Retrospective tracing of the developmental lineage of the mouse myotome

A niche for axial stem cells - A cellular perspective in amniotes

The head-tail axis in birds and mammals develops from a growth zone in the tail-end, which contains the node. This growth zone then forms the tailbud. Labelling experiments have shown that while many cells leave the node and tailbud to contribute to axial (notochord, floorplate) and paraxial (somite) structures, some cells remain resident in the node and tailbud. Could these cells be resident axial stem cells? If so, do the node and tailbud represent an instructive stem cell niche that specifies and maintains these stem cells? Serial transplantation and single cell labelling studies support the existence of self-renewing stem cells and heterotopic transplantations suggest that the node can instruct such self-renewing behaviour. However, only single cell manipulations can reveal whether self-renewing behaviour occurs at the level of a cell population (asymmetric or symmetric cell divisions) or at the level of single cells (asymmetric divisions only). We combine data on resident cells in the node and tailbud and review it in the context of axial development in chick and mouse, summarising our current understanding of axial stem cells and their niche and highlighting future directions of interest.

Resolving cell lineage contributions to the ventricular conduction system with a Cx40-GFP allele: a dual contribution of the first and second heart fields

BACKGROUND

The ventricular conduction system (VCS) coordinates the heartbeat and is composed of central components (the atrioventricular node, bundle, and right and left bundle branches) and a peripheral Purkinje fiber network. Conductive myocytes develop from common progenitor cells with working myocytes in a bimodal process of lineage restriction followed by limited outgrowth. The lineage relationship between progenitor cells giving rise to different components of the VCS is unclear.

RESULTS

Cell lineage contributions to different components of the VCS were analysed by a combination of retrospective clonal analysis, regionalized transgene expression studies, and genetic tracing experiments using Connexin40-GFP mice that precisely delineate the VCS. Analysis of a library of hearts containing rare large clusters of clonally related myocytes identifies two VCS lineages encompassing either the right Purkinje fiber network or left bundle branch. Both lineages contribute to the atrioventricular bundle and right bundle branch that segregate early from working myocytes. Right and left VCS lineages share the transcriptional program of the respective ventricular working myocytes and genetic tracing experiments discount alternate progenitor cell contributions to the VCS.

CONCLUSIONS

The mammalian VCS is comprised of cells derived from two lineages, supporting a dual contribution of first and second heart field progenitor cells.

Association of tissue lineage and gene expression: conservatively and differentially expressed genes define common and special functions of tissues

BACKGROUND

Embryogenesis is the process by which the embryo is formed, develops, and establishes developmental hierarchies of tissues. The recent advance in microarray technology made it possible to investigate the tissue specific patterns of gene expression and their relationship with tissue lineages. This study is focused on how tissue specific functions, tissue lineage, and cell differentiation are correlated, which is essential to understand embryonic development and organism complexity.

RESULTS

We performed individual gene and gene set based analysis on multiple tissue expression data, in association with the classic topology of mammalian fate maps of embryogenesis. For each sub-group of tissues on the fate map, conservatively, differentially and correlatively expressed genes or gene sets were identified. Tissue distance was found to correlate with gene expression divergence. Tissues of the ectoderm or mesoderm origins from the same segments on the fate map shared more similar expression pattern than those from different origins. Conservatively expressed genes or gene sets define common functions in a tissue group and are related to tissue specific diseases, which is supported by results from Gene Ontology and KEGG pathway analysis. Gene expression divergence is larger in certain human tissues than in the mouse homologous tissues.

CONCLUSION

The results from tissue lineage and gene expression analysis indicate that common function features of neighbor tissue groups were defined by the conservatively expressed genes and were related to tissue specific diseases, and differentially expressed genes contribute to the functional divergence of tissues. The difference of gene expression divergence in human and mouse homologous tissues reflected the organism complexity, i.e. distinct neural development levels and different body sizes.

Signaling gradients during paraxial mesoderm development

The sequential formation of somites along the anterior-posterior axis is under control of multiple signaling gradients involving the Wnt, FGF, and retinoic acid (RA) pathways. These pathways show graded distribution of signaling activity within the paraxial mesoderm of vertebrate embryos. Although Wnt and FGF signaling show highest activity in the posterior, unsegmented paraxial mesoderm (presomitic mesoderm [PSM]), RA signaling establishes a countergradient with the highest activity in the somites. The generation of these graded activities relies both on classical source-sink mechanisms (for RA signaling) and on an RNA decay mechanism (for FGF signaling). Numerous studies reveal the tight interconnection among Wnt, FGF, and RA signaling in controlling paraxial mesoderm differentiation and in defining the somite-forming unit. In particular, the relationship to a molecular oscillator acting in somite precursors in the PSM-called the segmentation clock-has been recently addressed. These studies indicate that high levels of Wnt and FGF signaling are required for the segmentation clock activity. Furthermore, we discuss how these signaling gradients act in a dose-dependent manner in the progenitors of the paraxial mesoderm, partly by regulating cell movements during gastrulation. Finally, links between the process of axial specification of vertebral segments and Hox gene expression are discussed.

