endocytic structures is still imperfectly understood. In particular, the question remains as to whether receptor tails initiate the assembly of the coat proteins or whether receptors migrate into preformed CCP. This question was approached through a dissection of the mechanisms implemented by Nef, an early protein of human and simian immunodeficiency virus (HIV and SIV, respectively), to accelerate the endocytosis of cluster of differentiation antigen type 4 (CD4), the major receptor for these viruses. Results collected showed that: (a) Nef promotes CD4 internalization via an increased association of CD4 with CCP; (b) the Nef-mediated increase of CD4 association with CCP is related to a doubling of the plasma membrane area occupied by clathrin-coated structures; (c) this increased CCP number at the plasma membrane has functional consequences preferentially on CD4 uptake and does not significantly affect transferrin receptor internalization or fluid-phase endocytosis; (d) the presence of a CD4 cytoplasmic tail including a critical dileucine motif is required to induce CCP formation via Nef; and (e) when directly anchored to the cytoplasmic side of the plasma membrane, Nef itself can promote CCP formation. Taken together, these observations lead us to propose that CD4 can promote CCP generation via the connector molecule Nef. In this model, Nef interacts on one side with CD4 through a dileucine-based motif present on CD4 cytoplasmic tail and on the other side with components of clathrin-coated surface domain (i.e., adaptins). These Nef-generated complexes would then initiate the nucleation of CCP.