Cranial neural crest-cell migration in the direct-developing frog, Eleutherodactylus coqui: molecular heterogeneity within and among migratory streams

ALX1-related frontonasal dysplasia results from defective neural crest cell development and migration

A pedigree of subjects presented with frontonasal dysplasia (FND). Genome sequencing and analysis identified a p.L165F missense variant in the homeodomain of the transcription factor ALX1 which was imputed to be pathogenic. Induced pluripotent stem cells (iPSC) were derived from the subjects and differentiated to neural crest cells (NCC). NCC derived from ALX1L165F/L165F iPSC were more sensitive to apoptosis, showed an elevated expression of several neural crest progenitor state markers, and exhibited impaired migration compared to wild-type controls. NCC migration was evaluated in vivo using lineage tracing in a zebrafish model, which revealed defective migration of the anterior NCC stream that contributes to the median portion of the anterior neurocranium, phenocopying the clinical presentation. Analysis of human NCC culture media revealed a change in the level of bone morphogenic proteins (BMP), with a low level of BMP2 and a high level of BMP9. Soluble BMP2 and BMP9 antagonist treatments were able to rescue the defective migration phenotype. Taken together, these results demonstrate a mechanistic requirement of ALX1 in NCC development and migration.

Migratory patterns and evolutionary plasticity of cranial neural crest cells in ray-finned fishes

The cranial neural crest (CNC) arises within the developing central nervous system, but then migrates away from the neural tube in three consecutive streams termed mandibular, hyoid and branchial, respectively, according to the order along the anteroposterior axis. While the process of neural crest emigration generally follows a conserved anterior to posterior sequence across vertebrates, we find that ray-finned fishes (bichir, sterlet, gar, and pike) exhibit several heterochronies in the timing and order of CNC emergence that influences their subsequent migratory patterns. First, emigration of the cranial neural crest in these fishes occurs prematurely compared to other vertebrates, already initiating during early neurulation and well before neural tube closure. Second, delamination of the hyoid stream occurs prior to the more anterior mandibular stream; this is associated with early morphogenesis of key hyoid structures like external gills (bichir), a large opercular flap (gar) or first forming cartilage (pike). In sterlet, the hyoid and branchial CNC cells form a single hyobranchial sheet, which later segregates in concert with second pharyngeal pouch morphogenesis. Taken together, the results show that despite generally conserved migratory patterns, heterochronic alterations in the timing of emigration and pattern of migration of CNC cells accompanies morphological diversity of ray-finned fishes.

Cranial muscle development in frogs with different developmental modes: direct development versus biphasic development

Normal development in anurans includes a free swimming larva that goes through metamorphosis to develop into the adult frog. We have investigated cranial muscle development and adult cranial muscle morphology in three different anuran species. Xenopus laevis is obligate aquatic throughout lifetime, Rana(Lithobates) pipiens has an aquatic larvae and a terrestrial adult form, and Eleutherodactylus coqui has direct developing juveniles that hatch from eggs deposited on leaves (terrestrial). The adult morphology shows hardly any differences between the investigated species. Cranial muscle development of E. coqui shows many similarities and only few differences to the development of Rana (Lithobates) and Xenopus. The differences are missing muscles of the branchial arches (which disappear during metamorphosis of biphasic anurans) and a few heterochronic changes. The development of the mandibular arch (adductor mandibulae) and hyoid arch (depressor mandibulae) muscles is similar to that observed in Xenopus and Rana (Lithobates), although the first appearance of these muscles displays a midmetamorphic pattern in E. coqui. We show that the mix of characters observed in E. coqui indicates that the larval stage is not completely lost even without a free swimming larval stage. Cryptic metamorphosis is the process in which morphological changes in the larva/embryo take place that are not as obvious as in normal metamorphosing anurans with a clear biphasic lifestyle. During cryptic metamorphosis, a normal adult frog develops, indicating that the majority of developmental mechanisms towards the functional adult cranial muscles are preserved.

