Academic literature on the topic 'Chick; Development; Ectoderm'

Classical embryological experiments have demonstrated that dorsal-ventral patterning of the vertebrate limb is dependent upon ectodermal signals. One such factor is Wnt-7a, a member of the Wnt family of secreted proteins, which is expressed in the dorsal ectoderm. Loss of Wnt-7a results in the appearance of ventral characteristics in the dorsal half of the distal limb. Conversely, En-1, a homeodomain transcription factor, is expressed exclusively in the ventral ectoderm, where it represses Wnt-7a. En-1 mutants have dorsal characteristics in the ventral half of the distal limb. Experiments in the chick suggest that the dorsalizing activity of Wnt-7a in the mesenchyme is mediated through the regulation of the LIM-homeodomain transcription factor Lmx-1. Here we have examined the relationship between Wnt-7a, En-1 and Lmx-1b, a mouse homolog of chick Lmx-1, in patterning the mammalian limb. We find that Wnt-7a is required for Lmx-1b expression in distal limb mesenchyme, and that Lmx-1b activation in the ventral mesenchyme of En-1 mutants requires Wnt-7a. Consistent with Lmx-1b playing a primary role in dorsalization of the limb, we find a direct correlation between regions of the anterior distal limb in which Lmx-lb is misregulated during limb development and the localization of dorsal-ventral patterning defects in Wnt-7a and En-1 mutant adults. Thus, ectopic Wnt-7a expression and Lmx-1b activation underlie the dorsalized En-1 phenotype, although our analysis also reveals a Wnt-7a-independent activity for En-1 in the repression of pigmentation in the ventral epidermis. Finally, we demonstrate that ectopic expression of Wnt-7a in the ventral limb ectoderm of En-1 mutants results in the formation of a second, ventral apical ectodermal ridge (AER) at the junction between Wnt-7a-expressing and nonexpressing ectoderm. Unlike the normal AER, ectopic AER formation is dependent upon Wnt-7a activity, indicating that distinct genetic mechanisms may be involved in primary and secondary AER formation.

The chicken eggshell supplies approximately 80% of the calcium found in the hatchling chick. The mobilization of eggshell calcium into the developing embryo involves the transepithelial transport of large amounts of calcium in a development-specific manner. The cells responsible for the transport of eggshell calcium into the embryonic circulation are the ectodermal cells of the chorioallantoic membrane. In this report, we present a method for the isolation and culture of chorioallantoic membrane ectodermal cells, which are amenable to direct experimental manipulation. Cell preparations are characterized with respect to the expression of an ectoderm-specific cell surface marker (transcalcin, a calcium-binding protein), and a specific enzymatic activity (elevated Ca(2+)-activated ATPase). Functional assessment of in vitro cellular calcium uptake by 45Ca2+ tracer kinetics indicates the persistence of a temperature-sensitive, rapid-influx pathway similar to that observed in vivo. The preparations of primary ectodermal cells present an in vitro system applicable to the experimental analysis of calcium metabolism and transport by the chick chorioallantoic membrane.

Peripheral nerves travel to their targets along precise routes, and it is likely that different cues provide guidance at different stages of the journey. In a developing chick limb, the cutaneous nerve fibres follow at first deep mixed nerve trunks, in company with motor axons; they branch from these trunks at predictable points and approach the skin; they then ramify profusely to form a plexus at a precisely defined depth beneath the ectoderm, at exactly the same level as the blood vascular plexus. To analyse the role of signals from the target patch of skin in regulating cutaneous nerve development, we have ablated patches of dorsal wing ectoderm using short-wave ultraviolet irradiation at E4 (embryonic day 4), approximately one day before nerves grow into the limb bud. The irradiated patches remain denuded of ectoderm for more than a week, by which time the cutaneous nerve plexus on the contralateral control side is well developed and can be revealed by whole-mount silver staining. Where the ectoderm has been ablated, no cutaneous nerve plexus forms, and the nerve branches that normally would have diverged from the neighbouring mixed nerve trunk to innervate the missing patch of skin are absent - ab initio, apparently. The routes of the mixed nerve trunks are not affected. Partial ablation of the territory of a cutaneous nerve branch often leads to loss of the whole nerve branch; the intact skin territory thus left vacant is invaded by ramifications from the remaining cutaneous branches, as expected if the normal extent of a cutaneous nerve's territory is regulated by competition. Where there is an ectodermal lesion, cutaneous innervation stops precisely at its boundary, even though the vascular plexus extends for some distance beyond this margin, beneath the denuded surface. The data suggest that the embryonic skin is required firstly to trigger divergence of cutaneous nerve branches from the mixed nerve trunks, and secondly, once the nerve fibres have reached the skin, to supply a trophic cue (probably NGF) encouraging growth of a plexus; at the same time, the embryonic skin generates a signal inhibiting nerves from approaching closer than about 70 microns to the surface.

The development of the chick face involves outgrowth of buds of tissue, accompanied by the differentiation of cartilage and bone in spatially defined patterns. To investigate the role of epithelial-mesenchymal interactions in facial morphogenesis, small fragments of facial tissue have been grafted to host chick wing buds to continue their development in isolation. Fragments of the frontonasal mass give rise to typical upper-beak-like structures: a long central rod of cartilage, the prenasal cartilage and an egg tooth. Meckel's cartilage, characteristic of the lower beak, develops from fragments of the mandible. Removal of the ectoderm prior to grafting leads to truncated development. In fragments of frontonasal mass mesenchyme only a small spur of cartilage differentiates and there is no outgrowth. The mandible is less affected; a rod of cartilage still forms but the amount of outgrowth is reduced. Retinoid treatment of chick embryos specifically affects the development of the upper beak and outgrowth and cartilage differentiation in the frontonasal mass are inhibited. The mandibles, however, are unaffected and develop normally. In order to investigate whether the epithelium or the mesenchyme of the frontonasal mass is the target of retinoid action, recombinations of retinoid-treated and untreated facial tissue have been grafted to host wing buds. Recombinations of retinoid-treated frontonasal mass ectoderm with untreated mesenchyme develop normally whereas recombinations of untreated ectoderm with retinoid-treated mesenchyme lead to truncations. The amount of outgrowth in fragments of mandibular tissue is slightly reduced when either the ectoderm or the mesenchyme has been treated with retinoids. These recombination experiments demonstrate that the mesenchyme of the frontonasal mass is the target of retinoid action. This suggests that retinoids interfere with the reciprocal epithelial-mesenchymal interactions necessary for outgrowth and normal upper beak development.