Subdivision of the lateral plate mesoderm and specification of the forelimb and hindlimb forming domains

Resolving the mechanisms underlying epithelial-to-mesenchymal transition of the lateral plate mesoderm

Formation of the vertebrate limb buds begins with a localized epithelial-to-mesenchymal transition (EMT) of the somatic lateral plate mesoderm (LPM). While the processes that drive proliferation and outgrowth of the limb mesenchyme are well established, the fundamental mechanisms that precede this process and initiate EMT are less understood. In this review, we outline putative drivers of EMT of the LPM, drawing from analyses across a range of vertebrates and developmental models. We detail the expression patterns of key EMT transcriptional regulators in the somatic LPM of the presumptive limb fields, and their potential role in producing a mesenchymal cell fate. These include a putative cooperative role between the EMT inducers PRRX1 and TWIST1, supported by evidence in zebrafish and chicken models but unconfirmed data from mice. As such, additional functional data are required to definitively determine the mechanisms that initiate and drive EMT of the somatic LPM, a critical transition preceding formation of the limb bud mesenchyme.

Lateral thinking in syndromic congenital cardiovascular disease

Syndromic birth defects are rare diseases that can present with seemingly pleiotropic comorbidities. Prime examples are rare congenital heart and cardiovascular anomalies that can be accompanied by forelimb defects, kidney disorders and more. Whether such multi-organ defects share a developmental link remains a key question with relevance to the diagnosis, therapeutic intervention and long-term care of affected patients. The heart, endothelial and blood lineages develop together from the lateral plate mesoderm (LPM), which also harbors the progenitor cells for limb connective tissue, kidneys, mesothelia and smooth muscle. This developmental plasticity of the LPM, which founds on multi-lineage progenitor cells and shared transcription factor expression across different descendant lineages, has the potential to explain the seemingly disparate syndromic defects in rare congenital diseases. Combining patient genome-sequencing data with model organism studies has already provided a wealth of insights into complex LPM-associated birth defects, such as heart-hand syndromes. Here, we summarize developmental and known disease-causing mechanisms in early LPM patterning, address how defects in these processes drive multi-organ comorbidities, and outline how several cardiovascular and hematopoietic birth defects with complex comorbidities may be LPM-associated diseases. We also discuss strategies to integrate patient sequencing, data-aggregating resources and model organism studies to mechanistically decode congenital defects, including potentially LPM-associated orphan diseases. Eventually, linking complex congenital phenotypes to a common LPM origin provides a framework to discover developmental mechanisms and to anticipate comorbidities in congenital diseases affecting the cardiovascular system and beyond.

Molecular Function and Contribution of TBX4 in Development and Disease

Over the past decade, recognition of the profound impact of the TBX4 (T-box 4) gene, which encodes a member of the evolutionarily conserved family of T-box-containing transcription factors, on respiratory diseases has emerged. The developmental importance of TBX4 is emphasized by the association of TBX4 variants with congenital disorders involving respiratory and skeletal structures; however, the exact role of TBX4 in human development remains incompletely understood. Here, we discuss the developmental, tissue-specific, and pathological TBX4 functions identified through human and animal studies and review the published TBX4 variants resulting in variable disease phenotypes. We also outline future research directions to fill the gaps in our understanding of TBX4 function and of how TBX4 disruption affects development.

Cell lineage specification and signalling pathway use during development of the lateral plate mesoderm and forelimb mesenchyme

The lateral plate mesoderm (LPM) is a transient tissue that produces a diverse range of differentiated structures, including the limbs. However, the molecular mechanisms that drive early LPM specification and development are poorly understood. In this study, we use single-cell transcriptomics to define the cell-fate decisions directing LPM specification, subdivision and early initiation of the forelimb mesenchyme in chicken embryos. We establish a transcriptional atlas and global cell-cell signalling interactions in progenitor, transitional and mature cell types throughout the developing forelimb field. During LPM subdivision, somatic and splanchnic LPM fate is achieved through activation of lineage-specific gene modules. During the earliest stages of limb initiation, we identify activation of TWIST1 in the somatic LPM as a putative driver of limb bud epithelial-to-mesenchymal transition. Furthermore, we define a new role for BMP signalling during early limb development, revealing that it is necessary for inducing a somatic LPM fate and initiation of limb outgrowth, potentially through activation of TBX5. Together, these findings provide new insights into the mechanisms underlying LPM development, somatic LPM fate choice and early initiation of the vertebrate limb.

