Ascidian homologs of mammalian thyroid peroxidase genes are expressed in the thyroid-equivalent region of the endostyle.

The endostyle is a pharyngeal organ for the internal filter feeding of urochordates, cephalochordates, and larval lamprey. This organ is also considered to be homologous to the follicular thyroid gland of higher vertebrates. Thyroglobulin (Tg) and thyroid peroxidase (TPO) are specifically expressed in the thyroid gland of higher vertebrates, and they play an important role in iodine metabolism for the synthesis of thyroid hormones. Previous histochemical observations showed that iodine-concentrating and peroxidase activities were detected in zones 7, 8, and 9 of the ascidian endostyle, suggesting that these zones contains cells that are equivalent to those in the vertebrate follicular thyroid. In order to investigate the molecular developmental mechanisms involved in the formation and function of the endostyle, with special reference to the evolution of the thyroid gland, in the present study, we isolated and characterized cDNA clones for TPO genes, CiTPO from Ciona intestinalis and HrTPO from Halocynthia roretzi. Northern blot and in situ hybridization analyses revealed that the expression of the ascidian TPO genes was restricted to zone 7, one of the elements equivalent to the thyroid. These results provide the first evidence at the gene expression level for shared function between a part of the ascidian endostyle and the vertebrate follicular thyroid gland. J. Exp. Zool. ( Mol. Dev. Evol. ) 285:158-169, 1999.     

Identification of the endostyle as a stem cell niche in a colonial chordate

Stem cell populations exist in "niches" that hold them and regulate their fate decisions. Identification and characterization of these niches is essential for understanding stem cell maintenance and tissue regeneration. Here we report on the identification of a novel stem cell niche in Botryllus schlosseri, a colonial urochordate with high stem cell-mediated developmental activities. Using in vivo cell labeling, engraftment, confocal microscopy, and time-lapse imaging, we have identified cells with stemness capabilities in the anterior ventral region of the Botryllus' endostyle. These cells proliferate and migrate to regenerating organs in developing buds and buds of chimeric partners but do not contribute to the germ line. When cells are transplanted from the endostyle region, they contribute to tissue development and induce long-term chimerism in allogeneic tissues. In contrast, cells from other Botryllus' regions do not show comparable stemness capabilities. Cumulatively, these results define the Botryllus' endostyle region as an adult somatic stem cell niche.     

C类花器官特征基因AGAMOUS(AG)调控植物花分生组织维持与终止研究进展

花分生组织的维持与终止在植物花器官发生和世代交替起着至关重要的作用。成功的花分生组织决定能够确保植物正常的生殖发育和生命周期进程。诸多研究表明AGAMOUS(AG)基因作为花器官分化和开花决定的主效调节因子,能够协调花发育过程中多种细胞命运决定。然而,关于AG参与调控植物世代交替及花分生组织维持与终止的分子调控机制尚不清晰。综述了近年来AG基因参与调控植物花分生组织维持与终止的研究进展及现状,以期为深入研究植物花器官分化过程中干细胞的维持和终止,以及干细胞活动与其他发育过程之间的分子调控过程提供参考。     

Changing gears in the Cell Cycle: Histoblasts and beyond

Although the molecular elements controlling cell cycle progression are well established, the mechanisms regulating how cell proliferation is triggered in response to extrinsic stimuli and how cell divisions change speed, particularly in stem or tumor cells or regenerative tissues, are poorly understood. One exceptional model system in which these events are precisely defined is Drosophila abdominal morphogenesis, in which stem-like histoblasts build the adult epidermis at metamorphosis by undergoing a series of sequential transitions from a non-proliferative to a growing, and finally to an invasive epithelium. We have recently uncovered in histoblasts an internal logic modulating cell cycle transitions that should constitute a reference paradigm for the study of other equivalent processes in stem cell, cancer or developmental biology.     

Lateral Root Initiation or the Birth of a New Meristem

Root branching happens through the formation of new meristems out of a limited number of pericycle cells inside the parent root. As opposed to shoot branching, the study of lateral root formation has been complicated due to its internal nature, and a lot of questions remain unanswered. However, due to the availability of new molecular tools and more complete genomic data in the model species Arabidopsis, the probability to find new and crucial elements in the lateral root formation pathway has increased. Increasingly more data are supporting the idea that lateral root founder cells become specified in young root parts before differentiation is accomplished. Next, pericycle founder cells undergo anticlinal asymmetric, divisions followed by an organized cell division pattern resulting in the formation of a new organ. The whole process of cell cycle progression and stimulation of the molecular pathway towards lateral root initiation is triggered by the plant hormone auxin. In this review, we aim to give an overview on the developmental events taking place from the very early specification of founder cells in the pericycle until the first anticlinal divisions by combining the knowledge originating from classical physiology studies with new insights from genetic-molecular analyses. Based on the current knowledge derived from recent genetic and developmental studies, we propose here a hypothetical model for LRI.     

