Journal: Developmental biology
In vitro human pluripotent stem cell (hPSC) derived tissues are excellent models to study certain aspects of normal human development. Current research in the field of hPSC derived tissues reveals these models to be inherently fetal-like on both a morphological and gene expression level. In this review we briefly discuss current methods for differentiating lung and intestinal tissue from hPSCs into individual 3-dimensional units called organoids. We discuss how these methods mirror what is known about in vivo signaling pathways of the developing embryo. Additionally, we will review how the inherent immaturity of these models lends them to be particularly valuable in the study of immature human tissues in the clinical setting of premature birth. Human lung organoids (HLOs) and human intestinal organoids (HIOs) not only model normal development, but can also be utilized to study several important diseases of prematurity such as respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD), and necrotizing enterocolitis (NEC).
The evolution of oral teeth is considered a major contributor to the overall success of jawed vertebrates. This is especially apparent in cartilaginous fishes including sharks and rays, which develop elaborate arrays of highly specialized teeth, organized in rows and retain the capacity for life-long regeneration. Perpetual regeneration of oral teeth has been either lost or highly reduced in many other lineages including important developmental model species, so cartilaginous fishes are uniquely suited for deep comparative analyses of tooth development and regeneration. Additionally, sharks and rays can offer crucial insights into the characters of the dentition in the ancestor of all jawed vertebrates. Despite this, tooth development and regeneration in chondrichthyans is poorly understood and remains virtually uncharacterized from a developmental genetic standpoint. Using the emerging chondrichthyan model, the catshark (Scyliorhinus spp.), we characterized the expression of genes homologous to those known to be expressed during stages of early dental competence, tooth initiation, morphogenesis, and regeneration in bony vertebrates. We have found that expression patterns of several genes from Hh, Wnt/β-catenin, Bmp and Fgf signalling pathways indicate deep conservation over ~450 million years of tooth development and regeneration. We describe how these genes participate in the initial emergence of the shark dentition and how they are re-deployed during regeneration of successive tooth generations. We suggest that at the dawn of the vertebrate lineage, teeth (i) were most likely continuously regenerative structures, and (ii) utilised a core set of genes from members of key developmental signalling pathways that were instrumental in creating a dental legacy redeployed throughout vertebrate evolution. These data lay the foundation for further experimental investigations utilizing the unique regenerative capacity of chondrichthyan models to answer evolutionary, developmental, and regenerative biological questions that are impossible to explore in classical models.
Preplacodal ectoderm (PPE) and neural crest (NC) are specified at the interface of neural and nonneural ectoderm and together contribute to the peripheral nervous system in all vertebrates. Bmp activates early steps for both fates during late blastula stage. Low Bmp activates expression of transcription factors Tfap2a and Tfap2c in the lateral neural plate, thereby specifying neural crest fate. Elevated Bmp establishes preplacodal competence throughout the ventral ectoderm by coinducing Tfap2a, Tfap2c, Foxi1 and Gata3. PPE specification occurs later at the end of gastrulation and requires complete attenuation of Bmp, yet expression of PPE competence factors continues well past gastrulation. Here we show that competence factors positively regulate each other’s expression during gastrulation, forming a self-sustaining network that operates independently of Bmp. Misexpression of Tfap2a in embryos blocked for Bmp from late blastula stage can restore development of both PPE and NC. However, Tfap2a alone is not sufficient to activate any other competence factors nor does it rescue individual placodes. On the other hand, misexpression of any two competence factors in Bmp-blocked embryos can activate the entire transcription factor network and support the development of NC, PPE and some individual placodes. We also show that while these factors are partially redundant with respect to PPE specification, they later provide non-redundant functions needed for development of specific placodes. Thus, we have identified a gene regulatory network that coordinates development of NC, PPE and individual placodes in zebrafish.
During embryogenesis, chordates pass through a tailbud stage in which the larval tail is formed. Since acquisition of a tadpole-like tail during tailbud stage is one of the key events in the evolution of chordates, understanding the anatomy of the tailbud stage chordate embryo is of special interest. In this study, to understand comprehensively the anatomy of the tailbud embryo at single-cell-level, real microscopic image stacks of the tailbud embryo in Ciona intestinalis were reconstructed into a 3D computer model. This comprehensive 3D model of the ascidian tailbud embryo was based on real images of confocal laser scanning microscope (CLSM) and therefore, cell shape, location and cell arrangement reflect real geometries of the tailbud embryo. We found that the tailbud embryo consists of 1579 cells, including 836 epidermal cells, 228 cells in the central nervous system, 218 mesenchymal cells, four trunk ventral cells, two B/B(⁎)8.11 cells, 36 muscle cells, 40 notochord cells, four primordial germ cells, and 199 endodermal cells. Moreover, we identified for the first time two populations of previously undefined cells (a total of 12 cells) in Ciona: one located in the lateral trunk and the other located under the tail dorsal epidermis. This information provides a first step for understanding how the body plan of the chordate tailbud embryo formed and evolved.
