Background: Turtles hold a unique place in vertebrate evolutionary history, making them critical assets in embryology research. Yet, they remain understudied as potential model organisms in the field. Here, to support experimental manipulations with turtle embryos, we have created a complete normal table of development for comprehensive embryonic staging of Trachemys scripta, one of the most common invasive turtle species worldwide.
Results: The development of T. scripta embryos from 0 days post-oviposition (DPO) to hatching (~60 DPO) was described from approximately 300 viable eggs collected at California State University, Northridge during the 2021-2024 nesting seasons. Thirty-one stages between oviposition and hatching were identified, and anatomical structures were cataloged using the Standard Event System (SES) chart. Morphological characteristics were imaged using bright-field microscopy and, for 4',6-diamidino-2-phenylindole-stained embryos, confocal microscopy.
Conclusion: To facilitate further research with Chelonian embryos, this staging series blends previously accepted staging practices with new details of T. scripta gastrulation, SES criteria, and a photographic annotated glossary.
Background: Echinoderms are invertebrate deuterostomes closely related to chordates and have become a tractable model for the study of the evolution of mechanisms involved in development, primordial germ cell specification, and regeneration. Sea urchins rely on inherited mechanisms for germline formation while sea stars rely instead on cell-cell inductive signaling mechanisms.
Results: Here, we present a single-cell RNA sequencing of the sea star Patiria miniata development (days 1, 2, 3, and 4 after fertilization). This resource focuses mainly on the day3 larva, but also presents an integrated dataset of the 4 days combined. We identified each cell cluster of the larva using marker genes for in situ RNA hybridization and found that, surprisingly, the primordial germ cells share many gene expression profiles with cells in the coelomic pouches, and that the ectodermal epithelium is quite heterogeneous.
Conclusion: This dataset from the sea star provides a developmental trajectory of gene expression leading to each major cell type in the larva, providing a foundation for comparative analysis with other echinoderm species in parsing out mechanisms of developmental specification, regeneration, and germ line formation.
Background: Activity of the receptor tyrosine kinase PDGFRα and the tyrosine phosphatase SHP2 is critical for vertebrate craniofacial development. SHP2 has been shown to both positively and negatively regulate PDGFR signaling through the recruitment of Grb2 and dephosphorylation of the receptor, respectively. We sought to determine the effect of SHP2 binding to PDGFRα in the facial mesenchyme via phenotypic and biochemical analyses of an allelic series of mouse embryos with combined loss of both proteins in the neural crest lineage.
Results: We demonstrated that SHP2 preferentially binds PDGFRα/α homodimers among the three PDGFR dimers. We showed that double-homozygous mutant embryos exhibit a combination, but not an improvement or worsening, of the phenotypes observed upon conditional ablation of PDGFRα or SHP2 in the neural crest lineage. We further revealed that cell death in the lateral nasal and maxillary processes underlies the upper jaw phenotypes in embryos with loss of SHP2. Finally, we showed that E10.5 Pdgfra+/fl;Shp2fl/fl;Wnt1-Cre+/Tg embryos have increased phosphorylation of PDGFRα and the downstream effector Erk1/2 compared to control and double-heterozygous embryos.
Conclusions: We propose a putative model in which SHP2 binds and dephosphorylates PDGFRα while simultaneously increasing survival through an Erk1/2-independent mechanism.
Background: The lungs of squamate reptiles (lizards and snakes) are highly diverse, exhibiting single chambers, multiple chambers, transitional forms with two to three chambers, along with a suite of other anatomical features, including finger-like epithelial projections into the body cavity known as diverticulae. During embryonic development of the simple, sac-like lungs of anoles, the epithelium is pushed through the openings of a pulmonary smooth muscle mesh by the forces of luminal fluid pressure. This process of stress ball morphogenesis generates the faveolar epithelium typical of squamate lungs.
Results: Here, we compared embryonic lung development in brown anoles, leopard geckos, and veiled chameleons to determine if stress ball morphogenesis is conserved across squamates and to understand the physical processes that generate transitional-chambered lungs with diverticulae. We found that epithelial protrusion through the holes in a pulmonary smooth muscle mesh is conserved across squamates. Surprisingly, however, we found that luminal inflation is not conserved. Instead, experimental and computational evidence suggests that leopard geckos and veiled chameleons may generate their faveolae via epithelial folding downstream of epithelial proliferation. Our data also suggest that the transitional chambers and diverticulae of veiled chameleon lungs may develop via apical constriction, a process known to be crucial for airway branching in the bird lung.