Principles of neural stem cell lineage progression: Insights from developing cerebral cortex

How to generate a brain of correct size and with appropriate cell-type diversity during development is a major question in Neuroscience. In the developing neocortex, radial glial progenitor (RGP) cells are the main neural stem cells that produce cortical excitatory projection neurons, glial cells, and establish the prospective postnatal stem cell niche in the lateral ventricles. RGPs follow a tightly orchestrated developmental program that when disrupted can result in severe cortical malformations such as microcephaly and megalencephaly. The precise cellular and molecular mechanisms instructing faithful RGP lineage progression are however not well understood. This review will summarize recent conceptual advances that contribute to our understanding of the general principles of RGP lineage progression.     

Cancer initiation and progression: involvement of stem cells and the microenvironment

The molecular events that lead to the cancer-initiating cell involve critical mutations in genes regulating normal cell growth and differentiation. Cancer stem cells, or cancer initiating cells have been described in the context of acute myeloid leukemia, breast, brain, bone, lung, melanoma and prostate. These cells have been shown to be critical in tumor development and should harbor the mutations needed to initiate a tumor. The origin of the cancer stem cells is not clear. They may be derived from stem cell pools, progenitor cells or differentiated cells that undergo trans-differentiation processes. It has been suggested that cell fusion and/or horizontal gene transfer events, which may occur in tissue repair processes, also might play an important role in tumor initiation and progression. Fusion between somatic cells that have undergone a set of specific mutations and normal stem cells might explain the extensive chromosomal derangements seen in early tumors. Centrosome deregulation can be an integrating factor in many of the mechanisms involved in tumor development. The regulation of the balance between cell renewal and cell death is critical in cancer. Increased knowledge of developmental aspects in relation to self-renewal and differentiation, both under normal and deregulated conditions, will probably shed more light on the mechanisms that lead to tumor initiation and progression.     

Social controls on cell proliferation in plants

Plant cell division occurs mainly in developing tissues and appears to be highly regulated in both space and time. Recently, genetic and molecular analyses have been able to dissect the function of cell proliferation in the processes of growth and development. Mutant studies have shown that plants have a compensatory mechanism whereby increased cell expansion can partially cover for defects in proliferation. Ectopic expression of developmental and cell-cycle regulators has indicated how growth rate is controlled at the molecular level in meristems and lateral organs.     

Friend cell variants temperature-sensitive for growth

Friend erythroleukemia cells, thermosensitive for growth, have been isolated by a novel selection procedure employing hypoxanthine, aminopterin and bromodeoxyuridine (HAB) with near-visible light. This reagent eliminates both wild-type cells replicating at the non-permissive temperature of 39 °C and cells lacking thymidine kinase activity unable to incorporate bromodeoxyuridine (BUdR), the lethal constituent of HAB. Clones growth arrested at the non-permissive temperature have a temperature-sensitive defect in progression through G1 of the cell cycle. At permissive temperatures these clones have a karyotype similar to that of wild-type cells and are inducible for synthesis of hemoglobin. Clones which have survived the selection by means of an extended generation time are almost tetraploid at permissive temperatures, are larger than wild-type cells and are inducible for hemoglobin synthesis. At 39 °C these cells are defective in accurate mitotic division. This results in a population of cells heterogeneous in size, having chromosome complements ranging from less than the mouse diploid number to approx. 150 chromosomes/ cell. In the latter giant cells, not all nuclei are in mitosis at any one time. Such cells may be defective in cytokinesis.The two distinct classes of ts variant obtained should be useful for

1. 1. the study of whether induction of hemoglobin synthesis is cell-cycle dependent;

2. 2. mapping the chromosomes important in controlling accurate mitotic division.

     

Patterning through differential endoreduplication in epithelial organogenesis of the chordate,Oikopleura dioica

The contributions that control of cell proliferation and cell growth make to developmental regulation of organ and body size remain poorly explored, particularly with respect to endocycles in polyploid tissues. The epithelium of the marine chordate Oikopleura dioica is composed of a fixed number of cells grouped in territories according to gene-specific expression and nuclear sizes and shapes. As the animal grows 10-fold during the life cycle, epithelial cells increase in size differentially as a function of their spatial position. We show that this cellular pattern reflected differences in ploidy levels ranging from 34 to 1,300 C. The diverse ploidy levels in defined cellular fields resulted both from different timing of entry into endocycles and from cell-specific regulation of endocycle lengths. Throughout the life cycle, differential cell size and ploidy increases were accompanied by field-specific profiles of progressive reductions in G-phase duration. Endocycles were asynchronous among cells of a given epithelial territory, but at the resolution of individual cells, both DNA replication timing and ploidy levels were bilaterally symmetric. The transparent, accessible, oikoplastic epithelium is a model of choice for the study of endoreduplication in the context of pattern formation and growth.     

