Ectomesenchyme

Abstract: To investigate the influence of hindbrain segmentation on craniofacial patterning we have studied the long term fate of neural crest (NC) subpopulations of individual rhombomeres (r), using quail-chick chimeras. Mapping of all skeletal and muscle connective tissues developing from these small regions revealed several novel features of the cranial neural crest. First, the mandibular arch skeleton has a composite origin in which the proximal elements are r1+r2 derived, whereas more distal ones are exclusively midbrain derived. The most proximal region of the lower jaw is derived from second arch (r4) NC. Second, both the lower jaw and tongue skeleton display an organisation which precisely reflects the rostrocaudal order of segmental crest deployment from the embryonic hindbrain. Third, cryptic intraskeletal boundaries, which do not correspond to anatomical landmarks, form sharply defined interfaces between r1+r2, r4 and r6+r7 crest. Cells that survive the early apoptotic elimination of premigratory NC in r3 and r5 are restricted to tiny contributions within the 2nd arch (r4) skeleton. Fourth, a highly constrained pattern of cranial skeletomuscular connectivity was found that precisely respects the positional origin of its constitutive crest: each rhombomeric population remains coherent throughout ontogeny, forming both the connective tissues of specific muscles and their respective attachment sites onto the neuro- and viscerocranium. Finally, focal clusters of crest cells, confined to the attachment sites of branchial muscles, intrude into the otherwise mesodermal cranial base. In the viscerocranium, an equally strict, rhombomere-specific matching of muscle connective tissues and their attachment sites is found for all branchial and tongue (hypoglossal) muscles. This coherence of segmental crest populations explains how cranial skeletomuscular pattern can be implemented and conserved despite evolutionary changes in the shapes of skeletal elements.

Abstract: Teeth develop from composite organ rudiments that are formed through the interaction of oral epithelium and mesenchyme of the first branchial arch; cells of the former differentiate into enamel-secreting ameloblasts whereas those of the latter differentiate into dentine-secreting odontoblasts. Experimental analysis of odontogenic tissue interactions in mammalian embryos has focused on the late developmental stages of morphogenesis and cytodifferentiation; little is known about initial pattern-forming events, during which presumptive tooth-forming cells are specified and the sites of tooth initiation become established. It requires to be shown, for example, whether the mesenchymal cells of mammalian teeth are derived, like those of amphibians, from the cranial neural crest, and if so, whether these form a specified subpopulation in the neural folds. Alternatively, are they specified after migration into the mandibular arch, possibly by interaction with the oral epithelium? The developmental potentials of mouse embryo premigratory cranial neural crest cells (CNC - explanted from the caudal mesencephalic and rostral metencephalic neural folds) have been studied in intraocular homograft recombinations with various regions of embryonic surface ectoderm. Cartilage, bone and neural tissue developed in all combinations of CNC and epithelium. Teeth formed in combinations of CNC with mandibular arch epithelium but not in combinations of CNC with limb bud epithelium. Teeth also formed in combinations of mandibular arch epithelium with neural crest explanted from the trunk level. These results indicate that mammalian neural crest has an odontogenic potential but that this is not restricted to the crest of presumptive tooth-forming levels. Normal migration appears not to be a prerequisite for expression of odontogenic potential but this does require an interaction with region-specific epithelium. It is reasonable to infer that during normal development the neural crest that enters the mandibular arch is odontogenically unspecified before or during migration and that the oral epithelium is the earliest known site of tooth pattern.

Abstract: A vital dye analysis of cranial neural crest migration in the chick embryo has provided a positional fate map of greater resolution than has been possible using labelled graft techniques. Focal injections of the fluorescent membrane probe DiI were made into the cranial neural folds at stages between 3 and 16 somites. Groups of neuroepithelial cells, including the premigratory neural crest, were labelled by the vital dye. Analysis of whole-mount embryos after 1-2 days further development, using conventional and intensified video fluorescence microscopy, revealed the pathways of crest cells migrating from mesencephalic and rhombencephalic levels of the neuraxis into the subjacent branchial region. The patterns of crest emergence and emigration correlate with the segmented disposition of the rhombencephalon. Branchial arches 1, 2 and 3 are filled by crest cells migrating from rhombomeres 2, 4 and 6 respectively, in register with the cranial nerve entry/exit points in these segments. The three streams of ventrally migrating cells are separated by alternating regions, rhombomeres 3 and 5, which release no crest cells. Rostrally, rhombomere 1 and the caudal mesencephalon also contribute crest to the first arch, primarily to its upper (maxillary) component. Both r3 and r5 are associated with enhanced levels of cell death amongst cells of the dorsal midline, suggesting that crest may form at these levels but is then eliminated. Organisation of the branchial region is thus related by the dynamic process of neural crest immigration to the intrinsic mechanisms that segment the neuraxis.

Abstract: To understand the role of twist during mammalian development, we generated twist-null mice. twist-null embryos died at embryonic day 11.5. Their most prominent phenotype was a failure of the cranial neural folds to fuse. Mutant embryos also had defects in head mesenchyme, somites, and limb buds. Chimera analysis suggested that head mesenchyme was required for cranial neural tube closure and that twist acted in a cell-autonomous manner in this tissue. In addition, in the head mesenchyme region of chimeras, twist-null cells were segregated from wild-type cells, and in the forebrain they lacked mesenchymal characteristics. These results suggest that twist regulates the cellular phenotype and behavior of head mesenchyme cells that are essential for the subsequent formation of the cranial neural tube.