Jan Stundl

Reproductive aging in vertebrates is commonly interpreted as a gradual decline; however, whether aging of the male gonad proceeds linearly or is characteri

The neural crest is an important stem cell population characterized by its multipotency, migratory behaviour and broad ability to differentiate into numerous derivatives throughout the vertebrate body, as diverse as cell types contributing to the cardiovascular system, craniofacial skeleton, peripheral nervous system and pigmentation of the skin. The developmental trajectory of the neural crest is governed by a complex gene regulatory network (GRN) that mediates induction and specification at the neural plate border, emergence of neural crest cells (NCCs) from the neural tube, their migration through the periphery and cell fate determination en route to different final destinations. In this Review, we discuss the significant progress in investigating the neural crest GRN, which has increased our understanding of how NCCs impact vertebrate development and evolution, their role in adult tissue regeneration and their contribution to diseases derived from abnormalities in NCCs.

Teeth are a key innovation that underpinned the adaptive radiation of jawed vertebrates; however, their evolutionary origin must lie with the diverse tooth‐like structures of living and fossil jawless vertebrates. Most previous studies have focussed on the extinct stem‐gnathostomes that phylogenetically intercalate the living jawed and jawless vertebrates. The only two extant jawless cyclostome lineages, the lampreys and hagfish bearing keratinous toothlets, have long been overlooked, though they possess complex (but unmineralised) toothlets that some have interpreted as precursors to the teeth of jawed vertebrates. Regardless of whether the toothlets of cyclostomes are homologous or convergent on the teeth of jawed vertebrates, they have the potential to offer unparalleled molecular developmental insights into the evolutionary origin of teeth. To that end, we provide a synthesis of classical literature on cyclostome toothlet structure and development, as a basis for informing future molecular studies, to which we add new insights from X‐ray microtomography of three parasitic lamprey species spanning the breadth of the lamprey crown group. Based on detailed morphological analysis we describe their toothlet replacement mechanism at tissue level and uncover a relationship between toothlet size and the number of replacement cones. All examined species reveal the presence of replacement toothlets, suggesting this replacement mode is a conserved feature of the lamprey crown group. We discuss these results in comparison to hagfish, and conclude that toothlet replacement is a symplesiomorphy of cyclostomes. By describing lamprey toothlet development and replacement and comparing it with gnathostome teeth, this study lays the foundation for research into the development and evolution of teeth and tooth‐like structures across vertebrate lineages.

Background: Dentitions have diversified enormously during vertebrate evolution, involving reductions, modifications, or allocations to prey seizing and processing regions. A combination of ancient and novel features related to dental and oropharyngeal apparatuses is found in extant lineages of non‐teleost fishes, such as the gars. While relevant to evolutionary‐developmental studies, gars have largely been overlooked regarding how their dentition arises, thus leaving our comprehension of the evolutionary history of vertebrate dentitions incomplete.Results: Here, we complement this knowledge gap by studying dental development in the tropical gar, Atractosteus tropicus. We follow ontogenetic changes from the initiation, tooth germ addition to the establishment of the larval replacing dentition. We pay attention to the progressive appearance of tooth fields, the emergence of dental patterns, the development of folded dentin morphology, and features related to tooth resorption and replacement. Furthermore, we identify snout elongation as the critical period when the general dentition layout becomes established.Conclusions: Our study depicts the gar oropharyngeal apparatus as a system that is established based on patterned initiation, differential growth, replacement, and complex shaping of teeth. These features form a reference standpoint for the likely developmental processes employed in dentitions of fossil stem and crown bony vertebrates, including ray‐finned fishes and tetrapods.

Summary: Neurofibromatosis Type 2 (NF-2) is a dominantly inherited genetic disorder that results from mutations in the tumor suppressor gene, neurofibromin 2 (NF2) gene. Here, we report the generation of a conditional zebrafish model of neurofibromatosis established by an inducible genetic knockout of nf2a/b , the zebrafish homolog of human NF2. Analysis of nf2a and nf2b expression reveals ubiquitous expression of nf2b in the early embryo, with overlapping expression in the neural crest and its derivatives and in the cranial mesenchyme. In contrast, nf2a displays lower expression levels. Induction of nf2a/b knockout at early stages increases the proliferation of larval Schwann cells and meningeal fibroblasts. Subsequently, in adult zebrafish, nf2a/b knockout triggers the development of a spectrum of tumors, including vestibular schwannomas, spinal schwannomas, meningiomas, and retinal hamartomas, mirroring the tumor manifestations observed in patients with NF-2. Collectively, these findings highlight the generation of a novel zebrafish model that mimics the complexities of the human NF-2 disorder. Consequently, this model holds significant potential for facilitating therapeutic screening and elucidating key driver genes implicated in NF-2 onset.