Comparison of Tendon Development Versus Tendon Healing and Regeneration

Tendon is a vital connective tissue in human skeletal muscle system, and tendon injury is very common and intractable in clinic. Tendon development and repair are two closely related but still not fully understood processes. Tendon development involves multiple germ layer, as well as the regulation of diversity transcription factors (Scx et al.), proteins (Tnmd et al.) and signaling pathways (TGFβ et al.). The nature process of tendon repair is roughly divided in three stages, which are dominated by various cells and cell factors. This review will describe the whole process of tendon development and compare it with the process of tendon repair, focusing on the understanding and recent advances in the regulation of tendon development and repair. The study and comparison of tendon development and repair process can thus provide references and guidelines for treatment of tendon injuries.

Tbx4 function during hindlimb development reveals a mechanism that explains the origins of proximal limb defects

We dissect genetically a gene regulatory network that involves the transcription factors Tbx4, Pitx1 and Isl1 acting cooperatively to establish the hindlimb bud, and identify key differences in the pathways that initiate formation of the hindlimb and forelimb. Using live image analysis of murine limb mesenchyme cells undergoing chondrogenesis in micromass culture, we distinguish a series of changes in cellular behaviours and cohesiveness that are required for chondrogenic precursors to undergo differentiation. Furthermore, we provide evidence that the proximal hindlimb defects observed in Tbx4 mutant mice result from a failure in the early differentiation step of chondroprogenitors into chondrocytes, providing an explanation for the origins of proximally biased limb defects.

Evolution of the locomotor skeleton in Anolis lizards reflects the interplay between ecological opportunity and phylogenetic inertia

Anolis lizards originated in continental America but have colonized the Greater Antillean islands and recolonized the mainland, resulting in three major groups (Primary and Secondary Mainland and Greater Antillean). The adaptive radiation in the Greater Antilles has famously resulted in the repeated evolution of ecomorphs. Yet, it remains poorly understood to what extent this island radiation differs from diversification on the mainland. Here, we demonstrate that the evolutionary modularity between girdles and limbs is fundamentally different in the Greater Antillean and Primary Mainland Anolis. This is consistent with ecological opportunities on islands driving the adaptive radiation along distinct evolutionary trajectories. However, Greater Antillean Anolis share evolutionary modularity with the group that recolonized the mainland, demonstrating a persistent phylogenetic inertia. A comparison of these two groups support an increased morphological diversity and faster and more variable evolutionary rates on islands. These macroevolutionary trends of the locomotor skeleton in Anolis illustrate that ecological opportunities on islands can have lasting effects on morphological diversification.

Regulation of vertebrate forelimb development and wing reduction in the flightless emu

The vertebrate limb is a dynamic structure which has evolved into many diverse forms to facilitate complex behavioral adaptations. The principle molecular and cellular processes that underlie development of the vertebrate limb are well characterized. However, how these processes are altered to drive differential limb development between vertebrates is less well understood. Several vertebrate models are being utilized to determine the developmental basis of differential limb morphogenesis, though these typically focus on later patterning of the established limb bud and may not represent the complete developmental trajectory. Particularly, heterochronic limb development can occur prior to limb outgrowth and patterning but receives little attention. This review summarizes the genetic regulation of vertebrate forelimb diversity, with particular focus on wing reduction in the flightless emu as a model for examining limb heterochrony. These studies highlight that wing reduction is complex, with heterochronic cellular and genetic events influencing the major stages of limb development. Together, these studies provide a broader picture of how different limb morphologies may be established during development.

