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Background The dual-specificity T-box/basic helix-loop-helix leucine zipper transcription factor MGA is part of the MAX-interacting network of proteins. In the mouse, MGA is necessary for the survival of the pluripotent epiblast cells of the peri-implantation embryo and a null, gene-trap allele MgaGt results in embryonic lethality shortly after implantation. We have used this allele to document expression of Mga in postimplantation embryos and also investigated a second, hypomorphic gene-trap allele, MgaInv. Results Compound heterozygotes, MgaGt/MgaInv, die prior to midgestation. The extraembryonic portion of the embryos appears to develop relatively normally while the embryonic portion, including the pluripotent cells of the epiblast, is severely retarded by E7.5. Mga expression is initially limited to the pluripotent inner cell mass of the blastocyst and epiblast, but during organogenesis it is widely expressed, notably in the central nervous system and sensory organs, reproductive and excretory systems, heart, somites and limbs. Conclusions Widespread yet specific areas of expression of Mga during organogenesis raise the possibility that the transcription factor may play roles in controlling proliferation and potency in the progenitor cell populations of different organ systems. Documentation of these patterns sets the stage for the investigation of specific progenitor cell types.

Ventral folding morphogenesis, a vital morphogenetic process in amniotes, mediates gut endoderm internalization, linear heart tube formation, ventral body wall closure and encasement of the fetus in extraembryonic membranes. Aberrant ventral folding morphogenesis underlies a number of birth defects, such as gastroschisis and ectopia cordis in human and misplacement of head and heart in mouse. Recent cell lineage-specific mouse mutant analyses identified the Bone Morphogenetic Protein (BMP) pathway and Anterior Visceral Endoderm (AVE) as key regulators of anterior ventral folding morphogenesis. Loss of BMP2 expression solely from embryonic visceral endoderm (EmVE) and the AVE blocks formation of foregut invagination, and simultaneously, aberrantly positions the heart anterior/dorsal to the head, suggesting a mechanistic link between foregut and head/heart morphogenesis.

The signaling pathways and cellular mechanisms that achieve alignment of dorsal and ventral midline structures remain a poorly understood aspect of vertebrate development. In this issue of Developmental Cell, Arraf et al. (2016) find a requisite role for bilaterally symmetrical BMP signaling in coordinating dorsal and ventral tissue morphogenesis.

Multipotent P19CL6 cells differentiate into cardiac myocytes or neural lineages when stimulated with dimethyl sulfoxide (DMSO) or retinoic acid (RA), respectively. Expression of the transcription factor Tbx6 was found to increase during cardiac myocyte differentiation and to decrease during neural differentiation. Overexpression of Tbx6 was not sufficient to drive P19CL6 cells to a cardiac myocyte fate or to accelerate DMSO-induced differentiation. In contrast, knockdown of Tbx6 dramatically inhibited DMSO-induced differentiation of P19CL6 cells to cardiac myocytes, as evidenced by the loss of striated muscle-specific markers and spontaneous beating. Tbx6 knockdown was also accompanied by almost complete loss of Nkx2.5, a transcription factor involved in the specification of the cardiac myocyte lineage, indicating that Nkx2.5 is downstream of Tbx6. In distinction to its positive role in cardiac myocyte differentiation, Tbx6 knockdown augmented RA-induced differentiation of P19CL6 cells to both neurons and glia, and accelerated the rate of neurite formation. Conversely, Tbx6 overexpression attenuated differentiation to neural lineages. Thus, in the P19CL6 model, Tbx6 is required for cardiac myocyte differentiation and represses neural differentiation. We propose a model in which Tbx6 is a part of a molecular switch that modulates divergent differentiation programs within a single progenitor cell.

Members of the T-box family of transcription factors are important regulators orchestrating the complex regionalization of the developing mammalian heart. Individual mutations in Tbx20 and Tbx3 cause distinct congenital heart abnormalities in the mouse: Tbx20 mutations result in failure of heart looping, developmental arrest and lack of chamber differentiation, while hearts of Tbx3 mutants progress further, loop normally but show atrioventricular convergence and outflow tract defects. The two genes have overlapping areas of expression in the atrioventricular canal and outflow tract of the heart but their potential genetic interaction has not been previously investigated. In this study we produced compound mutants to investigate potential genetic interactions at the earliest stages of heart development. We find that Tbx20; Tbx3 double heterozygous mice are viable and fertile with no apparent abnormalities, while double homozygous mutants are embryonic lethal by midgestation. Double homozygous mutant embryos display abnormal cardiac morphogenesis, lack of heart looping, expression patterns of cardiac genes and time of death that are indistinguishable from Tbx20 homozygous mutants. Prior to death, the double homozygotes show an overall developmental delay similar to Tbx3 homozygous mutants. Thus the effects of Tbx20 are epistatic to Tbx3 in the heart but Tbx3 is epistatic to Tbx20 with respect to developmental delay.

We report the derivation and characterization of two new human embryonic stem cells (hESC) lines (CU1 and CU2) from embryos with an irreversible loss of integrated organismic function. In addition, we analyzed retrospective data of morphological progression from embryonic day (ED) 5 to ED6 for 2480 embryos not suitable for clinical use to assess grading criteria indicative of loss of viability on ED5. Our analysis indicated that a large proportion ofin vitrofertilization (IVF) embryos not suitable for clinical use could be used for hESC derivation. Based on these combined findings, we propose that criteria commonly used in IVF clinics to determine optimal embryos for uterine transfer can be employed to predict the potential for hESC derivation from poor quality embryos without the destruction of vital human embryos.

Human embryonic stem cells (hESC) hold great promise for use in regenerative medicine. However, the extraordinary potential of hESC as therapeutic tools is tempered by ethical, moral and political issues surrounding their derivation from human embryos. It has previously been proposed that ethical criteria applied to essential organ donation could be employed for derivation of hESC from irreversibly arrested, and thus organismically dead, human embryos produced during routine IVF procedures. Here, it is shown that arrested embryos do not resume normal development during extended culture, yet most of them contain a substantial number of living cells on embryonic day 6 (72% have >1 viable cell, 47% have >5 viable cells), suggesting that this class of non-viable embryos could be a rich source of viable cells for derivation of hESC lines.