Progenitors of the protochordate ocellus as an evolutionary origin of the neural crest

Schwann cell precursors represent a neural crest-like state with biased multipotency

Schwann cell precursors (SCPs) are nerve-associated progenitors that can generate myelinating and non-myelinating Schwann cells but also are multipotent like the neural crest cells from which they originate. SCPs are omnipresent along outgrowing peripheral nerves throughout the body of vertebrate embryos. By using single-cell transcriptomics to generate a gene expression atlas of the entire neural crest lineage, we show that early SCPs and late migratory crest cells have similar transcriptional profiles characterised by a multipotent "hub" state containing cells biased towards traditional neural crest fates. SCPs keep diverging from the neural crest after being primed towards terminal Schwann cells and other fates, with different subtypes residing in distinct anatomical locations. Functional experiments using CRISPR-Cas9 loss-of-function further show that knockout of the common "hub" gene Sox8 causes defects in neural crest-derived cells along peripheral nerves by facilitating differentiation of SCPs towards sympathoadrenal fates. Finally, specific tumour populations found in melanoma, neurofibroma and neuroblastoma map to different stages of SCP/Schwann cell development. Overall, SCPs resemble migrating neural crest cells that maintain multipotency and become transcriptionally primed towards distinct lineages.

Riding the crest to get a head: neural crest evolution in vertebrates

In their seminal 1983 paper, Gans and Northcutt proposed that evolution of the vertebrate 'new head' was made possible by the advent of the neural crest and cranial placodes. The neural crest is a stem cell population that arises adjacent to the forming CNS and contributes to important cell types, including components of the peripheral nervous system and craniofacial skeleton and elements of the cardiovascular system. In the past few years, the new head hypothesis has been challenged by the discovery in invertebrate chordates of cells with some, but not all, characteristics of vertebrate neural crest cells. Here, we discuss recent findings regarding how neural crest cells may have evolved during the course of deuterostome evolution. The results suggest that there was progressive addition of cell types to the repertoire of neural crest derivatives throughout vertebrate evolution. Novel genomic tools have enabled higher resolution insight into neural crest evolution, from both a cellular and a gene regulatory perspective. Together, these data provide clues regarding the ancestral neural crest state and how the neural crest continues to evolve to contribute to the success of vertebrates as efficient predators.

Evolution of new cell types at the lateral neural border

During the course of evolution, animals have become increasingly complex by the addition of novel cell types and regulatory mechanisms. A prime example is represented by the lateral neural border, known as the neural plate border in vertebrates, a region of the developing ectoderm where presumptive neural and non-neural tissue meet. This region has been intensively studied as the source of two important embryonic cell types unique to vertebrates-the neural crest and the ectodermal placodes-which contribute to diverse differentiated cell types including the peripheral nervous system, pigment cells, bone, and cartilage. How did these multipotent progenitors originate in animal evolution? What triggered the elaboration of the border during the course of chordate evolution? How is the lateral neural border patterned in various bilaterians and what is its fate? Here, we review and compare the development and fate of the lateral neural border in vertebrates and invertebrates and we speculate about its evolutionary origin. Taken together, the data suggest that the lateral neural border existed in bilaterian ancestors prior to the origin of vertebrates and became a developmental source of exquisite evolutionary change that frequently enabled the acquisition of new cell types.

The gap junction protein connexin 43 controls multiple aspects of cranial neural crest cell development

Gap junctions are intercellular channels between cells that facilitate cell-cell communication. Connexin 43 (Cx43; also known as GJA1), the predominant gap junction protein in vertebrates, is expressed in premigratory cranial neural crest cells and is maintained throughout the neural crest cell epithelial-to-mesenchymal transition (EMT), but its function in these cells is unknown. To this end, we used a combination of in vivo and ex vivo experiments to assess gap junction formation, and Cx43 function, in chick cranial neural crest cells. Our results demonstrate that gap junctions exist between premigratory and migratory cranial neural crest cells and depend on Cx43 for their function. In the embryo, Cx43 knockdown just prior to EMT delays the emergence of Cx43-depleted neural crest cells from the neural tube, but these cells eventually successfully emigrate and join the migratory stream. This delay can be rescued by introduction of full-length Cx43 into Cx43-depleted cells. Furthermore, Cx43 depletion reduces the size of the premigratory neural crest cell domain through an early effect on neural crest cell specification. Collectively, these data identify new roles for Cx43 in chick cranial neural crest cell development.

Schwann cell precursor: a neural crest cell in disguise?

Schwann cell precursors (SCPs) are multipotent embryonic progenitors covering all developing peripheral nerves. These nerves grow and navigate with unprecedented precision, delivering SCP progenitors to almost all locations in the embryonic body. Within specific developing tissues, SCPs detach from nerves and generate neuroendocrine cells, autonomic neurons, mature Schwann cells, melanocytes and other cell types. These properties of SCPs evoke resemblances between them and their parental population, namely, neural crest cells. Neural crest cells are incredibly multipotent migratory cells that revolutionized the course of evolution in the lineage of early chordate animals. Given this similarity and recent data, it is possible to hypothesize that proto-neural crest cells are similar to SCPs spreading along the nerves. Here, we review the multipotency of SCPs, the signals that govern them, their potential therapeutic value, SCP's embryonic origin and their evolutionary connections. We dedicate this article to the memory of Wilhelm His, the father of the microtome and "Zwischenstrang", currently known as the neural crest.