Expression of cyclin D1, cyclin D2, and N-myc in embryos of the direct developing frog Eleutherodactylus coqui, with a focus on limbs

Species of frogs that develop directly have removed the tadpole from their ontogeny and form adult structures precociously. To see whether cell cycle regulators could be involved in this altered embryogenesis, we examined the expression of ccnd1, ccnd2, and mycn in embryos of the direct developing frog, Eleutherodactylus coqui. Notable differences compared to embryos of Xenopus laevis, a species with a tadpole, included prominent expression of ccnd2 in the midbrain and ccnd1 in the mandibular neural crest. The former may contribute to the precocious appearance of the adult-type visual system and the latter to the adult-type jaw. Large domains of ccnd2 and mycn presage the early appearance of limb buds, and ccnd1 and mycn are implicated in digit development.

Developmental diversity of amphibians

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes.

Tweaking the hinge and caps: testing a model of the organization of jaws

Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits-bauplan fidelity and elaboration of design-are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the "Hinge and Caps" model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model.

The fate of cranial neural crest cells in the Australian lungfish,Neoceratodus forsteri

The cranial neural crest has been shown to give rise to a diversity of cells and tissues, including cartilage, bone and connective tissue, in a variety of tetrapods and in the zebrafish. It has been claimed, however, that in the Australian lungfish these tissues are not derived from the cranial neural crest, and even that no migrating cranial neural crest cells exist in this species. We have earlier documented that cranial neural crest cells do migrate, although they emerge late, in the Australian lungfish. Here, we have used the lipophilic fluorescent dye, DiI, to label premigratory cranial neural crest cells and follow their fate until stage 43, when several cranial skeletal elements have started to differentiate. The timing and extent of their migration was investigated, and formation of mandibular, hyoid and branchial streams documented. Cranial neural crest was shown to contribute cells to several parts of the head skeleton, including the trabecula cranii and derivatives of the mandibular arch (e.g., Meckel's cartilage, quadrate), the hyoid arch (e.g., the ceratohyal) and the branchial arches (ceratobranchials I-IV), as well as to the connective tissue surrounding the myofibers in cranial muscles. We conclude that cranial neural crest migration and fate in the Australian lungfish follow the stereotyped pattern documented in other vertebrates.

Use of fluorescent dextran conjugates as a long-term marker of osteogenic neural crest in frogs

The neural crest is a population of multipotent stem cells unique to vertebrates. In the head, cranial neural crest (CNC) cells make an assortment of differentiated cell types and tissues, including neurons, melanocytes, cartilage, and bone. The earliest understanding of the developmental potentiality of CNC cells came from classic studies using amphibian embryos. Fate maps generated from these studies have been largely validated in recent years. However, a fate map for the most late-developing structures in amphibians, and especially anurans (frogs), has never been produced. One such tissue type, skull bone, has been among the most difficult tissues to study due to the long time required for its development during anuran metamorphosis, which in some species may not occur until several months, or even years, after hatching. We report a relatively simple technique for studying this elusive population of neural crest-derived osteogenic (bone-forming) cells in Xenopus laevis by using fluorescently labeled dextran conjugates.

Combined intrinsic and extrinsic influences pattern cranial neural crest migration and pharyngeal arch morphogenesis in axolotl

Cranial neural crest cells migrate in a precisely segmented manner to form cranial ganglia, facial skeleton and other derivatives. Here, we investigate the mechanisms underlying this patterning in the axolotl embryo using a combination of tissue culture, molecular markers, scanning electron microscopy and vital dye analysis. In vitro experiments reveal an intrinsic component to segmental migration; neural crest cells from the hindbrain segregate into distinct streams even in the absence of neighboring tissue. In vivo, separation between neural crest streams is further reinforced by tight juxtapositions that arise during early migration between epidermis and neural tube, mesoderm and endoderm. The neural crest streams are dense and compact, with the cells migrating under the epidermis and outside the paraxial and branchial arch mesoderm with which they do not mix. After entering the branchial arches, neural crest cells conduct an "outside-in" movement, which subsequently brings them medially around the arch core such that they gradually ensheath the arch mesoderm in a manner that has been hypothesized but not proven in zebrafish. This study, which represents the most comprehensive analysis of cranial neural crest migratory pathways in any vertebrate, suggests a dual process for patterning the cranial neural crest. Together with an intrinsic tendency to form separate streams, neural crest cells are further constrained into channels by close tissue apposition and sorting out from neighboring tissues.