Phylogenetic analysis of developmental and postnatal mouse cell lineages

Fate maps depict how cells relate together through past lineage relationships, and are useful tools for studying developmental and somatic processes. However, with existing technologies, it has not been possible to generate detailed fate maps of complex organisms such as the mouse. We and others have therefore proposed a novel approach, "phylogenetic fate mapping," where patterns of somatic mutation carried by the individual cells of an animal are used to retrospectively deduce lineage relationships through phylogenetic inference. Here, we have cataloged genomic polymorphisms at 324 mutation-prone polyguanine tracts for nearly 300 cells isolated from a single mouse, and have explored the cells' lineage relationships both phylogenetically and through a network-based approach. We present a model of mouse embryogenesis, where an early period of substantial cell mixing is followed by more coherent growth of clones later. We find that cells from certain tissues have greater numbers of close relatives in other specific tissues than expected from chance, suggesting that those populations arise from a similar pool of ancestral lineages. Finally, we have investigated the dynamics of cell turnover (the frequency of cell loss and replacement) in postnatal tissues. This work offers a longitudinal study of developmental lineages, from conception to adulthood, and provides insight into basic questions of mouse embryology as well as the somatic processes that occur after birth.

Redefining the progression of lineage segregations during mammalian embryogenesis by clonal analysis

Clonal lineage information is fundamental in revealing cell fate choices. Using genetic single-cell labeling in utero, we investigated lineage segregations during anteroposterior axis formation in mouse. We show that while endoderm and surface ectoderm segregate during gastrulation, neural ectoderm and mesoderm share a common progenitor persisting through all stages of axis elongation. These data challenge the paradigm that the three germ layers, formed by gastrulation, constitute the primary branchpoints in differentiation of the pluripotent epiblast toward tissue-specific precursors. Bipotent neuromesodermal progenitors show self-renewing characteristics and may represent the cellular substrate coupling sustained axial elongation and coordinated differentiation of these tissues. These findings have important implications for the interpretation of the phenotypic defects of several mouse mutants and the directed differentiation of embryonic stem (ES) cells in vitro.

Large-scale clonal analysis reveals unexpected complexity in surface ectoderm morphogenesis

BACKGROUND

Understanding the series of morphogenetic processes that underlie the making of embryo structures is a highly topical issue in developmental biology, essential for interpreting the massive molecular data currently available. In mouse embryo, long-term in vivo analysis of cell behaviours and movements is difficult because of the development in utero and the impossibility of long-term culture.

METHODOLOGY/PRINCIPAL FINDINGS

We improved and combined two genetic methods of clonal analysis that together make practicable large-scale production of labelled clones. Using these methods we performed a clonal analysis of surface ectoderm (SE), a poorly understood structure, for a period that includes gastrulation and the establishment of the body plan. We show that SE formation starts with the definition at early gastrulation of a pool of founder cells that is already dorso-ventrally organized. This pool is then regionalized antero-posteriorly into three pools giving rise to head, trunk and tail. Each pool uses its own combination of cell rearrangements and mode of proliferation for elongation, despite a common clonal strategy that consists in disposing along the antero-posterior axis precursors of dorso-ventrally-oriented stripes of cells.

CONCLUSIONS/SIGNIFICANCE

We propose that these series of morphogenetic processes are organized temporally and spatially in a posterior zone of the embryo crucial for elongation. The variety of cell behaviours used by SE precursor cells indicates that these precursors are not equivalent, regardless of a common clonal origin and a common clonal strategy. Another major result is the finding that there are founder cells that contribute only to the head and tail. This surprising observation together with others can be integrated with ideas about the origin of axial tissues in bilaterians.

