The developmental potentials of the caudalmost part of the neural crest are restricted to melanocytes and glia

Distal spinal nerve development and divergence of avian groups

The avian transition from long to short, distally fused tails during the Mesozoic ushered in the Pygostylian group, which includes modern birds. The avian tail embodies a bipartite anatomy, with the proximal separate caudal vertebrae region, and the distal pygostyle, formed by vertebral fusion. This study investigates developmental features of the two tail domains in different bird groups, and analyzes them in reference to evolutionary origins. We first defined the early developmental boundary between the two tail halves in the chicken, then followed major developmental structures from early embryo to post-hatching stages. Differences between regions were observed in sclerotome anterior/posterior polarity and peripheral nervous system development, and these were consistent in other neognathous birds. However, in the paleognathous emu, the neognathous pattern was not observed, such that spinal nerve development extends through the pygostyle region. Disparities between the neognaths and paleognaths studied were also reflected in the morphology of their pygostyles. The ancestral long-tailed spinal nerve configuration was hypothesized from brown anole and alligator, which unexpectedly more resembles the neognathous birds. This study shows that tail anatomy is not universal in avians, and suggests several possible scenarios regarding bird evolution, including an independent paleognathous long-tailed ancestor.

Deciphering the Neural Crest Contribution to Cephalic Development with Avian Embryos

For decades, the quail-chick system has been a gold standard approach to track cells and their progenies over complex morphogenetic movements and long-range migrations as well as to unravel their dialogue and interplays in varied processes of cell induction. More specifically, this model became decisive for the systematic explorations of the neural crest and its lineages and allowed a tremendous stride in understanding the wealth and complexity of this fascinating cell population. Much of our knowledge on craniofacial morphogenesis and vertebrate organogenesis was first gained in avian chimeras and later extended to mammalian models and humans. In addition, this system permits tissue and gene manipulations to be performed at once in the same cell population. Through the use of in ovo electroporation, this model became tractable for functional genomics, hence being even more resourceful for functional studies. Due to the ease of access and the possibility to combine micromanipulation of tissue anlagen and gene expression, this model offers the prospect of decrypting instructive versus permissive tissue interactions, to identify and crack the molecular codes underlying cell positioning and differentiation, with an unparalleled spatiotemporal accuracy.

Secondary neurulation of human embryos: morphological changes and the expression of neuronal antigens

PURPOSE

The morphological changes and expression patterns of neuronal antigens of human embryos, obtained from the therapeutic termination of pregnancy or from surgical procedures, were analyzed in order to characterize the secondary neurulation.

METHODS

A total of 21 human embryos from Carnegie stages 12 to 23 and two fetuses in early stages were studied. The markers used for immunohistochemical study were neural cell adhesion molecule (N-CAM), neuronal nuclear antigen (NeuN), neurofilament-associated protein (3A10), synaptophysin, and glial fibrillary acidic protein (GFAP).

RESULTS

The formation of the caudal neural tube to the tip of the caudal portion of the embryo was finished at stage 17. The postcloacal gut had completely disappeared at stage 18, and multiple cavities of the caudal neural tube were clearly visible. The caudal portion of the neural tube showed findings suggestive of involution at stage 19. The expression patterns of neuronal antigens were as follows: N-CAM and NeuN showed immunoreactivity at the germinal layer of the spinal cord at stages 17 and 18. Neurofilament-associated protein (3A10) showed persistent immunoreactivity at the caudal cell mass and notochord during the observation period, along with the spinal cord, and the positive reactions were mainly located at the dorsal white matter at stage 17. Synaptophysin showed a weak positive reaction at the caudal cell mass and notochord at stages 13 and 14, evident by staining observed at the spinal cord at stages 15 and 16. There was no definite positive reaction for GFAP.

CONCLUSIONS

These characteristic patterns might be helpful for the understanding of human congenital anomalies involving secondary neurulation processes.

A novel role for FOXA2 and SHH in organizing midbrain signaling centers

The floor plate (FP) is a midline signaling center, known to direct ventral cell fates and axon guidance in the neural tube. The recent identification of midbrain FP as a source of dopaminergic neurons has renewed interest in its specification and organization, which remain poorly understood. In this study, we have examined the chick midbrain and spinal FP and show that both can be partitioned into medial (MFP) and lateral (LFP) subdivisions. Although Hedgehog (HH) signaling is necessary and sufficient for LFP specification, it is not sufficient for MFP induction. By contrast, the transcription factor FOXA2 can execute the full midbrain and spinal cord FP program via HH-independent and dependent mechanisms. Interestingly, although HH-independent FOXA2 activity is necessary and sufficient for inducing MFP-specific gene expression (e.g., LMX1B, BMP7), it cannot confer ventral identity to midline cells without also turning on Sonic hedgehog (SHH). We also note that the signaling centers of the midbrain, the FP, roof plate (RP) and the midbrain-hindbrain boundary (MHB) are physically contiguous, with each expressing LMX1B and BMP7. Possibly as a result, SHH or FOXA2 misexpression can transform the MHB into FP and also suppress RP induction. Conversely, HH or FOXA2 knockdown expands the endogenous RP and transforms the MFP into a RP and/or MHB fate. Finally, combined HH blockade and FOXA2 misexpression in ventral midbrain induces LMX1B expression, which triggers the specification of the RP, rather than the MFP. Thus we identify HH-independent and dependent roles for FOXA2 in specifying the FP. In addition, we elucidate for the first time, a novel role for SHH in determining whether a midbrain signaling center will become the FP, MHB or RP.

