Background: Head or anterior body part regeneration is commonly associated with protostome, but not deuterostome invertebrates. However, it has been shown that the solitary hemichordate Ptychodera flava possesses the remarkable capacity to regenerate their entire nervous system, including their dorsal neural tube and their anterior head‐like structure, or proboscis. Hemichordates, also known as acorn worms, are marine invertebrate deuterostomes that have retained chordate traits that were likely present in the deuterostome ancestor, placing these animals in a vital position to study regeneration and chordate evolution. All acorn worms have a tripartite body plan, with an anterior proboscis, middle collar region, and a posterior trunk. The collar houses a hollow, dorsal neural tube in ptychoderid hemichordates and numerous chordate genes involved in brain and spinal cord development are expressed in a similar anterior–posterior spatial arrangement along the body axis. Results: We have examined anterior regeneration in the hemichordate Ptychodera flava and report the spatial and temporal morphological changes that occur. Additionally, we have sequenced, assembled, and analyzed the transcriptome for eight stages of regenerating P. flava, revealing significant differential gene expression between regenerating and control animals. Conclusions: Importantly, we have uncovered developmental steps that are regeneration‐specific and do not strictly follow the embryonic program. Developmental Dynamics 245:1159–1175, 2016. © 2016 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists

Enteropneust hemichordates share several characteristics with chordates, such as a Hox-specified anterior-posterior axis, pharyngeal gill slits, a dorsal central nervous system (CNS), and a juvenile postanal tail. Ptychoderid hemichordates, such as the indirect-developer Ptychodera flava, have feeding larvae and a remarkable capacity to regenerate their CNS. We compared neurulation of ptychoderid hemichordates and chordates using histological analyses, and found many similarities in CNS development. In ptychoderid hemichordates, which lack a notochord, the proboscis skeleton develops from endoderm after neurulation. The position of the proboscis skeleton directly under the nerve cord suggests that it serves a structural role similar to the notochord of chordates. These results suggest that either the CNS preceded evolution of the notochord or that the notochord has been lost in hemichordates. The evolution of the notochord remains ambiguous, but it may have evolved from endoderm, not mesoderm.

Engineered muscle tissues represent powerful tools for examining tissue level contractile properties of skeletal muscle. However, limitations in the throughput associated with standard analysis methods limit their utility for longitudinal study, high throughput drug screens, and disease modeling. Here we present a method for integrating 3D engineered skeletal muscles with a magnetic sensing system to facilitate non-invasive, longitudinal analysis of developing contraction kinetics. Using this platform, we show that engineered skeletal muscle tissues derived from both induced pluripotent stem cell and primary sources undergo improvements in contractile output over time in culture. We demonstrate how magnetic sensing of contractility can be employed for simultaneous assessment of multiple tissues subjected to different doses of known skeletal muscle inotropes as well as the stratification of healthy versus diseased functional profiles in normal and dystrophic muscle cells. Based on these data, this combined culture system and magnet-based contractility platform greatly broadens the potential for 3D engineered skeletal muscle tissues to impact the translation of novel therapies from the lab to the clinic.

Recent development of novel therapies has improved mobility and quality of life for people suffering from inheritable neuromuscular disorders. Despite this progress, the majority of neuromuscular disorders are still incurable, in part due to a lack of predictive models of neuromuscular junction (NMJ) breakdown. Improvement of predictive models of a human NMJ would be transformative in terms of expanding our understanding of the mechanisms that underpin development, maintenance, and disease, and as a testbed with which to evaluate novel therapeutics. Induced pluripotent stem cells (iPSCs) are emerging as a clinically relevant and non-invasive cell source to create human NMJs to study synaptic development and maturation, as well as disease modeling and drug discovery. This review will highlight the recent advances and remaining challenges to generating an NMJ capable of eliciting contraction of stem cell-derived skeletal muscle in vitro. We explore the advantages and shortcomings of traditional NMJ culturing platforms, as well as the pioneering technologies and novel, biomimetic culturing systems currently in use to guide development and maturation of the neuromuscular synapse and extracellular microenvironment. Then, we will explore how this NMJ-in-a-dish can be used to study normal assembly and function of the efferent portion of the neuromuscular arc, and how neuromuscular diseasecausing mutations disrupt structure, signaling, and function.

Severe injury to the central nervous system of chordates often results in permanent and irreversible mental and physical challenges. While some chordates are able to repair and/or regenerate portions of their nervous system, no chordate has been shown to be able to regenerate all regions of its central nervous system after catastrophic injury or amputation. Some hemichordates, on the other hand, are able to efficiently regenerate all neural structures, including their dorsal, hollow neural tube after complete ablation. Solitary hemichordates are marine acorn worms and a sister group to the echinoderms. The hemichordate Ptychodera flava progresses from a pelagic, feeding tornaria larva to a tripartite benthic worm with an anterior proboscis, a middle collar region, and a long posterior trunk. The adult worm regenerates all body parts when bisected in the trunk, but it was unknown whether the regeneration process was present in tornaria larvae. Now, we show that P. flava larvae are capable of robust regeneration after bisection through the sagittal, coronal, and axial planes. We also use antibody staining to show that the apical sensory organ regenerates a rich, serotonin-positive complex of cells within two weeks after amputation. Cells labeled with 5-ethynyl-2'-deoxyuridine confirm that regeneration is occurring through epimorphic processes as new cells are added at the cut site and throughout the regenerating tissue. This study verifies that P. flava larvae can be used for future functional studies aimed at identifying the genetic and morphological mechanisms controlling central nervous system regeneration in a stem deuterostome.