Spatiotemporal expression pattern of Connexin 43 during early chick embryogenesis

During embryogenesis, a single cell develops into new tissues and organs that are made up of a number of different cell types. The assembly of the trigeminal ganglion (cranial nerve V), an important component of the peripheral nervous system, typifies this process. The trigeminal ganglia perform key sensory functions, including sensing pain and touch in the face, and arise from cells of two different progenitor populations, the neural crest and the cranial placodes. One question that remains poorly understood is how these two populations of cells interact with each other during development to form a functional ganglion. Gap junctions are intercellular channels that allow for the passage of small solutes between connected cells and could serve as one potential mechanism by which neural crest and placode cells communicate to create the trigeminal ganglia. To this end, we have generated a comprehensive spatiotemporal expression profile for the gap junction protein Connexin 43, a highly expressed member of the Connexin protein family during development. Our results reveal that Connexin 43 is expressed in the neural folds during neural fold fusion and in premigratory neural crest cells prior to the epithelial-to-mesenchymal transition (EMT), during EMT, and in migratory neural crest cells. During trigeminal gangliogenesis, Connexin 43 is expressed in cranial neural crest cells and the mesenchyme but is strikingly absent in the placode-derived neurons. These data underscore the complexity of bringing two distinct cell populations together to form a new tissue during development and suggest that Connexin 43 may play a key role within neural crest cells during EMT, migration, and trigeminal gangliogenesis.

Nerve-associated neural crest: peripheral glial cells generate multiple fates in the body

Recent studies demonstrated that neural crest-derived Schwann cell precursors (SCPs) dwelling in the nerves are multipotent and can be recruited in the local tissue to provide building blocks of neural crest-derived nature. The variety of fates produced by SCPs is widening with every year and currently includes melanocytes/melanophores, parasympathetic and enteric neurons, endoneural fibroblast, mesenchymal stem cells and, of course, mature Schwann cells of different subtypes. However, it is still unclear if SCPs are, in fact, nerve-dwelling population of the neural crest or they are rather a different, more specialized, cell type. This review outlines the field and focuses on the capacity of nerve-associated glial progenitors to contribute to the development and regeneration of numerous tissues in various groups of vertebrates.

Revised lineage of larval photoreceptor cells in Ciona reveals archetypal collaboration between neural tube and neural crest in sensory organ formation

The Ciona intestinalis larva has two distinct photoreceptor organs, a conventional pigmented ocellus and a nonpigmented ocellus, that are asymmetrically situated in the brain. The ciliary photoreceptor cells of these ocelli resemble visual cells of the vertebrate retina. Precise elucidation of the lineage of the photoreceptor cells will be key to understanding the developmental mechanisms of these cells as well as the evolutionary relationships between the photoreceptor organs of ascidians and vertebrates. Photoreceptor cells of the pigmented ocellus have been thought to develop from anterior animal (a-lineage) blastomeres, whereas the developmental origin of the nonpigmented ocellus has not been determined. Here, we show that the photoreceptor cells of both ocelli develop from the right anterior vegetal hemisphere: those of the pigmented ocellus from the right A9.14 cell and those of the nonpigmented ocellus from the right A9.16 cell. The pigmented ocellus is formed by a combination of two lineages of cells with distinct embryonic origins: the photoreceptor cells originate from a medial portion of the A-lineage neural plate, while the pigment cell originates from the lateral edge of the a-lineage neural plate. In light of the recently proposed close evolutionary relationship between the ocellus pigment cell of ascidians and the cephalic neural crest of vertebrates, the ascidian ocellus may represent a prototypic contribution of the neural crest to a cranial sensory organ.

The Nervous System Orchestrates and Integrates Craniofacial Development: A Review

Development of a head is a dazzlingly complex process: a number of distinct cellular sources including cranial ecto- and endoderm, mesoderm and neural crest contribute to facial and other structures. In the head, an extremely fine-tuned developmental coordination of CNS, peripheral neural components, sensory organs and a musculo-skeletal apparatus occurs, which provides protection and functional integration. The face can to a large extent be considered as an assembly of sensory systems encased and functionally fused with appendages represented by jaws. Here we review how the developing brain, neurogenic placodes and peripheral nerves influence the morphogenesis of surrounding tissues as a part of various general integrative processes in the head. The mechanisms of this impact, as we understand it now, span from the targeted release of the morphogens necessary for shaping to providing a niche for cellular sources required in later development. In this review we also discuss the most recent findings and ideas related to how peripheral nerves and nerve-associated cells contribute to craniofacial development, including teeth, during the post- neural crest period and potentially in regeneration.

