The locations and migration of the embryonic primordia which form the heart are well known. However, the processes whereby the heart-forming mesoderm is induced, and the later mechanisms controlling the differentiation and morphogenesis of the heart are only poorly understood for any system. An important model system for studying heart induction and differentiation is the cardiac-lethal (c) mutant in the axolotl (Ambystoma mexicanum). Embryos homozygous for the c gene develop hearts which never begin to beat, become severely deformed due to the lack of circulation, and die shortly after hatching. The mutation was believed to affect the tissue responsible for heart induction, the anterior (pharyngeal) endoderm, rendering it incapable of supplying the appropriate inductive stimuli. The inductive failure hypothesis is largely based on an assumption. The assumption is that the timing of heart induction is the same in the axolotl as has been reported for another urodele species (Taricha torosa). As well, this hypothesis is based on the finding that wild-type heart mesoderm does not form beating hearts when transplanted into c/c embryos at late tailbud stages 28-29. On the basis of this evidence, it has been suggested that wild-type heart mesoderm does not receive the proper inductive signals in the mutant environment. However, it has also been suggested that the induction occurs much earlier in another species of Ambystoma than in T. torosa; the timing of the inductive process in the axolotl has never been determined. The presence of a specific activator and inhibitor of heart differentiation, both produced by the heart mesoderm itself, provides evidence that the later phases of heart formation (i.e. the organization of contractile proteins into functional sarcomeres, and possibly the early morphogenesis of the heart tube) are probably under the control of a two-morphogen reaction-diffusion system. Such systems have been demonstrated to control pattern formation in one invertebrate organism, and have been proposed to control morphogenesis in a variety of other systems. However, this study is the first direct, experimental evidence for a reaction-diffusion mechanism controlling the development of any vertebrate organ system. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/5940 |
| Date | January 1990 |
| Creators | Smith, Steven C. |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 185 p. |
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