A tail of sacral agenesis: delayed presentation of meningocele in sacral agenesis

INTRODUCTION

Sacral agenesis is a congenital condition associated with multiple orthopedic, spinal, abdominal and thoracic organ deformities. Meningocele is commonly found among patients with sacral agenesis.

DESCRIPTION

We present the first case in the literature describing a delayed presentation of terminal (posterior) meningocele in an adult patient born with sacral agenesis.

CONCLUSION

Surgical repair was performed and is the best treatment option for significantly large lesions, with postoperative CSF leak being the main complication.

The melanomas: a synthesis of epidemiological, clinical, histopathological, genetic, and biological aspects, supporting distinct subtypes, causal pathways, and cells of origin

Converging lines of evidence from varied scientific disciplines suggest that cutaneous melanomas comprise biologically distinct subtypes that arise through multiple causal pathways. Understanding the respective relationships of each subtype with etiologic factors such as UV radiation and constitutional factors is the first necessary step toward developing refined prevention strategies for the specific forms of melanoma. Furthermore, classifying this disease precisely into biologically distinct subtypes is the key to developing mechanism-based treatments, as highlighted by recent discoveries. In this review, we outline the historical developments that underpin our understanding of melanoma heterogeneity, and we do this from the perspectives of clinical presentation, histopathology, epidemiology, molecular genetics, and developmental biology. We integrate the evidence from these separate trajectories to catalog the emerging major categories of melanomas and conclude with important unanswered questions relating to the development of melanoma and its cells of origin.

Lack of motor neuron differentiation is an intrinsic property of the mouse secondary neural tube

The cranial part of the amniote neural tube is formed by folding and fusion of the ectoderm-derived neural plate (primary neurulation). After posterior neuropore closure, however, the caudal neural tube is formed by cavitation of tail bud mesenchyme (secondary neurulation). In mouse embryos, the secondary neural tube expresses several genes important in early patterning and induction, in restricted domains similar to the primary neural tube, yet it does not undergo neuronal differentiation, but subsequently degenerates. Although the secondary neural tube, isolated from surrounding tissues, is responsive to exogenous Sonic Hedgehog proteins in vitro, motor neuron differentiation is never observed. This cannot be attributed to the properties of the secondary notochord, since it is able to induce motor neuron differentiation in naive chick neural plate explants. Taken together, these results support that the lack of motor neuron differentiation is an intrinsic property of the mouse secondary neural tube.

Role of noggin as an upstream signal in the lack of neuronal derivatives found in the avian caudal-most neural crest

Neural crest cells (NCCs) arising from trunk neural tube (NT) during primary and secondary neurulation give rise to melanocytes, glia and neurons, except for those in the caudal-most region during secondary neurulation (somites 47 to 53 in the chick embryo), from which no neurons are formed, either in vivo or in vitro. To elucidate this discrepancy, we have specifically analyzed caudal-most NCC ontogeny. In this region, NCCs emerge at E5/HH26, one day after full cavitation of the NT and differentiation of flanking somites. The absence of neurons does not seem to result from a defect in NCC specification as all the usual markers, with the exception of Msx1, are expressed in the dorsal caudal-most NT as early as E4/HH24. However, Bmp4-Wnt1 signaling, which triggers trunk NCC delamination, is impaired in this region due to persistence of noggin (Nog) expression. Concomitantly, a spectacular pattern of apoptosis occurs in the NT dorsal moiety. Rostral transplantation of either the caudal-most somites or caudal-most NT reveals that the observed features of caudal-most NCCs relate to properties intrinsic to these cells. Furthermore, by forced Nog expression in the trunk NT, we can reproduce most of these particular features. Conversely, increased Bmp4-Wnt1 signaling through Nog inhibition in the caudal-most NT at E4/HH24 induces proneurogenic markers in migratory NCCs, suggesting that noggin plays a role in the lack of neurogenic potential characterizing the caudal-most NCCs.