Conclusions: Distinct morphogenetic mechanisms generate epithelial diversity in squamate lungs, which may underpin their species-specific physiological and ecological adaptations.
Neural crest cells are a transient cell population that emerges from the dorsal neural tube during neurulation and migrates extensively throughout the embryo. Among their diverse derivatives, glial cells (such as Schwann and satellite ganglionic cells) and melanocytes represent two major lineages. In vitro studies suggested they share a common progenitor yet follow distinct differentiation pathways. Hence, neural crest cells must choose between glia and melanocyte fates-a decision crucial for forming the peripheral nervous and pigmentary systems. A groundbreaking discovery revealed that Schwann cell precursors along peripheral nerves serve as a secondary source of melanocytes during development. This finding challenged the traditional view that melanocytes arise exclusively from direct neural crest migration and demonstrated remarkable plasticity in the glial lineage. This glia/melanocyte fate choice represents a well-characterized example of binary fate decisions in vertebrate development, involving complex interactions between transcriptional networks, signaling pathways, and environmental cues. Importantly, the glia/melanocyte decision has implications for cancer and injury-induced plasticity, where embryonic pathways may be reactivated. For example, during melanomagenesis, cells can exhibit both melanocytic and glial features. Understanding how neural crest cells decide between glial and melanocyte fates may offer new insights for regenerative medicine and cancer therapy.
Background: In vertebrate embryogenesis, cranial neural crest cells (CNCCs) migrate along discrete pathways. Analyses in the chick have identified key molecular candidates for the confinement of CNCC migration to stereotypical pathways as Colec12, Trail, and Dan. The effects of these factors on CNCCs in vitro are known, but how they confine migration to discrete streams in vivo remains poorly understood. Here, we propose and test several hypothetical mechanisms by which these factors confine cell streams and maintain coherent migration, simulating an expanded agent-based model for collective CNCC migration.
Results: Model simulations suggest that Trail enhances adhesion between CNCCs, facilitating movement towards stereotypical migratory pathways, whereas Colec12 confines CNCCs by inducing longer, branched filopodia that facilitate movement down Colec12 gradients and re-connections with streams. Moreover, we find that Trail and Colec12 facilitate the exchange of CNCCs and the formation of CNCC bridges between adjacent streams that are observed in vivo but poorly understood mechanistically. Finally, we predict that Dan increases the coherence of streams by modulating the speed of CNCCs at the leading edge of collectives to prevent escape.
Conclusions: Our work highlights the importance of Trail, Colec12, and Dan in CNCC migration and predicts novel mechanisms for the confinement of CNCCs to stereotypical pathways in vivo.
Background: Knockin mouse models expressing calbindin (Calb1), calretinin (Calb2), and peripherin (Prph) exhibit changes in hair cells (HCs), spiral ganglion neurons (SGN), vestibular ganglion neurons (VGNs), and their central projections.
Results: Developing cristae HCs show strong Calb1-positive expression, but adult HCs are mainly Calb2-positive. Utricle and saccule initially have Calb2-positive HCs and later develop Calb1-positive HCs in the striola region. Inner hair cells (IHCs) and outer hair cells (OHCs) in the cochlea express Calb2 early on. Calb1 expression in OHCs overlaps with Calb2; the expression of Myo7a, Calb1, and Calb2 reaches the apex later. SGNs and VGNs exhibit distinct Calb1 and Calb2 patterns but include a subpopulation with mixed expression. Central fibers are Calb1- and Calb2-positive early in the developing cochlear nuclei (CN) and vestibular nuclei (VN) but remain highly Prph-positive. VGNs innervate the lateral and VN, which are positive for Calb2 and Prph. Distinct Calb1-positive neurons overlap with the anterior (A) and ventral (V) cochlear nuclei (AVCN, PVCN) with Calb2, while the dorsal cochlear nucleus (DCN) shows segregation of Calb2 and Calb1.
Conclusion: We offer insights into the timing of how neuronal identity and connectivity are regulated in the auditory and vestibular systems, as shown by the expression of Calb1, Calb2, and Prph.
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