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).