Auswahl der wissenschaftlichen Literatur zum Thema „Cranial mesoderm“

A combination of micromanipulative cell grafting and fluorescent cell labelling techniques were used to examine the developmental fate of the cranial paraxial mesoderm of the 8.5-day early-somite-stage mouse embryo. Mesodermal cells isolated from seven regions of the cranial mesoderm, identified on the basis of their topographical association with specific brain segments were assessed for their contribution to craniofacial morphogenesis during 48 hours of in vitro development. The results demonstrate extensive cell mixing between adjacent but not alternate groups of mesodermal cells and a strict cranial-to-caudal distribution of the paraxial mesoderm to craniofacial structures. A two-segment periodicity similar to the origins of the branchial motor neurons and the distribution of the rhombencephalic neural crest cells was observed as the paraxial mesoderm migrates during formation of the first three branchial arches. The paraxial mesoderm colonises the mesenchymal core of the branchial arches, consistent with the location of the muscle plates. A dorsoventral regionalisation of cell fate similar to that of the somitic mesoderm is also found. This suggests evolution has conserved the fate of the murine cranial paraxial mesoderm as a multiprogenitor population which displays a predominantly myogenic fate. Heterotopic transplantation of cells to different regions of the cranial mesoderm revealed no discernible restriction in cell potency in the craniocaudal axis, reflecting considerable plasticity in the developmental fate of the cranial mesoderm at least at the time of experimentation. The distribution of the different groups of cranial mesoderm matches closely with that of the cranial neural crest cells suggesting the two cell populations may share a common segmental origin and similar destination.

Cells of the cranial paraxial mesoderm give rise to parts of the skull and muscles of the head. Some mesoderm cells migrate from locations close to the hindbrain into the branchial arches where they undergo muscle differentiation. We have characterised these migratory pathways in chick embryos either by DiI-labelling cells before migration or by grafting quail cranial paraxial mesoderm orthotopically. These experiments demonstrate that depending on their initial rostrocaudal position, cranial paraxial mesoderm cells migrate to fill the core of specific branchial arches. A survey of the expression of myogenic genes showed that the myogenic markers Myf5, MyoD and myogenin were expressed in branchial arch muscle, but at comparatively late stages compared with their expression in the somites. Pax3 was not expressed by myogenic cells that migrate into the branchial arches despite its expression in migrating precursors of limb muscles. In order to test whether segmental plate or somitic mesoderm has the ability to migrate in a cranial location, we grafted quail trunk mesoderm into the cranial paraxial mesoderm region. While segmental plate mesoderm cells did not migrate into the branchial arches, somitic cells were capable of migrating and were incorporated into the branchial arch muscle mass. Grafted somitic cells in the vicinity of the neural tube maintained expression of the somitic markers Pax3, MyoD and Pax1. By contrast, ectopic somitic cells located distal to the neural tube and in the branchial arches did not express Pax3. These data imply that signals in the vicinity of the hindbrain and branchial arches act on migrating myogenic cells to influence their gene expression and developmental pathways.

The transcription factors, MesP1 and MesP2, sharing an almost identical bHLH motif, have an overlapping expression pattern during gastrulation and somitogenesis. Inactivation of the Mesp1 gene results in abnormal heart morphogenesis due to defective migration of heart precursor cells, but somitogenesis is not disrupted because of normal expression of the Mesp2 gene. To understand the cooperative functions of MesP1 and MesP2, either a deletion or sequential gene targeting strategy was employed to inactivate both genes. The double-knockout (dKO) embryos died around 9.5 days postcoitum (dpc) without developing any posterior structures such as heart, somites or gut. The major defect in this double-knockout embryo was the apparent lack of any mesodermal layer between the endoderm and ectoderm. The abnormal accumulation of cells in the primitive streak indicates a defect in the migratory activity of mesodermal cells. Molecular markers employed to characterize the phenotype revealed a lack of the cranio-cardiac and paraxial mesoderm. However, the axial mesoderm, as indicated by brachyury (T) expression, was initially generated but anterior extension was halted after 8.5 dpc. Interestingly, a headfold-like structure developed with right anterior-posterior polarity; however, the embryos lacked any posterior neural properties. The persistent and widely distributed expression of Cerberus-like-1(Cer1), Lim1 and Otx2 in the anterior endoderm might be responsible for the maintenance of anterior neural marker expression. We also performed a chimera analysis to further study the functions of MesP1 and MesP2 in the development of mesodermal derivatives. In the chimeric embryos, dKO cells were scarcely observed in the anterior-cephalic and heart mesoderm, but they did contribute to the formation of the somites, notochord and gut. These results strongly indicate that the defect in the cranial-cardiac mesoderm is cell-autonomous, whereas the defect in the paraxial mesoderm is a non-cell-autonomous secondary consequence.