The lateral plate mesoderm

The lateral plate mesoderm (LPM) forms the progenitor cells that constitute the heart and cardiovascular system, blood, kidneys, smooth muscle lineage and limb skeleton in the developing vertebrate embryo. Despite this central role in development and evolution, the LPM remains challenging to study and to delineate, owing to its lineage complexity and lack of a concise genetic definition. Here, we outline the processes that govern LPM specification, organization, its cell fates and the inferred evolutionary trajectories of LPM-derived tissues. Finally, we discuss the development of seemingly disparate organ systems that share a common LPM origin.

Summary: The lateral plate mesoderm is the origin of several major cell types and organ systems in the vertebrate body plan. How this mesoderm territory emerges and partitions into its downstream fates provides clues about vertebrate development and evolution.

Body Cavity Development Is Guided by Morphogen Transfer between Germ Layers

The body cavity is a space where internal organs develop and are placed. Despite the importance of this internal space, how the body cavity forms specifically within the mesoderm remains largely unknown. Here, we report that upon the onset of dorsal mesodermal cell polarization and initial lumen formation, mesodermal cells form filamentous projections that are directed toward the ectoderm. This cell behavior enables the dorsal population of mesodermal cells to receive BMP7 that is expressed by the ectoderm. Suppression of ectodermal BMP7 diminishes mesodermal cell projection and results in the loss of body cavity development. The data reveal that body cavity induction depends on signaling factor transfer from ectoderm to mesoderm.

Methylome and transcriptome maps of human visceral and subcutaneous adipocytes reveal key epigenetic differences at developmental genes

Adipocytes support key metabolic and endocrine functions of adipose tissue. Lipid is stored in two major classes of depots, namely visceral adipose (VA) and subcutaneous adipose (SA) depots. Increased visceral adiposity is associated with adverse health outcomes, whereas the impact of SA tissue is relatively metabolically benign. The precise molecular features associated with the functional differences between the adipose depots are still not well understood. Here, we characterised transcriptomes and methylomes of isolated adipocytes from matched SA and VA tissues of individuals with normal BMI to identify epigenetic differences and their contribution to cell type and depot-specific function. We found that DNA methylomes were notably distinct between different adipocyte depots and were associated with differential gene expression within pathways fundamental to adipocyte function. Most striking differential methylation was found at transcription factor and developmental genes. Our findings highlight the importance of developmental origins in the function of different fat depots.

Timed Collinear Activation of Hox Genes during Gastrulation Controls the Avian Forelimb Position

Limb position along the body is highly consistent within one species but very variable among vertebrates. Despite major advances in our understanding of limb patterning in three dimensions, how limbs reproducibly form along the antero-posterior axis remains largely unknown. Hox genes have long been suspected to control limb position; however, supporting evidences are mostly correlative and their role in this process is unclear. Here, we show that limb position is determined early in development through the action of Hox genes. Dynamic lineage analysis revealed that, during gastrulation, the forelimb, interlimb, and hindlimb fields are progressively generated and concomitantly patterned by the collinear activation of Hox genes in a two-step process. First, the sequential activation of Hoxb genes controls the relative position of their own collinear domains of expression in the forming lateral plate mesoderm, as demonstrated by functional perturbations during gastrulation. Then, within these collinear domains, we show that Hoxb4 anteriorly and Hox9 genes posteriorly, respectively, activate and repress the expression of the forelimb initiation gene Tbx5 and instruct the definitive position of the forelimb. Furthermore, by comparing the dynamics of Hoxb genes activation during zebra finch, chicken, and ostrich gastrulation, we provide evidences that changes in the timing of collinear Hox gene activation might underlie natural variation in forelimb position between different birds. Altogether, our results that characterize the cellular and molecular mechanisms underlying the regulation and natural variation of forelimb positioning in avians show a direct and early role for Hox genes in this process.

