Regulation of ADMP and BMP2/4/7 at opposite embryonic poles generates a self-regulating morphogenetic field.

Embryos have the ability to self-regulate and regenerate normal structures after being sectioned in half. How is such a morphogenetic field established? We discovered that quadruple knockdown of ADMP and BMP2/4/7 in Xenopus embryos eliminates self-regulation, causing ubiquitous neural induction throughout the ectoderm. ADMP transcription in the Spemann organizer is activated at low BMP levels. When ventral BMP2/4/7 signals are depleted, Admp expression increases, allowing for self-regulation. ADMP has BMP-like activity and signals via the ALK-2 receptor. It is unable to signal dorsally because of inhibition by Chordin. The ventral BMP antagonists Sizzled and Bambi further refine the pattern. By transplanting dorsal or ventral wild-type grafts into ADMP/BMP2/4/7-depleted hosts, we demonstrate that both poles serve as signaling centers that can induce histotypic differentiation over considerable distances. We conclude that dorsal and ventral BMP signals and their extracellular antagonists expressed under opposing transcriptional regulation provide a molecular mechanism for embryonic self-regulation.

Induction of cardiogenesis by Hensen's node and fibroblast growth factors.

The earliest events underlying cardiac induction and morphogenesis remain largely unknown. In the present study, we show that Hensen's node, the organizer of the avian embryo, induces cardiogenesis. Specifically, following heterotopic transplantation, Hensen's node induces ectopic host tissue that expresses two early cardiac markers ( cNkx-2.5 and cNkx-2.8), as well as a ventricular marker ( VMHC1), but not an atrial marker ( AMHC1). Moreover, we examine the potential roles of candidate growth factors known to be secreted by Hensen's node. Our results show that fibroblast growth factors (FGF-2 and FGF-4) when ectopically expressed can initiate cardiac development, inducing host tissue to express the two cardiac transcription factors cNkx-2.5 and cNkx-2.8, as well as the cardiac-restricted structural gene VMHC1, but not AMHC1. In contrast to FGFs, TGFbeta family members fail to induce ectopic tissue and expression of cardiac marker genes. We also examined the effects of growth factors on the morphogenesis of the host embryo's heart. Both exogenous FGFs and TGFbeta family members perturb normal morphogenesis of the early cardiac tube and alter patterns of ventricular and atria gene expression in characteristic ways. Namely, exogenous FGFs expand areas expressing the ventricular marker VMHC1 at the expense of areas expressing the atrial marker AMHC1. Conversely, exogenous TGFbeta1 inhibits expression of VMHC1, expanding AMHC1 expression. We show here that Hensen's node and FGFs induce ectopic expression of cardiac lineage markers, and that FGF and TGFbeta family members can modulate early development of the heart. Collectively, these data suggest that the organizer plays a crucial role in cardiac induction and morphogenesis, mediated in part by endogenous members of the FGF and TGFbeta families.

Post-nodal mesoblast caudalizes the host axis and inhibits cell population growth, and induces new primitive streaks in chick embryos.

One to eight post-nodal fragments (PN) or Hensen's nodes (HN) from full primitive streak stage chick embryos were transplanted onto the area pellucida or area opaca of stage 4 embryos and cultured for 20 h. Thirteen morphological and numerical parameters were affected in the host embryos and analyzed by multiple logistic regression for parametric hierarchy. In the area pellucida, both PN and HN transplants inhibited cell population growth while only PN caudalized the host axis and induced supernumerary primitive streaks expressing the mesoderm-specific gene Brachyury. In the area opaca, neither grafts influenced host axis morphogenesis, but PN inhibited the cell population growth significantly. Tracking [(3)H]TdR labeled grafts showed that PN cells migrated towards the host axis and participated in the formation of supernumerary somites and hearts. When placed near the host axis, PN caudalized it and inhibited cell population growth.

The neural inductive response of competent chick ectoblast decreases away from the host axis and correlates with an increased proliferative activity.

We have assessed the quality and quantity of the neural inductive response of the chick gastrula ectoblast located at increasing distancefrom the host axis. In a stage 4 gastrula, entire ectoblast exhibits neural competence. The quality of induced neural tissue shifts from deuterencephalic type in the area pellucida to archencephalic type in the area opaca and primitive medullary or palisade type atthe margin of overgrowth with a concomitant reduction in the number of induced neural cells. In contrast, the mitotic and 3H-TdR labelling frequencies in the competent ectoblast increase with increasing distance from the host axis and in a proportion inverse to the amount of induced neural tissue. It is suggested that the strong neural inductive response is correlated with low proliferative activity, or longer cell cycle time, of the competent ectoblast.

The neural inductive signal is transferred to ectoblast in 1-2 h but a continued contact with mesoblast for 2-3 h is essential for neuralization in the chick area pellucida.

In the area pellucida of the chick gastrula, the Hensen's node (HN) graft must contact the competent ectoblast for at least 4 h to promote neural induction. When we removed the grafted HN after 1 to 3 h and replaced it by a non-inducing post nodal (PN) fragment, a 1-2 h contact with HN was found to be sufficient to promote neural induction. When HN graft was removed after 3 or 4 h and replaced by PN, the neural inductive response was substantially improved towards formation of archencephalic structures. Thus, our results indicate that neural induction takes place in two steps. In the first step, a contact with HN for 1-2 h is sufficient to transferthe inductive signal which is stabilized through a second step involving continued cell-cell contact with even non-inducing PN mesoblast.

Effects of follistatin and BMP4 proteins on early dorso-ventral patterning in chick.

In Xenopus and zebrafish certain bone morphognetic proteins (BMPs), and proteins that antagonise these by preventing their interaction with receptors, constitute a morphogen system in primary dorso-ventral patterning. This system may be directly involved in the parallel processes, within mesoderm and ectoderm, whereby the boundaries of the dorsal (paraxial) mesoderm and the neural plate are established. The bird blastoderm, amenable to grafting techniques and to direct exposure to specific proteins, has provided an opportunity to explore the phylogenetic conservation of such antagonistic system. We have grafted the gastrular organiser (node) into hosts, testing the effects of prior exposure of either grafted or host tissue to Follistatin (a known antagonist of TGFbeta superfamily ligands including BMP4) or to BMP4 protein. Strong, converse effects are seen from the two agents, the most consistent being on the sizes of new dorsalised areas (second neural plates) induced in host epiblast. Follistatin also enhances extension movements due to grafts, though without clear effect upon the rostro-caudal completeness of new patterns. Neural induction in chick epiblast by grafted mouse nodes are also more extensive, after their pre-incubation in Follistatin. Follistatin potentiates other, unknown but distinctive signals coming from the node, being unable to convert other non-inducing pieces of blastoderm into organisers on grafting. Pre-incubation of early blastoderms in BMP4 has such profound effects on normal dorsal axial development that host responsiveness of these blastoderms as hosts to node grafts is difficult to assess. Follistatin has no such overt effect on host development, but greatly enhances the competence of host epiblast to grafts of untreated nodes. Early chick BMP4 and BMP7 expressions are consistent with the proposed roles, though Follistatin is probably an experimental tool only in the present study.