AbstractThe vertebrate cranium consists of three parts: neuro-, viscero- and dermatocranium, which differ in both developmental and phylogenetic origin. Traditionally, developmental origin has been used as a criterion for homology, but this becomes problematic when skull elements such as the parietal bone are now shown, by modern fate-mapping studies, to have different developmental origins in different groups of tetrapods. This indicates a flexibility of developmental programmes and regulatory pathways which has probably been very important in cranial evolution. The trabecula cranii is an intriguing cranial element in the anterior cranial base in vertebrates. It forms a viscerocranial part of the neurocranium and is believed to be neural crest-derived in gnathostomes, but a similarly named structure in lampreys has been shown to have a mesodermal origin. Topographically, this trabecula seems to be homologous to the gnathostome trabecula cranii, and might also have the same function: to form a border between adjacent morphogenetic domains, to constrain and redirect growth of both brain and stomodeum and thus to refine developmental schedules of both. We suggest that such a border zone can recruit cells from either the mesoderm (as in the lamprey) or from the neural crest (as in the gnathostomes investigated), and still retain its homology. In our view, the trabecula is an interface element that integrates the respective divergent morphogenetics programs of the preotic head into a balanced unit; we suggest that such a definition can be used to define "the sameness" of this element throughout vertebrates.

The prospective fate of cells in the primitive streak was examined at early, mid and late stages of mouse gastrula development to determine the order of allocation of primitive streak cells to the mesoderm of the extraembryonic membranes and to the fetal tissues. At the early-streak stage, primitive streak cells contribute predominantly to tissues of the extraembryonic mesoderm as previously found. However, a surprising observation is that the erythropoietic precursors of the yolk sac emerge earlier than the bulk of the vitelline endothelium, which is formed continuously throughout gastrula development. This may suggest that the erythropoietic and the endothelial cell lineages may arise independently of one another. Furthermore, the extraembryonic mesoderm that is localized to the anterior and chorionic side of the yolk sac is recruited ahead of that destined for the posterior and amnionic side. For the mesodermal derivatives in the embryo, those destined for the rostral structures such as heart and forebrain mesoderm ingress through the primitive streak early during a narrow window of development. They are then followed by those for the rest of the cranial mesoderm and lastly the paraxial and lateral mesoderm of the trunk. Results of this study, which represent snapshots of the types of precursor cells in the primitive streak, have provided a better delineation of the timing of allocation of the various mesodermal lineages to specific compartments in the extraembryonic membranes and different locations in the embryonic anteroposterior axis.

Bones of the cranial vault appear to be highly conserved among tetrapod vertebrates. Moreover, bones identified with the same name are assumed to be evolutionarily homologous. However, recent developmental studies reveal a key difference in the embryonic origin of cranial vault bones between representatives of two amniote lineages, mammals and birds, thereby challenging this view. In the mouse, the frontal is derived from cranial neural crest (CNC) but the parietal is derived from mesoderm, placing the CNC–mesoderm boundary at the suture between these bones. In the chicken, this boundary is located within the frontal. This difference and related data have led several recent authors to suggest that bones of the avian cranial vault are misidentified and should be renamed. To elucidate this apparent conflict, we fate-mapped CNC and mesoderm in axolotl to reveal the contributions of these two embryonic cell populations to the cranial vault in a urodele amphibian. The CNC–mesoderm boundary in axolotl is located between the frontal and parietal bones, as in the mouse but unlike the chicken. If, however, the avian frontal is regarded instead as a fused frontal and parietal (i.e. frontoparietal) and the parietal as a postparietal, then the cranial vault of birds becomes developmentally and topologically congruent with those of urodeles and mammals. This alternative hypothesis of cranial vault homology is also phylogenetically consistent with data from the tetrapod fossil record, where frontal, parietal and postparietal bones are present in stem lineages of all extant taxa, including birds. It further implies that a postparietal may be present in most non-avian archosaurs, but fused to the parietal or supraoccipital as in many extant mammals.