The hair follicle contains stem/progenitor cells that supply progeny for skin development and the hair cycle. Several signaling molecules belonging to the Wnt, BMP, shh, and transforming growth factor β (TGF-β) signaling cascades are involved in the normal hair follicle cycle. However, the systemic mechanism of how these humoral factors are controlled remains largely unknown. Previously, we reported that Tsukushi (TSK), a member of the small leucine-rich repeat proteoglycan family, functions extracellularly as a key coordinator of multiple signaling networks. Here, we show that TSK is expressed at the restricted areas of hair follicle during the morphogenesis and the hair cycle. Targeted disruption of the TSK gene causes the hair cycle to be delayed with low levels of TGF-β1 and phosphorylated Smad2/3 (pSmad2/3) expression. Biochemical analysis indicates that TSK directly binds to TGF-β1. Our data suggest that TSK controls the hair cycle by regulating TGF-β1 signaling.
We describe the internal organization of murine embryoid bodies (EBs) in terms of the structures and cell types formed as Oct4 expression becomes progressively lost. This is done by making the EBs from iPS cells carrying a novel Oct4 reporter (Oct4-MerCreMer;mTmG) which is inducible, sensitive, and permanent in all cellular progeny. When these EBs are treated with tamoxifen, the Oct4 expressing cells switch from a red to a green fluorescence color, and this is maintained thereafter by all their progeny. We show that there is no specific pattern in which Oct4 is downregulated, rather it appears to be spatially random. Many of the earliest cells to lose Oct4 expression stain positive for markers of visceral endoderm (DAB2, α-fetoprotein (AFP), HNF4). These are randomly located, although if endoderm differentiation is allowed to commence before EB formation then an external layer is formed. This is true both of EBs made from the reporter iPS cells, or from an embryo-derived mouse ES line (R1 cells). Markers of the early body axis, Brachyury (BRA) and FOXA2, usually showed a concentration of positive cells in one region of the EB, but the morphology is not predictable and there are also scattered cells expressing these markers. These patterns are similar in R1 cells. Use of the Oct4 reporter showed a difference between BRA and FOXA2. BRA, which marks the early mesoderm, node and notochord, arises in Oct4 expressing cells on days 3-4. FOXA2, which marks the floor plate of the neural tube and definitive endoderm, as well as the node and notochord, arises at the same time but mostly in cells that have already lost Oct4 expression. Several clumps of cardiomyocytes are visible by days 7-8 of EB development, both in our iPS cells and in R1 cells. Using the Oct4 reporter we show that the cells forming these clumps lose Oct4 expression between days 3 and 5. Overall, our results indicate that EBs recapitulate normal development quite well in terms of the tempo of events and the appearance of specific markers, but they do not resemble embryos in terms of their morphology.
Insect embryo segmentation is largely divided into long and short germ types. In the long germ type, each segment primordium is represented on a large embryonic rudiment of the blastoderm, and segmental patterning occurs nearly simultaneously in the syncytium. In the short germ type, however, only anterior segments are represented in the small embryonic rudiment, usually located on the egg posterior, and the rest of the segments are added sequentially from the posterior growth zone in a cellular context. The long germ type is thought to have evolved from the short germ type. It is proposed that this transition, which appears to have occurred multiple times over the course of evolution, was realized through the acquisition of a localized anterior instruction center. Here, I examined the early segmentation process in the silkmoth Bombyx mori, a lepidopteran insect, in which the mechanisms of anterior-posterior (AP) axis formation have not been well analyzed. In this insect, both the long germ and short germ features have been reported. The mRNAs for two key genes involved in insect AP axis formation, orthodenticle (Bm-otd) and caudal (Bm-cad), are localized maternally in the germ anlage, where they act as anterior and posterior instruction centers, respectively. RNAi studies indicate that, while Bm-cad affects the formation of all the even skipped (Bm-eve) stripes, there is also anterior Bm-eve stripe formation activity that involves Bm-otd. Thus, there is redundancy in Bm-eve stripe formation activity that must be coordinated. Some genetic interactions, identified either experimentally or hypothetically, are also introduced, which might enable robust AP formation in this organism.
