Evolution of the vertebrate head is revealed by the rosette mesoderm

The vertebrate head comprises characteristic combinations of the cranium, brain, cranial nerves and head muscles. However, there have long been arguments about the developmental and evolutionary origins and the possible segmental nature of the head muscles, particularly those anterior to the otic vesicle. In gnathostomes, the presence of pre-otic segments (trunk somite homologs) has been denied by anti-segmentalists, but championed by segmentalists, who have focused on marginally detectable somitomeres or on more obvious head cavities. This metameric ideology has generated various definitions of segments, causing great controversy1,2, and the evaluation of such arguments has been impeded by the relative paucity of relevant work on the head mesoderm of cyclostomes (hagfishes and lampreys). Here, we demonstrate the presence of rosettes (which are reminiscent of somites) in the head mesoderm of lamprey (Lethenteron camtschaticum) embryos using confocal laser scanning and transmission electron microscopy. These transient rosettes, which were not segmented by acellular fissures and were genetically distinct from trunk somites, emerged several times during character individualization of the head muscles. This specialty of the rosette dynamics suggests that the lamprey head mesoderm evolved de novo, rather than from somites, and molecular comparison among deuterostomes supported this perspective.

Developmental fates of shark head cavities reveal mesodermal contributions to tendon progenitor cells in extraocular muscles

Vertebrate extraocular muscles (EOMs) function in eye movements. The EOMs of modern jawed vertebrates consist primarily of four recti and two oblique muscles innervated by three cranial nerves. The developmental mechanisms underlying the establishment of this complex and the evolutionarily conserved pattern of EOMs are unknown. Chondrichthyan early embryos develop three pairs of overt epithelial coeloms called head cavities (HCs) in the head mesoderm, and each HC is believed to differentiate into a discrete subset of EOMs. However, no direct evidence of these cell fates has been provided due to the technical difficulty of lineage tracing experiments in chondrichthyans. Here, we set up an in ovo manipulation system for embryos of the cloudy catshark Scyliorhinus torazame and labeled the epithelial cells of each HC with lipophilic fluorescent dyes. This experimental system allowed us to trace the cell lineage of EOMs with the highest degree of detail and reproducibility to date. We confirmed that the HCs are indeed primordia of EOMs but showed that the morphological pattern of shark EOMs is not solely dependent on the early pattern of the head mesoderm, which transiently appears as tripartite HCs along the simple anteroposterior axis. Moreover, we found that one of the HCs gives rise to tendon progenitor cells of the EOMs, which is an exceptional condition in our previous understanding of head muscles; the tendons associated with head muscles have generally been supposed to be derived from cranial neural crest (CNC) cells, another source of vertebrate head mesenchyme. Based on interspecies comparisons, the developmental environment is suggested to be significantly different between the two ends of the rectus muscles, and this difference is suggested to be evolutionarily conserved in jawed vertebrates. We propose that the mesenchymal interface (head mesoderm vs CNC) in the environment of developing EOM is required to determine the processes of the proximodistal axis of rectus components of EOMs.

Zebrafish embryonic explants undergo genetically encoded self-assembly

Embryonic stem cell cultures are thought to self-organize into embryoid bodies, able to undergo symmetry-breaking, germ layer specification and even morphogenesis. Yet, it is unclear how to reconcile this remarkable self-organization capacity with classical experiments demonstrating key roles for extrinsic biases by maternal factors and/or extraembryonic tissues in embryogenesis. Here, we show that zebrafish embryonic tissue explants, prepared prior to germ layer induction and lacking extraembryonic tissues, can specify all germ layers and form a seemingly complete mesendoderm anlage. Importantly, explant organization requires polarized inheritance of maternal factors from dorsal-marginal regions of the blastoderm. Moreover, induction of endoderm and head-mesoderm, which require peak Nodal-signaling levels, is highly variable in explants, reminiscent of embryos with reduced Nodal signals from the extraembryonic tissues. Together, these data suggest that zebrafish explants do not undergo bona fide self-organization, but rather display features of genetically encoded self-assembly, where intrinsic genetic programs control the emergence of order.

Cardiac competence of the head mesoderm fades concomitant with a shift towards the head skeletal muscle programme

The vertebrate head mesoderm provides the heart, the great vessels, smooth and most head skeletal muscle, and parts of the skull base. The ability to generate cardiac and smooth muscle is thought to be the evolutionary ground-state of the tissue, and initially the head mesoderm has cardiac competence throughout, even in the paraxial region that normally does not engage in cardiogenesis. How long this competence lasts, and what happens as cardiac competence fades, is not clear.Using a wide palette of marker genes in the chicken embryo, we show that the paraxial head mesoderm has the ability to respond to Bmp, a known cardiac inducer, for a long time. However, Bmp signals are interpreted differently at different time points. Bmp triggers cardiogenesis up to early head fold stages; the ability to upregulate smooth muscle markers is retained slightly longer. Notably, as cardiac competence fades, Bmp activates the head skeletal muscle programme instead.Summary statementThe head mesoderm has generic cardiac competence until head fold stages. Thereafter, cardiac competence fades in the paraxial region, and Bmp activates head skeletal muscle programmes instead of cardiac programmes.

Head Mesoderm Tissue Growth, Dynamics and Neural Crest Cell Migration

ABSTRACTVertebrate head morphogenesis involves orchestrated cell growth and tissue movements of the mesoderm and neural crest to form the distinct craniofacial pattern. To better understand structural birth defects, it is important that we learn how these processes are controlled. Here, we examine this question during chick head morphogenesis using time-lapse imaging, computational modeling, and experiment. We find that head mesodermal cells are inherently dynamic in culture and alter cell behaviors in the presence of either ectoderm or neural crest cells. Mesodermal cells in vivo display large-scale whirling motions that rapidly transition to lateral, directed movements after neural crest cells emerge. Computer model simulations predict distinct changes in neural crest migration as the spatio-temporal growth profile of the mesoderm is varied. BrdU-labeling and photoconversion combined with cell density measurements then reveal non-uniform mesoderm growth in space and time. Chemical inhibition of head mesoderm proliferation or ablation of premigratory neural crest alters mesoderm growth and neural crest migration, implying a dynamic feedback between tissue growth and neural crest cell signaling to confer robustness to the system.Summary StatementDynamic feedback between tissue growth and neural crest cell migration ensures robust neural crest stream formation and head morphogenesis shown by time-lapse microscopy, mathematical modeling and embryo perturbations.

Neural induction by the node and placode induction by head mesoderm share an initial state resembling neural plate border and ES cells

Around the time of gastrulation in higher vertebrate embryos, inductive interactions direct cells to form central nervous system (neural plate) or sensory placodes. Grafts of different tissues into the periphery of a chicken embryo elicit different responses: Hensen’s node induces a neural plate whereas the head mesoderm induces placodes. How different are these processes? Transcriptome analysis in time course reveals that both processes start by induction of a common set of genes, which later diverge. These genes are remarkably similar to those induced by an extraembryonic tissue, the hypoblast, and are normally expressed in the pregastrulation stage epiblast. Explants of this epiblast grown in the absence of further signals develop as neural plate border derivatives and eventually express lens markers. We designate this state as “preborder”; its transcriptome resembles embryonic stem cells. Finally, using sequential transplantation experiments, we show that the node, head mesoderm, and hypoblast are interchangeable to begin any of these inductions while the final outcome depends on the tissue emitting the later signals.