An approach to the problem of embryonic induction.
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At early embryonal and larval stages of development 7 species of amphibia have been studied. The ultimobranchial anlage and processes resulted in formation of the secretory follicle are investigated. Dynamics on changes of amount of the gland cells and the first appearance of capillaries are analyzed. In Anura and Urodela the anlage of the ultimobranchial gland develops from the epithelial lining of the pharynx behind the last branchial pocket rudiment. The gland is asymmetric and can be laid either in the right or in the left side of the body. Death of calcitonin-secreting cells is compensated at the expense of repeated anlage of follicles from the pharyngeal epithelium. The newly formed follicles can either incorporate into the existing gland, or form independent follicles. For amphibia formation of the capillary network around the gland after beginning of the follicular secretion is specific. Owing to these data, it is possible to conclude that the stimulus for the gland to become overgrown with capillaries is the beginning of calcitonin secretion.
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Within the fertilized egg lies the information necessary to generate a diversity of cell types in the precise pattern of tissues and organs that comprises the vertebrate body. Seminal embryological experiments established the importance of induction, or cell interactions, in the formation of embryonic tissues and provided a foundation for molecular studies. In recent years, secreted gene products capable of inducing or patterning embryonic tissues have been identified. Despite these advances, embryologists remain challenged by fundamental questions: What are the endogenous inducing molecules? How is the action of an inducer spatially and temporally restricted? How does a limited group of inducers give rise to diversity of tissues? In this review, the focus is on the induction and patterning of mesodermal and neural tissues in the frog Xenopus laevis, with an emphasis on families of secreted molecules that appear to underlie inductive events throughout vertebrate embryogenesis.
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