Homeotic transformation of tails into limbs in anurans

Anuran tadpoles can regenerate their tails after amputation. However, they occasionally form ectopic limbs instead of the lost tail part after vitamin A treatment. This is regarded as an example of a homeotic transformation. In this phenomenon, the developmental fate of the tail blastema is apparently altered from that of a tail to that of limbs, indicating a realignment of positional information in the blastema. Morphological observations and analyses of the development of skeletal elements during the process suggest that positional information in the blastema is rewritten from tail to trunk specification under the influence of vitamin A, resulting in limb formation. Despite the extensive information gained from morphological observations, a comprehensive understanding of this phenomenon also requires molecular data. We review previous studies related to anuran homeotic transformation. The findings of these studies provide a basis for evaluating major hypotheses and identifying molecular data that should be prioritized in future studies. Finally, we argue that positional information for the tail blastema changes to that for a part of the trunk, leading to homeotic transformations. To suggest this hypothesis, we present published data that favor the rewriting of positional information.

The Palm-Heart Diameter: A Prospective Simple Screening Tool for Identifying Heart Enlargement

BACKGROUND:

Several speculations have linked the size of the fist to be equal to the size of the heart. However, the substantial scientific report still lacks to support this theory.

AIM:

This study aims to provide the validity of the fist-heart assumption by correlating the palm and heart diameters while benchmarking it as a reference tool for determining the normal heart size.

MATERIALS AND METHOD:

Volunteers from the public were recruited during a health fair organised by the school. A self-administered questionnaire for necessary information was distributed after the volunteers signed the consent forms. The palm of both hands was measured in duplicates using a flexible ruler. Ultrasound examination was used in measuring the diameter of the heart with the landmark being from the anterior fibrous pericardium to the lowest part of the posterior fibrous pericardium. The level of significance was kept at P < 0.05.

RESULTS:

A total of 275 people, consisting of 123 males and 152 females participated in the study. The age range was from 15 to 80 years with a mean age of 28.16 ± 16.18. The measurement showed that the size of both palms correlated with the heart diameter, p < 0.05. Other factors such as age and height showed a substantial level of correlation. However, this correlation ceased with older participants. Palm size did not correlate among participants with previously diagnosed prehypertension. However, participants with previously diagnosed hypertension with good medication compliance maintained the correlation.

CONCLUSION:

This study establishes the correlation between the palm and heart diameters. Since the heart tissue and the upper limb share a similar embryonic origin, being the mesoderm, this study prospects the fact that heart enlargement could be preliminarily identified by measuring the size of the hand.

The embryonic origins and genetic programming of emerging haematopoietic stem cells

Haematopoietic stem cells (HSCs) emerge from the haemogenic endothelium (HE) localised in the ventral wall of the embryonic dorsal aorta (DA). The HE generates HSCs through a process known as the endothelial to haematopoietic transition (EHT), which has been visualised in live embryos and is currently under intense study. However, EHT is the culmination of multiple programming events, which are as yet poorly understood, that take place before the specification of HE. A number of haematopoietic precursor cells have been described before the emergence of definitive HSCs, but only one haematovascular progenitor, the definitive haemangioblast (DH), gives rise to the DA, HE and HSCs. DHs emerge in the lateral plate mesoderm (LPM) and have a distinct origin and genetic programme compared to other, previously described haematovascular progenitors. Although DHs have so far only been established in Xenopus embryos, evidence for their existence in the LPM of mouse and chicken embryos is discussed here. We also review the current knowledge of the origins, lineage relationships, genetic programming and differentiation of the DHs that leads to the generation of HSCs. Importantly, we discuss the significance of the gene regulatory network (GRN) that controls the programming of DHs, a better understanding of which may aid in the establishment of protocols for the de novo generation of HSCs in vitro.