Seminars in Developmental Biology最新文献

The bacterium Caulobacter crescentus undergoes a simple developmental program within each cell cycle, resulting in the formation of two different daughter cells: a motile swarmer cell and a non-motile stalked cell. The generation of two cell types is a consequence of the asymmetric positioning of proteins, compartmentalized gene expression and differential programming of DNA replication capacities in the predivisional cell. The localization of a developmentally expressed chemotaxis receptor to the swarmer pole is attributable to two distinct mechanisms: polar targeting, followed by pole-specific proteolysis. In contrast, swarmer pole-specific transcription of flagellar genes results from the pole-specific activation of a transcription factor.

Volvox carteri is a spheroidal green alga in which each young adult has ∼2000 small, terminally differentiated somatic cells at the surface and ∼16 large, potentially immortal asexual reproductive cells, or gonidia, on the interior. When mature, each gonidium initiates a stereotyped cleavage program, during which prospective gonidia and somatic cells of the next generation are set apart by asymmetric divisions. At the end of cleavage, prospective gonidia are ∼30 X the volume of prospective somatic cells, and a variety of studies lead to the conclusion that it is this difference in size, not an accompanying difference in cytoplasmic quality, that triggers entirely different programs of gene expression and differentiation in the two cell lineages. Genes that appear to play key roles in differentiation include gls, which is required for asymmetric division, regA, which suppresses reproductive development in small cells, and the lag genes which suppress somatic development in large cells. The prospects for analysing the nature of these genes and their actions have been brightened by recent molecular-genetic developments, but the mechanism by which differences in cell size are transduced into differences in gene expression remains obscure.

The flagella of the unicellular green alga Chlamydomonas reinhardtii are controlled by a number of sophisticated homeostatic mechanisms which ensure that the flagella are maintained at a specific length, and that each cell has two flagella of equal length. Mutants with defects in flagellar length control have been obtained, defining at least nine genes that are involved in the control of flagellar length and the equality of flagellar length. The active machinery involved in flagellar length control requires that cells precisely measure the length of their flagella and drastically alter flagellar protein production and assembly when necessary to maintain desired lengths.

The generation of specialized cell types during sporulation in Bacillus subtilis occurs in conjunction with formation of an asymmetrically positioned septum that partitions the sporangium into dissimilar-sized progeny. Differentiation is governed by the activation in the smaller cell of transcription factor σ^F, which sets in motion a chain of events leading to the cell-specific appearance of three additional RNA polymerase sigma factors. Understanding the establishment of cell type requires elucidation of the role of asymmetric septation in the cell-specific activation of σ^F and of the mechanisms that govern the placement of the sporulation septum.

The sexual phase of the life cycle in ciliates represents a developmental program with several parallels to multicellular development. During this pathway an undifferentiated zygotic nucleus gives rise to two lineages, a germinal micronuclear lineage and a somatic macronuclear lineage. The development of nascent macronuclei (or ‘anlagen’) from micronuclei involves a highly regulated set of DNA rearrangements which include chromosomal breakage, telomere addition, DNA elimination and gene amplification. Here we review recent progress in identifying stage-specific polypeptides from Tetrahymena analgen that are likely to be involved in these rearrangements. One of the more abundant of these polypeptides, p65, participates in the formation of DNA-containing structures that resemble developing nucleoli. We propose a simple model in which the micronuclear gene segments that are not to be included in the mature macronuclear genome are first digested in these p65-based particles, and then the resulting nucleotides are ‘recycled’ by using them to amplify rDNA. Our ‘intranuclear recycling’ model suggests a possible compartmentalization strategy that functions to ensure adequate rDNA/rRNA production during macronuclear development. Implications of the model for programmed DNA rearrangements and nucleolar biogenesis are discussed.

Fucus zygotes are symmetrical at fertilization. During the first cell cycle, a polar axis is generated that can be oriented by external gradients. This polar axis defines the orientation of zygotic growth, the plane of the first cell division, the distinct fates of the asymmetric daughter cells and the polarity of the organism. Establishment of polarity involves the generation of labile asymmetries in the zygote (axis formation), the stabilization of the asymmetries (axis fixation), and the separation of these asymmetries ito two daughter cells (fate determination). The actin cytoskeleton and the cell wall paly critical roles in these processes. A model postulating signaling between the cell wall and cytoplasm is presented.

Topographic matching between pre- and postsynaptic ganglion cell sheets appears to be a typical way in which different regions of the vertebrate brain are interconnected. The visual system provides a well-suited model to study the processes underlying the generation of 'topographic projections' during development. In this review, we will discuss several mechanisms which are possibly involved in the sorting of retinal axon terminals within their target field, the optic tectum. These mechanisms include target recognition by 'cytochemical matching' between axons and target cells, fiber-fiber interactions, and synapse stabilization and elimination based on impulse activity.

Studies of fiber guidance in invertebrates and at decision regions in vertebrates, show that three different sets of processes intervene. The first, is complex interactions of growing fibers with cells, often neuronal, that exist at strategic locations, at special borders or at decision points. The second is a modulation of growth cone motility, which manifests itself by a stop-start pattern of advance. The third, is response of growth cones to guidance cues, which are presumed to be distributed in graded or abrupt spatial distributions. In this paper, I will review the process of retinal fiber decussation from this point of view, with the aim of demonstrating how each of these processes contributes to fiber growth in this and other decision regions.

One of the earliest and most crucial steps in the development of connectivity within the CNS is the acquisition of specific identities by developing neural cells. In this review, we discuss how a neural cell may come to acquire its unique identity and some of the genes that may be involved in this process. Experimental evidence suggests that ectodermal cells may pass through several phases at which their potential fates become progressively more restricted. An initial step occurs during neural induction when ectodermal cells become restricted to either a neural or non-neural fate. A little later in development, a further set of interactions determine which of the neural cells become postmitotic and begin a programme of differentiation. The differentiation phase may itself involve several steps at which the postmitotic neuron progressively advances towards its final identity.