The spatial distribution of the cranial paraxial mesoderm and the neural crest cells during craniofacial morphogenesis of the mouse embryo was studied by micromanipulative cell grafting and cell labelling. Results of this study show that the paraxial mesoderm and neural crest cells arising at the same segmental position share common destinations. Mesodermal cells from somitomeres I, III, IV and VI were distributed to the same craniofacial tissues as neural crest cells of the forebrain, the caudal midbrain, and the rostral, middle and caudal hindbrains found respectively next to these mesodermal segments. This finding suggests that a basic meristic pattern is established globally in the neural plate ectoderm and paraxial mesoderm during early mouse development. Cells from these two sources mixed extensively in the peri-ocular, facial, periotic and cervical mesenchyme. However, within the branchial arches a distinct segregation of these two cell populations was discovered. Neural crest cells colonised the periphery of the branchial arches and enveloped the somitomere-derived core tissues on the rostral, lateral and caudal sides of the arch. Such segregation of cell populations in the first three branchial arches is apparent at least until the 10.5-day hindlimb bud stage and could be important for the patterning of the skeletal and myogenic derivatives of the arches.

Craniofacial mesenchyme is composed of three mesodermal populations – prechordal plate, lateral mesoderm and paraxial mesoderm, which includes the segmented occipital somites and the incompletely segmented somitomeres – and the neural crest. This paper outlines the fates of each of these, as determined using quail–chick chimaeras, and presents similarities and differences between these cephalic populations and their counterparts in the trunk. Prechordal and paraxial mesodermal populations are the sources of all voluntary muscles of the head. The latter also provides most of the connective precursors of the calvaria, occipital, otic–parietal and basisphenoid tissues. Lateral mesoderm is the source of peripharyngeal connective tissues; the most rostral skeletal tissues it forms are the laryngeal and tracheal cartilages. When migrating neural crest cells encounter segmented paraxial mesoderm (occipital and trunk somites), most move into the region between the dermamyotome and sclerotome in the cranial half of each somite. In contrast, most cephalic crest cells migrate superficial to somitomeres. There is, however, a small subpopulation of the head crest that invades somitomeric mesoderm. These cells subsequently segregate presumptive myogenic precursors of visceral arch voluntary muscles from underlying mesenchyme. In the neurula-stage avian embryo, all paraxial and lateral mesodermal populations contain precursors of vascular endothelial cells, which can be detected in chimaeric embryos using anti-quail endothelial antibodies. Some of these angioblasts differentiate in situ, contributing directly to pre-existing vessels or forming isolated, nonpatent, cords that subsequently vesiculate and fuse with nearby vessels. Many angioblasts migrate in all directions, invading embryonic mesenchymal and epithelial tissues and participating in new blood vessel formation in distant sites. The interactions leading to proper spatial patterning of craniofacial skeletal, muscular, vascular and peripheral neural tissues has been studied by performing heterotopic transplants of each of these mesodermal and neural crest populations. The results consistently indicate that connective tissue precursors, regardless of their origin, contain spatial information used by the precursors of muscles and blood vessels and by outgrowing peripheral nerves. Some of these connective tissue precursors (e.g. the neural crest, paraxial mesoderm) acquire their spatial programming while in association with the central nervous system or developing sensory epithelia (e.g. otic, optic, nasal epithelia).