The epiblast (EPI) and the primitive endoderm (PE), which constitute foundations for the future embryo body and yolk sac, build respectively deep and surface layers of the inner cell mass (ICM) of the blastocyst. Before reaching their target localization within the ICM, the PE and EPI precursor cells, which display distinct lineage-specific markers, are intermingled randomly. Since the ICM cells are produced in two successive rounds of asymmetric divisions at the 8→16 (primary inner cells) and 16→32 cell stage (secondary inner cells) it has been suggested that the fate of inner cells (decision to become EPI or PE) may depend on the time of their origin. Our method of dual labeling of embryos allowed us to distinguish between primary and secondary inner cells contributing ultimately to ICM. Our results show that the presence of two generations of inner cells in the 32-cell stage embryo is the source of heterogeneity within the ICM. We found some bias concerning the level of Fgf4 and Fgfr2 expression between primary and secondary inner cells, resulting from the distinct number of cells expressing these genes. Analysis of experimental aggregates constructed using different ratios of inner cells surrounded by outer cells revealed that the fate of cells does not depend exclusively on the timing of their generation, but also on the number of cells generated in each wave of asymmetric division. Taking together, the observed regulatory mechanism adjusting the proportion of outer to inner cells within the embryo may be mediated by FGF signaling.
Centralspindlin, a complex composed of the subunits ZEN-4 and CYK-4, recruits and regulates proteins that modulate the actin cytoskeleton to promote cleavage furrow formation and progression during cytokinesis. The ZEN-4 subunit is a kinesin that is proposed to function primarily by bundling microtubules and promoting transport of the complex to the midzone. ZEN-4 and CYK-4 are mutually dependent for localization to the midzone during cytokinesis. Once at the midzone, the CYK-4 subunit functions to recruit actin regulators and the scaffold anillin as well as to regulate RhoA and Rac via its intrinsic GAP domain, ultimately promoting actomyosin contractile ring assembly. We have revealed a distinct mechanism for centralspindlin localization and function at a stable, postmitotic intercellular bridge in the Caenorhabditis elegans gonad. Loss of zen-4 or cyk-4 function disrupts germ cell progression postmitotically. In contrast to the localization and recruitment relationships during mitosis, centralspindlin is maintained at the intercellular bridge by anillin, and CYK-4 is localized independently of ZEN-4 but not vice versa. We present evidence that centralspindlin function at the rachis bridge involves ZEN-4 action on the microtubules as opposed to the regulation of the actin cytoskeleton mediated by CYK-4 during cytokinesis.
MicroRNAs (miRNAs) are known to play diverse roles in the regulation of vertebrate development. To investigate miRNA-target mRNA relationships in embryonic development, we have carried out small-RNA sequencing to identify miRNAs expressed in the early gastrula of Xenopus laevis. We identify a total of 180 miRNAs, and we have identified the locations of the miRNA precursor sequences in the X. laevis genome. Of these miRNAs, 141 represent miRs previously identified in X. tropicalis. Alignment to human miRNAs led to the identification of 39 miRNAs that have not previously been described for Xenopus. We have also used a biochemical approach to isolate mRNAs that are associated with the RNA-Induced Silencing Complex (RISC) in early gastrulae and thus candidate targets of miRNA-dependent regulation. Interrogation of this RISC-associated mRNA pool by RT-PCR indicates that a number of genes essential for early patterning and specification may be under regulation by miRNAs. Smad1 transcripts are associated with the RISC; target prediction algorithms identify a single miRNA-binding site for miR-26, which is common to the 3'UTRs of Smad1a and Smad1b. Disruption of the interaction between miR-26 and the Smad1 3'UTR via a Target Protector Morpholino Oligonucleotide (TPMO) leads to a 2-fold increase in Smad1 protein accumulation, moderate increases in the expression of BMP4/Smad1 target genes, and a reduction in organizer gene expression, as well as a partially ventralized phenotype in approximately 25% of embryos. Overexpression of miR-26 resulted in moderately decreased expression of Smad1-dependent genes and an expansion of the region expressing the Organizer gene not1. Our findings indicate that interactions between miR-26 and the Smad1 3'UTR modulate Smad1 function in the establishment of axial patterning; they also establish a foundation for the functional analysis of miRNAs and their regulatory interactions during gastrulation.