Phylogenetic fate mapping: theoretical and experimental studies applied to the development of mouse fibroblasts

Mutations are an inevitable consequence of cell division. Similarly to how DNA sequence differences allow inferring evolutionary relationships between organisms, we and others have recently demonstrated how somatic mutations may be exploited for phylogenetically reconstructing lineages of individual cells during development in multicellular organisms. However, a problem with such "phylogenetic fate maps" is that they cannot be verified experimentally; distinguishing actual lineages within clonal populations requires direct observation of cell growth, as was used to construct the fate map of Caenorhabditis elegans, but is not possible in higher organisms. Here we employ computer simulation of mitotic cell division to determine how factors such as the quantity of cells, mutation rate, and the number of examined marker sequences contribute to fidelity of phylogenetic fate maps and to explore statistical methods for assessing accuracy. To experimentally evaluate these factors, as well as for the purpose of investigating the developmental origins of connective tissue, we have produced a lineage map of fibroblasts harvested from various organs of an adult mouse. Statistical analysis demonstrates that the inferred relationships between cells in the phylogenetic fate map reflect biological information regarding the origin of fibroblasts and is suggestive of cell migration during mesenchymal development.

A phylogenetic approach to mapping cell fate

Recent, surprising, and controversial discoveries have challenged conventional concepts regarding the origins and plasticity of stem cells, and their contributions to tissue regeneration, and highlight just how little is known about mammalian development in comparison to simpler model organisms. In the case of the transparent worm, Caenorhabditis elegans, Sulston and colleagues used a microscope to record the birth and death of every cell during its life, and the compilation of this "fate map" represents a milestone achievement of developmental biology. Determining a fate map for mammals or other higher organisms is more complicated because they are opaque, take a long time to mature, and have a tremendous number of cells. Consequently, fate mapping experiments have relied on tagging a progenitor cell with a dye or genetic marker in order to later identify its descendants. This approach, however, extracts little information because it demonstrates that a population of cells, all having inherited the same label, shares a common ancestor, but it does not reveal how cells in that population are related to one another. To avoid that problem, as well as technical limitations of current methods for mapping cell fate, we, and others, have developed a new strategy for retrospectively deriving cell fate maps by using phylogenetics to infer the order in which somatic mutations have arisen in the genomes of individual cells during development in multicellular organisms. DNA replication inevitably introduces mutations, particularly at repetitive sequences, every time a cell divides. It is thus possible to deduce the history of cell divisions by cataloging somatic mutations and phylogenetically reconstructing cell lineage. This approach has the potential to produce a complete mammalian cell fate map that, in principle, could describe the developmental lineage of any cell and help resolve outstanding questions of stem cell biology, tissue repair and maintenance, and aging.

Phylogenetic fate mapping

Cell fate maps describe how the sequence of cell division, migration, and apoptosis transform a zygote into an adult. Yet, it is only in Caenorhabditis elegans where microscopic observation of each cell division has allowed for construction of a complete fate map. More complex, and opaque, animals prove less yielding. DNA replication, however, generates somatic mutations. Consequently, multicellular organisms comprise mosaics where most cells acquire unique genomes that are potentially capable of delineating their ancestry. Here we take a phylogenetic approach to passively retrace embryonic relationships by deducing the order in which mutations have arisen during development. We show that polyguanine repeat DNA sequences are particularly useful genetic markers, because they frequently change length during mitosis. To demonstrate feasibility, we phylogenetically reconstruct the lineage of cultured mouse NIH 3T3 cells based on mutations affecting the length of polyguanine markers. We then employ whole genome amplification to genotype polyguanine markers in single cells taken from a mouse and use phylogenetics to infer the developmental relationships of the sampled tissues. The result is consistent with the present understanding of embryogenesis and demonstrates the large scale potential of this method for producing a complete mammalian cell fate at the resolution of a single cell.

Patterns of myocardial histogenesis as revealed by mouse chimeras

In order to study the pattern of clonal myocyte distribution during mammalian heart development, we have exploited embryo aggregation chimeras using, as cellular markers, an enhanced jellyfish green fluorescent protein (eGFP) transgene and a desmin-promoter-driven, nuclear-localized beta-galactosidase (nlacZ) knock-in. In neonatal, weanling, and adult chimeric atria and ventricles, irregularly formed patches of various sizes rather than highly dispersed cardiomyocytes were observed. Most of the smaller patches and single cardiomyocytes were found in spatial neighborhood of large patches. This indicated largely coherent clonal growth during myocardial histogenesis combined with tangential displacement or active migration of myocytes. The patterns of ventricular walls were simpler than those of the septum and the atria. In the adult heart, large myocardial volumes devoid of eGFP-positive cardiomyocytes indicated a lack of secondary immigration of blood-borne stem cells into the myocardium. The patterns of oligoclonal expansions revealed in this work might be helpful in detecting and analyzing cell-lineage-based pathological processes in the heart.