The transcription factor Twist1 is a key regulator of craniofacial development. Deletion of Twist1 in the mouse embryo leads to neural tube defects, abnormal head development and mid-gestational lethality. To dissect the function of Twist1 in the cranial mesoderm (CM) beyond mid-gestation, the Mesp1-Cre transgenic line was used to delete Twist1 in the anterior mesoderm, including the progenitors of the CM. Loss of Twist1 in CM cells resulted in loss and malformations of the cranial mesoderm-derived skeleton and failure to fully segregate the mesoderm and the neural crest cells. The development of extraocular muscles was compromised whereas the differentiation of branchial arch muscles was not affected, indicating a differential requirement for Twist1 in these two types of craniofacial muscle. Surprisingly, loss of Twist1 led to the inability of the mesodermal cells to maintain their mesenchymal characteristics followed by acquisition of an epithelial-like morphology. Microarray analysis of the Twist1 deficient embryos revealed gene expression changes relating to cell–matrix interaction, blood vessel morphogenesis and regulation of Epithelial to Mesenchymal Transition (EMT). Combining the microarray data set with ChIP-sequencing identified Prrx1 and Ddr2at least two Twist1 transcriptional targets Prrx1 and Ddr2, both involved in EMT and the propagation of the mesenchymal cell characteristics. The findings presented in this thesis point to a central role of role of Twist1 in maintaining the mesenchymal architecture and the progenitor state of the mesoderm necessary for proper craniofacial development.

The vertebrate head is a complex structure derived from all three embryonic germ layers. Cranial mesoderm forms most of the neurocranium, cardiovascular tissues and voluntary muscles required for intake of food and oxygenated fluid. Despite its essential role in shaping cranial and neck anatomy, long-term fate maps of cranial mesoderm are known only from the mouse and chicken, as effective labeling techniques for use in other species have been developed only recently. Data from additional species are needed to determine the embryonic origin of features absent in amniotes but present in other vertebrates and to evaluate the extent of conservation in the development of homologous structures. This dissertation examines the role of cranial mesoderm as well as its interactions with neural crest in shaping the tetrapod craniofacial and neck region, focusing on the skull and head muscles in the axolotl, Ambystoma mexicanum. I demonstrate a dual embryonic origin of the pharyngeal skeleton, including derivation of basibranchial 2 from mesoderm closely associated with the second heart field. Additionally, heterotopic transplantation experiments reveal lineage restriction of mesodermal cells that contribute to pharyngeal cartilage. The entire parietal bone is derived from mesoderm. Several structures arise from both mesoderm and cranial neural crest, including the squamosal, parasphenoid and stapes. The mesodermal contribution to the dorsal portion of the squamosal bone supports the homology of the corresponding dorsal ossification center, which fuses to the ventral center early in development, to the supratemporal, a bone lost repeatedly in tetrapods. I locate the posterior limit of myogenic cranial mesoderm, extending the head-trunk boundary to the axial level of the third somite. Using fate mapping, gene expression and comparative anatomy, I provide evidence that the cucullaris muscle, a homologue of the mammalian trapezius, is a cranial muscle allied with the gill levators of anamniotes. Finally, I generate two novel transgenic lines of Xenopus tropicalis that will be used to fate map neural crest and mesoderm. Taken together, these results add to our understanding of cranial homologies and point to a larger role for cranial mesoderm in the evolution of a mobile neck.
Biology, Organismic and Evolutionary