The neural crest is an embryonic stem cell population unique to vertebrates¹ whose expansion and diversification are thought to have promoted vertebrate evolution by enabling emergence of new cell types and structures such as jaws and peripheral ganglia². Although jawless vertebrates have sensory ganglia, convention has it that trunk sympathetic chain ganglia arose only in jawed vertebrates^3,4,5,6,7,8. Here, by contrast, we report the presence of trunk sympathetic neurons in the sea lamprey, Petromyzon marinus, an extant jawless vertebrate. These neurons arise from sympathoblasts near the dorsal aorta that undergo noradrenergic specification through a transcriptional program homologous to that described in gnathostomes. Lamprey sympathoblasts populate the extracardiac space and extend along the length of the trunk in bilateral streams, expressing the catecholamine biosynthetic pathway enzymes tyrosine hydroxylase and dopamine β-hydroxylase. CM-DiI lineage tracing analysis further confirmed that these cells derive from the trunk neural crest. RNA sequencing of isolated ammocoete trunk sympathoblasts revealed gene profiles characteristic of sympathetic neuron function. Our findings challenge the prevailing dogma that posits that sympathetic ganglia are a gnathostome innovation, instead suggesting that a late-developing rudimentary sympathetic nervous system may have been characteristic of the earliest vertebrates.

The vertebrate brain emerged more than ~500 million years ago in common evolutionary ancestors. To systematically trace its cellular and molecular origins, we established a spatially resolved cell type atlas of the entire brain of the sea lamprey—a jawless species whose phylogenetic position affords the reconstruction of ancestral vertebrate traits—based on extensive single-cell RNA-seq and in situ sequencing data. Comparisons of this atlas to neural data from the mouse and other jawed vertebrates unveiled various shared features that enabled the reconstruction of cell types, tissue structures and gene expression programs of the ancestral vertebrate brain. However, our analyses also revealed key tissues and cell types that arose later in evolution. For example, the ancestral brain was probably devoid of cerebellar cell types and oligodendrocytes (myelinating cells); our data suggest that the latter emerged from astrocyte-like evolutionary precursors in the jawed vertebrate lineage. Altogether, our work illuminates the cellular and molecular architecture of the ancestral vertebrate brain and provides a foundation for exploring its diversification during evolution.

The vertebrate pharynx is a key embryonic structure with crucial importance for the metameric organization of the head and face. The pharynx is primarily built upon progressive formation of paired pharyngeal pouches that typically develop in post-oral (mandibular, hyoid and branchial) domains. However, in the early embryos of non-teleost fishes, we have previously identified pharyngeal pouch-like outpocketings also in the pre-oral domain of the cranial endoderm. This pre-oral gut (POG) forms by early pouching of the primitive gut cavity, followed by the sequential formation of typical (post-oral) pharyngeal pouches. Here, we tested the pharyngeal nature of the POG by analysing expression patterns of selected core pharyngeal regulatory network genes in bichir and sturgeon embryos. Our comparison revealed generally shared expression patterns, including Shh, Pax9 , Tbx1, Eya1 , Six1 , Ripply3 or Fgf8, between early POG and post-oral pharyngeal pouches . POG thus shares pharyngeal pouch-like morphogenesis and a gene expression profile with pharyngeal pouches and can be regarded as a pre-mandibular pharyngeal pouch. We further suggest that pre-mandibular pharyngeal pouches represent a plesiomorphic vertebrate trait inherited from our ancestor's pharyngeal metameric organization, which is incorporated in the early formation of the pre-chordal plate of vertebrate embryos.

Bone is an evolutionary novelty of vertebrates, likely to have first emerged as part of ancestral dermal armor that consisted of osteogenic and odontogenic components. Whether these early vertebrate structures arose from mesoderm or neural crest cells has been a matter of considerable debate. To examine the developmental origin of the bony part of the dermal armor, we have performed in vivo lineage tracing in the sterlet sturgeon, a representative of nonteleost ray-finned fish that has retained an extensive postcranial dermal skeleton. The results definitively show that sterlet trunk neural crest cells give rise to osteoblasts of the scutes. Transcriptional profiling further reveals neural crest gene signature in sterlet scutes as well as bichir scales. Finally, histological and microCT analyses of ray-finned fish dermal armor show that their scales and scutes are formed by bone, dentin, and hypermineralized covering tissues, in various combinations, that resemble those of the first armored vertebrates. Taken together, our results support a primitive skeletogenic role for the neural crest along the entire body axis, that was later progressively restricted to the cranial region during vertebrate evolution. Thus, the neural crest was a crucial evolutionary innovation driving the origin and diversification of dermal armor along the entire body axis.