Although mammalian hearts show virtually no ability to regenerate, there is a growing initiative to determine whether existing cardiomyocytes or progenitor cells can be coaxed into eliciting a regenerative response. In contrast to mammals, a number of non-mammalian vertebrate species are able to regenerate their hearts1–3, including the zebrafish4,5, which can fully regenerate its heart following amputation of up to 20% of the ventricle. To directly address the source of newly formed cardiomyocytes during zebrafish heart regeneration, we first established a genetic strategy to lineage-trace cardiomyocytes in the adult fish, based on the Cre/lox system widely used in the mouse6. Using this system, we show here that regenerated heart muscle cells are derived from the proliferation of differentiated cardiomyocytes. Furthermore, we show that proliferating cardiomyocytes undergo limited dedifferentiation characterized by the disassembly of their sarcomeric structure, detachment from one another and expression of regulators of cell cycle progression. Specifically, we show that polo-like kinase1 (plk1) is an essential component of cardiomyocyte proliferation during heart regeneration. Our data provides the first direct evidence for the source of proliferating cardiomyocytes during zebrafish heart regeneration and indicates that stem/progenitor cells are not significantly involved in this process.

The cellular and molecular bases allowing tissue regeneration are not well understood. By performing gain-and loss-of-function experiments of specific members of the Wnt pathway during appendage regeneration, we demonstrate that this pathway is not only necessary for regeneration to occur, but it is also able to promote regeneration in axolotl, Xenopus, and zebrafish. Furthermore, we show that changes in the spatiotemporal distribution of ␤-catenin in the developing chick embryo elicit apical ectodermal ridge and limb regeneration in an organism previously thought not to regenerate. Our studies may provide valuable insights toward a better understanding of adult tissue regeneration.Supplemental material is available at http://www.genesdev.org.

Decreasing ECM stiffness rescues the regeneration ability of the neonatal mouse heart.

Characterization of pluripotent stem cells is required for the registration of stem cell lines and allows for an impartial and objective comparison of the results obtained when generating multiple lines. It is therefore crucial to establish specific, fast and reliable protocols to detect the hallmarks of pluripotency. Such protocols should include immunocytochemistry (takes 2 d), identification of the three germ layers in in vitro-derived embryoid bodies by immunocytochemistry (immunodetection takes 3 d) and detection of differentiation markers in in vivo-generated teratomas by immunohistochemistry (differentiation marker detection takes 4 d). Standardization of the immunodetection protocols used ensures minimum variations owing to the source, the animal species, the endogenous fluorescence or the inability to collect large amounts of cells, thereby yielding results as fast as possible without loss of quality. This protocol provides a description of all the immunodetection procedures necessary to characterize mouse and human stem cell lines in different circumstances.

Transcription-factor-induced reprogramming of somatic cells to pluripotency is a very inefficient process, probably due to the existence of important epigenetic barriers that are imposed during differentiation and that contribute to preserving cell identity. In an effort to decipher the molecular nature of these barriers, we followed a genome-wide approach, in which we identified macrohistone variants (macroH2A) as highly expressed in human somatic cells but downregulated after reprogramming to pluripotency, as well as strongly induced during differentiation. Knockdown of macrohistone variants in human keratinocytes increased the efficiency of reprogramming to pluripotency, whereas overexpression had opposite effects. Genome-wide occupancy profiles show that in human keratinocytes, macroH2A.1 preferentially occupies genes that are expressed at low levels and are marked with H3K27me3, including pluripotency-related genes and bivalent developmental regulators. The presence of macroH2A.1 at these genes prevents the regain of H3K4me2 during reprogramming, imposing an additional layer of repression that preserves cell identity.

The protocol followed results in high rates of survival and potential for in-vitro maturation, but has a deleterious effect on the organization of the meiotic spindle of human oocytes cryopreserved at both the GV and MII stages.

SUMMARYThe transcriptional basis of vertebrate limb initiation, which is a well-studied system for the initiation of organogenesis, remains elusive. Specifically, involvement of the -catenin pathway in limb initiation, as well as its role in hindlimb-specific transcriptional regulation, are under debate. Here, we show that the -catenin pathway is active in the limb-forming area in mouse embryos. Furthermore, conditional inactivation of -catenin as well as Islet1, a hindlimb-specific factor, in the lateral plate mesoderm results in a failure to induce hindlimb outgrowth. We further show that Islet1 is required for the nuclear accumulation of -catenin and hence for activation of the -catenin pathway, and that the -catenin pathway maintains Islet1 expression. These two factors influence each other and function upstream of active proliferation of hindlimb progenitors in the lateral plate mesoderm and the expression of a common factor, Fgf10. Our data demonstrate that Islet1 and -catenin regulate outgrowth and Fgf10-Fgf8 feedback loop formation during vertebrate hindlimb initiation. Our study identifies Islet1 as a hindlimb-specific transcriptional regulator of initiation, and clarifies the controversy regarding the requirement of -catenin for limb initiation.

Organ shape and size, and, ultimately, organ function, relate in part to the cell and tissue spatial arrangement that takes place during embryonic development. Despite great advances in the genetic regulatory networks responsible for tissue and organ development, it is not yet clearly understood how specific gene functions are linked to the specific morphogenetic processes underlying the internal organ asymmetries found in vertebrate animals. During female chick embryogenesis, and in contrast to males where both testes develop symmetrically, asymmetrical gonad morphogenesis results in only one functional ovary. The disposition of paired organs along the left-right body axis has been shown to be regulated by the activity of the homeobox containing gene pitx2. We have found that pitx2 regulates cell adhesion, affinity, and cell recognition events in the developing gonad primordium epithelia. This in turn not only allows for proper somatic development of the gonad cortex but also permits the proliferation and differentiation of primordial germ cells. We illustrate how Pitx2 activity directs asymmetrical gonad morphogenesis by controlling mitotic spindle orientation of the developing gonad cortex and how, by modulating cyclinD1 expression during asymmetric ovarian development, Pitx2 appears to control gonad organ size. All together our observations indicate that the effects elicited by Pitx2 during the development of the female chick ovary are critical for cell topology, growth, fate, and ultimately organ morphogenesis and function.cyclin D1 ͉ left-right asymmetry ͉ chick ovary development ͉ mitotic orientation ͉ primordial germ cell D espite the initial bilateral symmetry of the vertebrate body plan, internal organs, such as the heart, stomach, and intestines, all have a characteristic asymmetric structure and are asymmetrically positioned in the body cavity. The acquisition of proper left-right asymmetries is achieved in a highly conserved manner during embryo development. Since the seminal work more than a decade ago that identified a functional link between asymmetrical gene expression and organ asymmetry (1), our understanding of the molecular and cellular events involved in the establishment of left-right asymmetry in vertebrates has increased enormously. This is indeed the case in the initial establishment of left-right asymmetrical differences that occur during gastrulation as well as during the immediately subsequent stages of embryo development when the local asymmetrical cues in and around the node are deployed to broader domains of gene expression in the lateral plate mesoderm. Once side-specific gene expression domains are established and stabilized in the lateral plate mesoderm, left-right information is transferred to the organ primordial, where left and right side-specific morphogenetic programs are executed (reviewed in refs. 2-6).The identification of several genes that display side-specific patterns of expression within the developing organs has provided an entry point for understanding the molecu...