How can we understand the construction of an organism?

Since a dozen years, biology is in a state of permanent technical and conceptual excitement. The pendulum is swinging back from the selectionist populationist biology of Darwin--the organism is a black box: viewed from the outside--to the mechanistic embryology of Aristotle--viewed from the inside of the organism--leading to a new interpretation of old concepts. This short text tries to get to the meaning of these events by putting them in historical and epistemological perspectives, through such concepts as teleology and differentiation, on the way describing several paradoxes: experimental results yield a detailed description of purposeless mechanical devices, explaining Nature, which, to us, appears purposeful.

Intermingling versus clonal coherence during skeletal muscle development: Mosaicism in eGFP/nLacZ-labeled mouse chimeras

To study the relative contributions of clonal coherence vs. myoblast intermingling to the formation of mammalian skeletal muscles, enhanced green fluorescent protein (eGFP) and nLacZ labels were used to analyze neonatal and adult mouse embryo aggregation chimeras. The eGFP marker allowed us to estimate absolute levels of the eGFP transgenic parental contributions as well as variances of levels within a chimeric individual; nLacZ served as a counter-label in neonatal chimeras. In mature chimeric muscle, free intracellular diffusion of eGFP led to an averaging of the eGFP level in individual myofibers. Chimerism, i.e., differing eGFP levels, was evident between corresponding right and left muscles, between adjacent back, body wall, limb (shank, foot), and extraocular muscles and among myofibers within these muscles. Inhomogeneities in muscles far from their somitic origin indicated unbalanced sampling from small founder pools. The results obtained with adult eGFP<-->0 chimeras were corroborated by neonatal chimeras with complementary eGFP<-->nLacZ labels.

Myotome meanderings. Cellular morphogenesis and the making of muscle

The formation of muscles within the vertebrate embryo is a tightly orchestrated and complex undertaking. Beyond the initial specification of cells to become muscle are several complex cellular movements and migrations, which lead to the positioning of muscle precursors at specific locations within the embryo. The consequent differentiation, elongation and striation of these cells results in the formation of individual muscles. Investigation of the in vivo morphogenesis of individual vertebrate muscle cells has only recently begun, and is being approached through the use of sophisticated cell labelling and lineage analysis techniques. However, a consensus about the mechanisms involved has yet to be achieved. This review outlines vertebrate embryonic muscle formation in chick, fish and mice, focusing on the embryonic myotome, which generates both the axial musculature and the appendicular muscle of the fins and limbs. We highlight the points of consensus about, and the complexity of, this developmental system, and propose an evolutionary context for the basis of these understandings.

Myogenic regulatory factors and the specification of muscle progenitors in vertebrate embryos

Embryological and genetic studies of mouse, bird, zebrafish, and frog embryos are providing new insights into the regulatory functions of the myogenic regulatory factors, MyoD, Myf5, Myogenin, and MRF4, and the transcriptional and signaling mechanisms that control their expression during the specification and differentiation of muscle progenitors. Myf5 and MyoD genes have genetically redundant, but developmentally distinct regulatory functions in the specification and the differentiation of somite and head muscle progenitor lineages. Myogenin and MRF4 have later functions in muscle differentiation, and Pax and Hox genes coordinate the migration and specification of somite progenitors at sites of hypaxial and limb muscle formation in the embryo body. Transcription enhancers that control Myf5 and MyoD activation in muscle progenitors and maintain their expression during muscle differentiation have been identified by transgenic analysis. In epaxial, hypaxial, limb, and head muscle progenitors, Myf5 is controlled by lineage-specific transcription enhancers, providing evidence that multiple mechanisms control progenitor specification at different sites of myogenesis in the embryo. Developmental signaling ligands and their signal transduction effectors function both interactively and independently to control Myf5 and MyoD activation in muscle progenitor lineages, likely through direct regulation of their transcription enhancers. Future investigations of the signaling and transcriptional mechanisms that control Myf5 and MyoD in the muscle progenitor lineages of different vertebrate embryos can be expected to provide a detailed understanding of the developmental and evolutionary mechanisms for anatomical muscles formation in vertebrates. This knowledge will be a foundation for development of stem cell therapies to repair diseased and damaged muscles.