Les muscles squelettiques sont présents dans tout le corps et présentent un niveau surprenant d'hétérogénéité, dans leur susceptibilité aux maladies, potentiel de régénération ou capacités métaboliques. Cette diversité est également retrouvée au cours du développement embryonnaire où les cellules myogéniques et non myogéniques établissent le système musculo-squelettique. La tête et le cou sont constitués d'une grande variété de muscles qui remplissent des fonctions essentielles, mais nous en savons peu sur la biologie des muscles craniofaciaux. Ces structures sont associées à l'émergence de cellules de la crête neurale (CCN) qui donnent naissance à la plupart des tissus non myogéniques crâniens et qui sont cruciales à la formation des muscles. Cependant, certains muscles crâniens sont privés de CCN, et nous ignorons comment les cellules myogéniques et non myogéniques contribuent à ces domaines. Cette thèse fournit des preuves démontrant que les progéniteurs en amont du muscle se détournent du programme myogénique pour donner naissance au tissu conjonctif. Nous avons utilisé une approche de single-cell RNAseq non biaisée et restreinte avec différentes lignées transgéniques de souris à des stades embryonnaires distincts, des marquages in situ et de nouvelles méthodes analytiques, et avons montré que les progéniteurs bipotents issus du mésoderme exprimant le gène de détermination musculaire Myf5 donnent naissance au muscle squelettique et au tissu conjonctif anatomiquement associé dans les muscles partiellement privés de CCN. Cette transition est caractérisée par une complémentarité de signalisation de récepteurs tyrosine kinase entre les cellules musculaires et non musculaires, ainsi que par des modules régulateurs distincts. Les muscles crâniens proviennent également de différentes lignées qui impliquent l'activité de cascades de régulation génique spécifiques. Ici, nous avons utilisé une approche non biaisée et large pour découvrir des modules de régulation spécifiques qui sous-tendent différentes populations de cellules myogéniques dans la tête et à travers plusieurs stades de développement. Certaines de ces « tâches de naissance génétiques » uniques sont des facteurs de transcription spécifiques et sont conservées dans les cellules souches musculaires adultes, ce qui indique que leur importance potentielle est de fournir les propriétés uniques qui ont été signalées pour différentes populations de cellules souches musculaires. Enfin, ces études utilisent des méthodes analytiques inédites qui bénéficient des dernières avancées algorithmiques et offrent de nouvelles perspectives pour la découverte de processus biologiques à partir de données à haut débit
Skeletal muscles are found throughout the body and they display a surprising level of heterogeneity in properties and function. For example, some muscles are specifically susceptible to diseases, and some have better regenerative potential or different metabolic capacities. Diversity is also found during embryonic development where myogenic and non-myogenic cells establish the musculoskeletal system. The head and neck are comprised of a wide variety of muscles that perform essential functions such as feeding, breathing and vocalising, yet little is known about craniofacial muscle biology. Novel structures are associated with the emergence of neural crest cells (NCC) which give rise to most craniofacial connective tissue, cartilage and bone and are crucial for muscle morphogenesis. However, some cranial muscles are deprived of NCC, and it is unclear how myogenic and non-myogenic cells contribute to those domains. This thesis provides evidence demonstrating that upstream progenitors redirect from the myogenic program to give rise to the muscle-associated connective tissue that supports the formation of muscular structures. We employed unbiased and lineage-restricted single-cell RNAseq using different mouse transgenic lines at distinct embryonic stages, in situ labelling, and new analytical methods, and show that bipotent progenitors expressing the muscle determination gene Myf5 give rise to skeletal muscle and anatomically associated connective tissue in distinct muscle groups spatiotemporally. Notably, this property was restricted to muscles with only partial contribution from NCCs suggesting that in their absence, the balance of myogenic and connective tissue cells is undertaken by somite-derived or cranial-derived mesoderm. This transition is characterised by a complementarity of tyrosine kinase receptor signalling between muscle and non-muscle cells, as well as distinct regulatory modules. Cranial muscles also originate from different lineages that involve the activity of specific gene regulatory cascades. Here, we used an all-inclusive unbiased approach to uncover specific regulatory modules that underlie different myogenic cell populations in the head and across multiple developmental stages. Some of these unique “genetic birthmarks” are specific transcription factors, and are retained in adult muscle stem cells pointing to their potential importance is delivering the unique properties that have been reported for different muscle stem cell populations. Finally, these studies employ novel computational methods that benefit from the latest algorithmic advancements and they provide prospects for the discovery of new biological processes from high throughput data

To determine whether exposure to simulated microgravity (SMG) affects cranial neural crest (CNC)-derived tissues, zebrafish embryos were exposed to SMG starting at one of three developmental stages corresponding to CNC migration. Juvenile and adult fish were analyzed after exposure to SMG using statistics and geometric morphometrics for changes in melanophore surface area and number, and changes in skull morphology. Analyses reveal an initial increase in the surface area of melanophores present on the dorsal view of the juvenile skull and a decrease in melanophore number over the period of a week. Additionally, buckling is observed in CNC-derived frontal bones in juvenile fish after exposure. The effects on the melanophores are transient and the effects on CNC-derived bones are short-term. Surprisingly, severe long-term effects occurred in mesoderm-derived bones, such as the parasphenoid. In summary, exposure to SMG affects both CNC- and mesoderm-derived tissues in the juvenile and adult zebrafish head.

