| 1. |
- Ericsson, Rolf
(författare)
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A Comparative Study of Head Development in Mexican Axolotl and Australian Lungfish: Cell Migration, Cell Fate and Morphogenesis
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of the vertebrate head is a complex process involving interactions between a multitude of cell types and tissues. This thesis describes the development of the cranial neural crest and of the visceral arch muscles in the head of two species. One, the Mexican axolotl (Ambystoma mexicanum), is a basal tetrapod, whereas the other, the Australian lungfish (Neoceratodus forsteri), belongs to the Dipnoi, the extant sister group of the Tetrapoda. The migration of neural crest cells, which form most of the bones and connective tissues in the head, and the morphogenesis of the jaw, was determined in the Mexican axolotl. It was shown that both the upper and lower jaws form from ventral condensations of neural crest cells in the mandibular arch. The dorsal condensation, earlier considered to give rise to the upper jaw, was shown to form the trabecula cranii.The normal spatio-temporal development of visceral arch muscles was investigated in both the Mexican axolotl and the Australian lungfish. In axolotl, the muscles tended to start forming almost simultaneously in all visceral arches at their future origins and extend towards their future insertions at the onset of muscle fibre formation. In lungfish, fibres formed simultaneously throughout most of each muscle anlage in the first and second visceral arch, but were delayed in the branchial arches. The anlagen were first observed at their future insertion, from which they developed towards future origins. To test the ability of neural crest cells to pattern the visceral arch muscles, migrating crest cells were extirpated from axolotl embryos, which resulted in a wide range of muscle malformations. In most cases, the muscles appeared in the right position but were small and extended in abnormal directions. This shows that neural crest cells are responsible not for the position of the muscles but for their correct anatomical pattern. Fate mapping showed that connective tissue surrounding myofibers is, at least partly, neural crest derived.In conclusion, the work presented in this thesis shows that although early development may map out the patterns of later development, the differences between axolotl and lungfish head development are not seen until during morphogenesis. Further investigation of morphogenesis is needed to explain the great variation of head morphology seen in vertebrates today.
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| 3. |
- Olsson, Tomas, et al.
(författare)
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Force Control and Visual Servoing Using Planar Surface Identification
- 2002
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Ingår i: Proceedings. ICRA '02.. - 0780372727 ; 4, s. 4211-4216
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Konferensbidrag (refereegranskat)abstract
- When designing flexible multi-sensor based robot systems, one important problem is how to combine the measurements fromdisparate sensors such as cameras and force sensors. In this paper,we present a method for combining direct force control and visualservoing in the presence of unknown planar surfaces. The controlalgorithm involves a force feedback control loop and a vision basedreference trajectory as a feed-forward signal. The vision system isbased on a constrained image-based visual servoing algorithm designedfor surface following, wherethe location and orientation of the planar constraint surface is estimated online using position-, force- and visual data.We show how data from a simple and efficient camera calibration method can be used in combination withforce and position data to improve the estimation and referencetrajectories.The method is validated through experiments involving force controlleddrawing on an unknown surface. The robot will grasp a pen and use itto draw lines between a number of markers drawn ona white-board, while the contact force is kept constant. Despite its simplicity, the performance of the method is satisfactory.
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