Cellular patterning of the vertebrate embryo

Recent studies show that cell dispersal is a widespread phenomenon in the development of early vertebrate embryos. These cell movements coincide with major decisions for the spatial organization of the embryo, and they parallel genetic patterning events. For example, in the central nervous system, cell dispersal is first mainly anterior-posterior and subsequently dorsal-ventral. Thus, genes expressed in signaling centers of the embryo probably control cell movements, tightly linking cellular and genetic patterning. Cell dispersal might be important for the correct positioning of cells and tissues involved in intercellular signaling. The emergence of cell dispersal at the onset of vertebrate evolution indicates a shift from early, lineage-based cellular patterning in small embryos to late, movement-based cellular patterning of polyclones in large embryos. The conservation of the same basic body plan by invertebrate and vertebrate chordates suggests that evolution of the embryonic period preceding the phylotypic stage was by intercalary co-option of basic cell activities present in the ancestral metazoan cell.

Vertebrate somitogenesis

In vertebrates, the paraxial mesoderm corresponds to the bilateral strips of mesodermal tissue flanking the notochord and neural tube and which are delimited laterally by the intermediate mesoderm and the lateral plate. The paraxial mesoderm comprises the head or cephalic mesoderm anteriorly and the somitic region throughout the trunk and the tail of the vertebrates. Soon after gastrulation, the somitic region of vertebrates starts to become segmented into paired blocks of mesoderm, termed somites. This process lasts until the number of somites characteristic of the species is reached. The somites later give rise to all skeletal muscles of the body, the axial skeleton, and part of the dermis. In this review I discuss the processes involved in the formation of the paraxial mesoderm and its segmentation into somites in vertebrates.

Morphogenetic cell movements in the middle region of the dermomyotome dorsomedial lip associated with patterning and growth of the primary epaxial myotome

The morphogenetic cell movements responsible for growth and morphogenesis in vertebrate embryos are poorly understood. Myotome precursor cells undergo myotomal translocation; a key morphogenetic cell movement whereby myotomal precursor cells leave the dermomyotome epithelium and enter the subjacent myotome layer where myogenic differentiation ensues. The precursors to the embryonic epaxial myotome are concentrated in the dorsomedial lip (DML) of the somite dermomyotome (W. F. Denetclaw, B. Christ and C. P. Ordahl (1997) Development 124, 1601–1610), a finding recently substantiated through surgical transplantation studies (C. P. Ordahl, E. Berdougo, S. J. Venters and W. F. Denetclaw, Jr (2001) Development 128, 1731–1744). Confocal microscopy was used here to analyze the location and pattern of myotome cells whose precursors had earlier been labeled by fluorescent dye injection into the middle region of the DML, a site that maximizes the potential to discriminate among experimental outcomes. Double-dye injection experiments conducted at this site demonstrate that cells fated to form myotome do not involute around the recurved epithelium of the DML but rather are displaced laterally where they transiently intermingle with cells fated to enter the central epithelial sheet region of the dermomyotome. Time- and position-dependent labeling experiments demonstrated that myotome precursor cells translocate directly from the middle region of the DML without prior intra-epithelial ‘translational’ movements of precursor cells to either the cranial or caudal lips of the dermomyotome epithelium, nor were any such translational movements evident in these experiments. The morphogenetic cell movements demonstrated here to be involved in the directional growth and segmental patterning of the myotome and dermomyotome bear interesting similarities with those of other morphogenetic systems.

Different clonal dispersion in the rostral and caudal mouse central nervous system

We have performed a systematic clonal analysis to describe the modes of growth, dispersion and production of cells during the development of the mouse neural system. We have used mice expressing a LaacZ reporter gene under the control of the neuron specific enolase promoter to randomly generate LacZ clones in the central nervous system (CNS). We present evidence for (1) a pool of CNS founder cells that is not regionalized, i.e. give descendants dispersed along the entire A-P axis, (2) an early separation between pools of precursors for the anterior and posterior CNS and (3) distinct modes of production of progenitors in these two domains. More specifically, cell growth and dispersion of the progenitors follow a relatively coherent pattern throughout the anterior CNS, a mode that leads to a progressive regionalization of cell fates. In contrast, cell growth of progenitors of the SC appears to involve self-renewing stem cells that progress caudally during regression of the mode. Therefore, at least part of the area surrounding the node is composed of precursors with self-renewing properties and the development of the trunk is dependent on pools of stem cells regressing from A to P. Taken together with our analysis of the cell growth changes associated with neuromere formation (Mathis, L., Sieur, J., Voiculescu, O., Charnay, P. and Nicolas, J. F. (1999) Development 126, 4095–4106), our results suggest that major transitions in CNS development correspond to changes in cell behavior and may provide a link between morphogenesis and genetic patterning mechanisms (i.e. formation of the body plan).