The ins and outs of the plant cell cycle

Plant growth and development are driven by the continuous generation of new cells. Whereas much has been learned at a molecular level about the mechanisms that orchestrate progression through the different cell-cycle phases, little is known about how the cell-cycle machinery operates in the context of an entire plant and contributes to growth, cell differentiation and the formation of new tissues and organs. Here, we discuss how intrinsic developmental signals and environmental cues affect cell-cycle entry and exit.     

Pericycle cell division competence underlies various developmental programs

Pericycle cells possess proliferative activity long after leaving the root apical meristem. Depending on the developmental stage and external stimuli, pericycle cell division leads to the production of lateral roots, vascular cambium and periderm, and callus. Therefore, pericycle cell division competence underlies root branching and secondary growth, as well as plant regeneration capacity. In this review, we first briefly present an overview of the molecular pathways of the four developmental programs originated, exclusively or partly, from pericycle cells. Then, we provide a review of up-to-date knowledge in the mechanisms determining pericycle cells’ competence to undergo cell division. Furthermore, we discuss directions of future research to further our understanding of the pericycle’s characteristics and functions.     

Molecular and cellular aspects of auxin-transport-mediated development

The plant hormone auxin is frequently observed to be asymmetrically distributed across adjacent cells during crucial stages of growth and development. These auxin gradients depend on polar transport and regulate a wide variety of processes, including embryogenesis, organogenesis, vascular tissue differentiation, root meristem maintenance and tropic growth. Auxin can mediate such a perplexing array of developmental processes by acting as a general trigger for the change in developmental program in cells where it accumulates and by providing vectorial information to the tissues by its polar intercellular flow. In recent years, a wealth of molecular data on the mechanism of auxin transport and its regulation has been generated, providing significant insights into the action of this versatile coordinative signal.     

Transformation of the endostyle of the anadromous sea lamprey,Petromyzon marinus L., during metamorphosis. II. Electron microscopy

Electron microscopy was used to follow the transformation of the endostyle to a thyroid gland in the anadromous sea lamprey, Petromyzon marinus L., throughout metamorphosis (stages 1–7). Transformation of the larval (ammocoete) endostyle begins at the first signs of external change (stages 1–2), and the adult form of the gland is reached by stage 5. Only slight modifications of the gland accompany further development to the end of metamorphosis. Development of the thyroid gland involves degeneration, proliferation, and reorganization of the cells in the endostyle, and changes in their fine structure. Ultrastructural changes during early stages are most obvious in the type 1 cells that make up the shrinking glandular tracts, and involves the accumulation of cytoplasmic microfilaments and a variety of cytoplasmic inclusions. The glandular tracts and their cells gradually disappear through autolysis and, apparently, through phagocytosis by neighboring epithelial cells and macrophages. Although the fine structure of the type 2, 3, 4, and 5 cells is not altered in the early stages, by stage 3, many of these cells become either vacuolated, undergo autolysis, or are extruded. Phagocytosis of some of each of these cell types likely occurs. Thyroid follicles are first observed during stage 4. Some of their lumina seem to arise from the accumulation of material in intercellular spaces and from vacuoles among cell clusters. Other lumina may represent a portion of the original lumen of the endostyle. Many follicles appear to be comprised of cells with cytological characteristics similar to those of larval cell types 3 and 2c. Some of the other larval cell types, such as type 5, may also be involved. In young adult lampreys follicles are composed of cuboidal to columnar cells that lack the dilated cisternae of rough endoplasmic reticulum seen in follicular cells of higher vertebrates. Dense collagenous connective tissue surrounding the follicles contains relatively few blood vessels. The transformation process described may have some relevance to our understanding of the development and evolution of the vertebrate thyroid gland.     

Cell cycle control and plant morphogenesis: is there an essential link?

Plant morphogenesis has some interesting features that may have consequences for the regulation of cell division. In particular, the immobility of plant cells implies the necessity for highly accurate controls, in contrast with the flexibility of many developmental processes in animals. An important question in plant development concerns the status of the relationship between plant morphogenesis and cell division. In this review, we discuss the current knowledge of the molecular mechanisms controlling the plant cell cycle and how this could be differentially regulated during plant morphogenesis. The plant genes involved are homologous to those of other higher eukaryotes, suggesting a similar cell cycle machinery. A variety of mechanisms control these genes, reflecting the complexity of internal and environmental signals to which plants should respond. This intricate network requires an upstream control mechanism to function as a failsafe system and to govern cell division and growth to produce the correct plant shape. BioEssays 21:29–37, 1999. © 1999 John Wiley & Sons, Inc.