This chapter focuses on ectodermal placodes, which are areas of columnar-shaped cells. It illustrates how cranial placodes in the head contribute to the sense organs forming the olfactory epithelium, the inner ear, and the lens of the eye, and to the cranial sensory ganglia. It also explains how the pre-placodal region separates into individual placodes, a process controlled by local signals from the neural tube and underlying mesoderm or endoderm. The chapter discusses eye development and shows how it starts with the specification of the eye field in the ventral diencephalon. The chapter also shows the major role Pax6 plays in eye formation. It mentions the enamel knot, which is the signaling center for tooth shape and development.

Evaluating the adrenal gland with imaging can be challenging. The adrenal glands may be morphologically within normal limits even in the presence of clear hyperfunction. Hyperplasia and small nodules may coexist. Nonfunctioning nodules are frequent and need to be differentiated from culpable hyperfunctioning adenomas or carcinomas. However, the increasingly sophisticated anatomical imaging provided by CT and MRI, together with the functional characterization afforded by radionuclide imaging, allows good correlation with clinical and endocrine parameters. Embryologically, the adrenal cortex derives from coelomic mesoderm and the medulla from neural crest cells. Development is independent of the kidney and adrenal glands will normally be present in the absence of a kidney. In the newborn the adrenal glands are large structures, being one-third of the size of the kidneys. They involute rapidly, however, and in the adult are small structures. They are situated immediately above and anteromedial to the upper pole of the kidneys, although the left is less suprarenal. The right lies immediately behind the cava, alongside the right diaphragmatic crus. The left lies behind the splenic vein, lateral to the left crus. The normal adrenal has a characteristic inverted Y- or V-shape with the two limbs fusing anteromedially. The most cranial section has a triangular appearance. Cross-sectional appearance varies according to the exact level. Each limb measures 2.5–4 cm in length and 3–6 mm in thickness. Greater than 1 cm thickness is definitely abnormal. Accessory adrenal tissue (rests) may be found in the kidney, testis, or ovary, and elsewhere in the retroperitoneum. Arterial supply is from three sources: superior–multiple arteries from the inferior phrenic; middle from the aorta; and inferior from the renal artery. A single vein drains each adrenal. The left is a tributary of the left renal vein, the right leads directly to the cava, although rarely may join a hepatic vein first. The right adrenal vein is shorter and narrower.

Abstract The primary palate is the keystone to the upper lip and anterior portion of the definitive palate. Its embryogenesis is fundamental to normal development of the midface, and its maldevelopment has profound clinical and sociological consequences upon breathing, suckling, swallowing, mastication, osculation, speech, and facial physiognomy. Recent advances in molecular biology and genetics have provided significant insights into craniofacial embryology. Orofacial development in the embryo is first demarcated by the appearance of the prechordal plate at the cranial end of the embryonic disk at the 14th day postconception (Sperber, 2001). This plate designates the site of the future mouth or stomodeum. The mesenchyme that provides the facial primordia is peculiarly of ectodermal derivation, arising from neural crest cells at the apices (crests) of the neural folds prior to neural tube formation (Fig. 1.1) (La Bonne and Bronner-Fraser, 1999; Lawson et al., 2001). The neural crest cells peculiarly disrupt the ectodermal-mesodermal boundary and migrate into the subjacent tissue as ectomesenchymal cells. Their migration and proliferation are fundamental in facial development. During their migration, they interact with the extracellular matrix and adjacent epithelia, which partly determines the patterning and nature of the derivative tissues they will form. These derivatives include neural, skeletal, connective, and muscular tissues (Sarkar et al., 2001).

Head fold morphogenesis constitutes the first discernible epithelial folding event in the embryonic development of the chick. It arises at Hamburger and Hamilton (HH) stage 6 (approximately 24 hours into a 21-day incubation period) and establishes the anterior extent of the embryo [1]. At this stage, the embryonic blastoderm is composed of three germ layers (endoderm, mesoderm, and ectoderm), which are organized into a flat layered sheet that overlies the fibrous vitelline membrane (VM). Within this blastodermal sheet, a crescent-shaped head fold develops just anterior to the elongating notochord, spanning across the embryonic midline at the rostral end of neural plate. At the crest of this fold, the bilateral precardiac plates fuse in a cranial to caudal direction and give rise to the primitive heart tube and foregut [2, 3]. An understanding of head fold morphogenesis may thus offer insight into how embryonic tissues are arranged to make ready for proper cardiac formation.