Evolution of vertebrates as viewed from the crest

The origin of vertebrates was accompanied by the advent of a novel cell type: the neural crest. Emerging from the central nervous system, these cells migrate to diverse locations and differentiate into numerous derivatives. By coupling morphological and gene regulatory information from vertebrates and other chordates, we describe how addition of the neural-crest-specification program may have enabled cells at the neural plate border to acquire multipotency and migratory ability. Analysis of the topology of the neural crest gene regulatory network can serve as a useful template for understanding vertebrate evolution, including elaboration of neural crest derivatives.

Vertebrate cranial placodes as evolutionary innovations--the ancestor's tale

Evolutionary innovations often arise by tinkering with preexisting components building new regulatory networks by the rewiring of old parts. The cranial placodes of vertebrates, ectodermal thickenings that give rise to many of the cranial sense organs (ear, nose, lateral line) and ganglia, originated as such novel structures, when vertebrate ancestors elaborated their head in support of a more active and exploratory life style. This review addresses the question of how cranial placodes evolved by tinkering with ectodermal patterning mechanisms and sensory and neurosecretory cell types that have their own evolutionary history. With phylogenetic relationships among the major branches of metazoans now relatively well established, a comparative approach is used to infer, which structures evolved in which lineages and allows us to trace the origin of placodes and their components back from ancestor to ancestor. Some of the core networks of ectodermal patterning and sensory and neurosecretory differentiation were already established in the common ancestor of cnidarians and bilaterians and were greatly elaborated in the bilaterian ancestor (with BMP- and Wnt-dependent patterning of dorsoventral and anteroposterior ectoderm and multiple neurosecretory and sensory cell types). Rostral and caudal protoplacodal domains, giving rise to some neurosecretory and sensory cells, were then established in the ectoderm of the chordate and tunicate-vertebrate ancestor, respectively. However, proper cranial placodes as clusters of proliferating progenitors producing high-density arrays of neurosecretory and sensory cells only evolved and diversified in the ancestors of vertebrates.

The ascidian pigmented sensory organs: structures and developmental programs

The recent advances on ascidian pigment sensory organ development and function represent a fascinating platform to get insight on the basic programs of chordate eye formation. This review aims to summarize current knowledge, at the structural and molecular levels, on the two main building blocks of ascidian light sensory organ, i.e. pigment cells and photoreceptor cells. The unique features of these structures (e.g., simplicity and well characterized cell lineage) are indeed making it possible to dissect the developmental programs at single cell resolution and will soon provide a panel of molecular tools to be exploited for a deep developmental and comparative-evolutionary analysis.

Ephrin-mediated restriction of ERK1/2 activity delimits the number of pigment cells in the Ciona CNS

Recent evidence suggests that ascidian pigment cells are related to neural crest-derived melanocytes of vertebrates. Using live-imaging, we determine a revised cell lineage of the pigment cells in Ciona intestinalis embryos. The neural precursors undergo successive rounds of anterior-posterior (A-P) oriented cell divisions, starting at the blastula 64-cell stage. A previously unrecognized fourth A-P oriented cell division in the pigment cell lineage leads to the generation of the post-mitotic pigment cell precursors. We provide evidence that MEK/ERK signals are required for pigment cell specification until approximately 30min after the final cell division has taken place. Following each of the four A-P oriented cell divisions, ERK1/2 is differentially activated in the posterior sister cells, into which the pigment cell lineage segregates. Eph/ephrin signals are critical during the third A-P oriented cell division to spatially restrict ERK1/2 activation to the posterior daughter cell. Targeted inhibition of Eph/ephrin signals results in, at neurula stages, anterior expansion of both ERK1/2 activation and a pigment cell lineage marker and subsequently, at larval stages, supernumerary pigment cells. We discuss the implications of these findings with respect to the evolution of the vertebrate neural crest.

The lamprey: a jawless vertebrate model system for examining origin of the neural crest and other vertebrate traits

Lampreys are a group of jawless fishes that serve as an important point of comparison for studies of vertebrate evolution. Lampreys and hagfishes are agnathan fishes, the cyclostomes, which sit at a crucial phylogenetic position as the only living sister group of the jawed vertebrates. Comparisons between cyclostomes and jawed vertebrates can help identify shared derived (i.e. synapomorphic) traits that might have been inherited from ancestral early vertebrates, if unlikely to have arisen convergently by chance. One example of a uniquely vertebrate trait is the neural crest, an embryonic tissue that produces many cell types crucial to vertebrate features, such as the craniofacial skeleton, pigmentation of the skin, and much of the peripheral nervous system (Gans and Northcutt, 1983). Invertebrate chordates arguably lack unambiguous neural crest homologs, yet have cells with some similarities, making comparisons with lampreys and jawed vertebrates essential for inferring characteristics of development in early vertebrates, and how they may have evolved from nonvertebrate chordates. Here we review recent research on cyclostome neural crest development, including research on lamprey gene regulatory networks and differentiated neural crest fates.