Developmental basis of vascular smooth muscle diversity

The origins of vascular smooth muscle are far more diverse than previously thought. Lineage mapping studies show that the segmental organization of early vertebrate embryos leaves footprints on the adult vascular system in the form of a mosaic pattern of different smooth muscle types. Moreover, evolutionarily conserved tissue forming pathways produce vascular smooth muscle from a variety of unanticipated sources. A closer look at the diversity of smooth muscle origins in vascular development provides new perspectives about how blood vessels differ from one another and why they respond in disparate ways to common risk factors associated with vascular disease. The origins of vascular smooth muscle are far more diverse than previously thought. A closer look at the diversity of smooth muscle origins in vascular development provides new perspectives about how blood vessels differ from one another and why they respond in disparate ways to common risk factors associated with vascular disease.

Sonic Hedgehog and Retinoic Acid are not sufficient to induce motoneuron generation in the avian caudal neural tube

The caudal neural tube (CNT) of the avian embryo is devoid of both dorsal and ventral roots. We show that the lack of ventral roots in the CNT, from somite 48 caudalwards, is due to an absence of post-mitotic motoneurons (MNs). The absence of MNs is not due to a defective notochordal induction since Sonic Hedgehog (SHH) signaling is intact and the caudal notochord is able to induce ectopic MNs when grafted laterally to a host neural tube. The transcription factors involved in MN specification (Pax6, Nkx6.1, and Olig2) are all expressed in the CNT, despite the lower expression level of Pax6, but an overlap between Olig2 and the ventrally expressed transcription factor Nkx2.2 is observed in the CNT. Grafting a quail CNT into the cervical level of a chick host rescues MN generation, demonstrating both the CNT potential for MN generation and the key role of the caudal environment in the MN differentiation blockade. The transplantation of the CNT-flanking somites into the cervical level does not inhibit MN generation. Furthermore, implantation of a retinoic-acid-soaked bead laterally to the CNT does not rescue MN generation. Together, these data indicate that the rostral environment contains a signal different from both SHH and Retinoic Acid that acts on MN differentiation.

The caudal end of the rat spinal cord: transformation to and ultrastructure of the filum terminale

Contrary to the current belief, the spinal cord of the rat does not terminate with the conus terminalis (CT), but its basic components (central canal, gray matter, white matter) continue in the filum terminale (FT). Proceeding caudally in the conus terminalis, first the motoneuron cell column discontinues in the ventral horn. More caudally the dorsal horns separate from the intermediate zone, and discontinue. The ensuing filum terminale consists of the slit-like central canal lined by ciliated ependymal cells, the periventricular gray matter and the peripheral white matter. Uniform small size neurons and glial cells populate the gray matter. Ultrastructural analysis revealed various types of axodendritic and axosomatic synapses as well as fine unmyelinated axons. The white matter consists mainly of myelinated nerve fibers. The neuronal components of the filum terminale, if they occur also in the human spinal cord, should be involved in the diagnosis and treatment of various diseases, e.g. tethered spinal cord syndrome, vascular malformations and disraphysm.

Fgf4 positively regulates scleraxis and tenascin expression in chick limb tendons

In vertebrates, tendons connect muscles to skeletal elements. Surgical experiments in the chick have underlined developmental interactions between tendons and muscles. Initial formation of tendons occurs autonomously with respect to muscle. However, further tendon development requires the presence of muscle. The molecular signals involved in these interactions remain unknown. In the chick limb, Fgf4 transcripts are located at the extremities of muscles, where the future tendons will attach. In this paper, we analyse the putative role of muscle-Fgf4 on tendon development. We have used three general tendon markers, scleraxis, tenascin, and Fgf8 to analyse the regulation of these tendon-associated molecules by Fgf4 under different experimental conditions. In the absence of Fgf4, in muscleless and aneural limbs, the expression of the three tendon-associated molecules, scleraxis, tenascin, and Fgf8, is down-regulated. Exogenous implantation of Fgf4 in normal, aneural, and muscleless limbs induces scleraxis and tenascin expression but not that of Fgf8. These results indicate that Fgf4 expressed in muscle is required for the maintenance of scleraxis and tenascin but not Fgf8 expression in tendons.

Genetic control of caudal development

Several lines of evidence suggest that caudal development involves a distinct programme. This is illustrated by the fact that a specific pattern of malformations affects the caudal end of the human embryo. In addition, neurulation, the process leading to the formation of the neural tube, proceeds through different morphogenetic movements caudally. In mammals, as in birds, the caudal neural tube arises from cavitation and not from folding of the neural plate as in more rostral levels. However, recent fate mapping studies have suggested that the two modes of neurulation represent a continuous programme, possibly involving similar cellular or molecular mechanisms. Finally, analyses of mutant mice have shown that T-box transcription factors and components of the Wnt signalling pathway control cellular migration and the promotion of mesoderm formation in the caudal embryo. In humans, mutation in the HLXB9 transcription factor causes an autosomal dominant form of sacral agenesis. Thus, the combination of classical embryological and molecular genetics approaches has provided critical reference points for the delineation of the developmental programme of the caudal embryo.