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Pharyngeal arches are bulges found on the lateral surface of the head of vertebrate embryos. They are lined externally by ectoderm and internally by endoderm, with a mesenchymal core of neural crest cells and mesoderm. Lateral expansion of pockets of endoderm form pharyngeal pouches at specific locations along the pharynx. Each one of these aligns with invaginating portions of overlying ectoderm to form the anterior and posterior border of each pharyngeal arch. Current studies suggest endoderm plays a prominent role in patterning the arches, but little is known about how this tissue develops and is organised. Investigation of pharyngeal pouch morphogenesis revealed morphological differences between anterior and posterior pouches. These region-based differences are also evident during epithelial interaction at each ectoderm/endoderm interface, where the first interface does not sustain direct contact in contrast to those posteriorly. This results in the fusion and subsequent breakdown of their basement membranes and cell death of the overlying ectoderm. I have revealed that this morphogenetic program for posterior pouch development is conserved in vertebrates and outpocketing of the pouch endoderm represents an early conserved stage of ‘gill’ development. To molecularly characterise the differences between anterior and posterior pharyngeal regions I examined Hox gene expression revealing alignment with specific pouches, thereby separating the pharynx into anterior and posterior regions. Furthermore, the most posterior pouch is demarcated by Hoxb1 expression, so as new pouches form this expression ‘moves’ posteriorly. This dynamic expression pattern is conserved and therefore may underlie how pouch number is controlled within each species. Moreover, a general trend toward a reduction in the number of pharyngeal arches has occurred with vertebrate evolution and I have localised this reduction to the posterior pharyngeal region. By using morphogenetic, molecular and comparative anatomical data I have characterised pharyngeal development and highlighted key differences in anterior and posterior regions reflecting the two main functions of the pharynx: feeding and respiration. As vertebrates transitioned from water to land, their method of respiration was adapted and this is reflected by the reduction in posterior pharyngeal segments in tetrapod species.

A large diversity of tetrapods - amphibians, reptiles, mammals and birds - populate this planet today, all of which share a common ancestor. Tetrapods show an exponential pattern of growth, punctuated by losses during major extinctions over the last 400 million years. Detailed study of three major evolutionary events and a macroscopic analysis to identify broad patterns helps us understand how vertebrates have evolved. expanded and diversified on land. After the evolution of tetrapods in the Devonian, a sparse record is present in the early Carboniferous, punctuated by a hiatus termed Ramer's Gap. Growing tetrapod diversity was impacted in the Late Pennsylvanian by the Carboniferous Rainforest Collapse of Euramerica. Changes in taxa, communities, and tTophic structure indicate that amphibian diversity stagnated in the drier climate while reptiles invaded empty habitats, explored new feeding strategies, and created endemic faunas. Reptiles continued to flourish until the Permian-Triassic Boundary extinction. Ecosystems were destroyed by a multi-pulse event, communities were restructured and though taxa recovered quickly, full ecological recovery occurred in the Late Triassic. Patterns of mass extinction and recovery have reoccurred throughout tetrapod history: large c1ades were wiped out, disaster taxa become widespread, animals possessing unique and advantageous adaptations thrived, and communities took many millions of years to recover. A macroevolutionary analysis supports the Court Jester model of diversification where external, abiotic elements have had a broader influence than the intrinsic, biotic factors of the Red Queen model. Tetrapod taxonomic and ecological diversity are strongly linked, having been driven at an increasing rate by the different tetrapod classes. The Court Jester is supported by the unrestricted taxonomic and ecological expansion of tetrapods, the ability of taxa to invade empty ecospace and rare catastrophic events which have devastated life.

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No description available.

No description available.

No description available.

Cells analogous to vertebrate endocrine cells have been described in the gut epithelium of the ascidian Styela clava. As well as some histochemical similarities, ultrastructural correlations have been demonstrated, particularly the presence of electron dense granules, clustered around and mainly below the nucleus, thinning out towards the apex. Like the endocrine cells of the vertebrate gut, the cells are often pyramidal, with a narrow apex which is occasionally observed to extend to the lumen of the gut. In addition, strong secretin immunofluorescence was observed in the endocrine-like (E-L) cells of the mucous cap of the gastric ridges. Because of these observations, acid extracts of Styela gut were assayed for secretin in the rat and in the turkey. The Styela extracts as prepared were inactive but it is possible that this reflected faults in the extraction technique. Development of a perfused Styela gut preparation, however, produced evidence to support the hypothesis that a CCK-like peptide is released into the circulation, presumably from the E-L cap cells, although CCK-like immunoreactivity is not demonstrable in these cells. The observation that in addition to CCK, bombesin and physalaemin also induce enzyme release suggests that the pre-pancreatic zymogen cells contain a rich complement of receptors, corresponding to all the classes of PI stimulating receptors which have been found on vertebrate acinar cells. There is therefore the implication that these hormones or analogues may be present in Styela. As secretin was found not to act as a secretagogue in this system, the significance of its production is unclear. By analogy with vertebrate systems it may exert some control over the secretion of mucus by the cap cells.

Nicholas J. Pioch. / "June 1995." / Includes bibliographical references (p. 207-211). / Supported in part by the Naval Air Warfare Center, Training Systems Division. N61339-94-C-0087 Supported in part by the U.S. Department of the Navy, Office of Naval Research. N00014-93-1-1399

Im Rahmen der Diskussion zu den immer höher werdenden Personalkosten , vor allem in Deutschland und Österreich, ist die Gewährung von betrieblichen Sozial- und Zusatzleistungen in Unternehmen nach dem Gießkannenprinzip nicht mehr zeitgemäß und kosteneffizient. Dem entgegen wirken würde eine individualisierte und bedürfnisorientierte Vergabe von Zusatzleistungen an die Mitarbeiterin bzw. an den Mitarbeiter, ausgerichtet an deren bzw. dessen jeweiligem Nutzenempfinden. Erwartet wird hierbei eine äußerst heterogene Nutzen- bzw. Präferenzstruktur unter den MitarbeiterInnen, beeinflusst von Faktoren wie Sozio- und Demographie. Das Ziel dieser Arbeit ist die Entwicklung einer Methode, die es den Unternehmen selbst ermöglicht, Präferenzmessungen betrieblicher Zusatzleistungen, basierend auf unterschiedlichen Mitarbeitertypen, durchzuführen bzw. die Ergebnisse dieser Messungen zu verwenden, um das Zusatzleistungsportfolio bedürfnisgerecht zu optimieren bzw. diejenigen Leistungen aufzunehmen, von denen erwartet werden kann, dass sie den höchsten Nutzen für die in diesem Unternehmen beschäftigten MitarbeiterInnen (bzw. Mitarbeitertypen) stiften. Zusätzlich wird versucht, eine Quantifizierung und eine Monetarisierung der Präferenzvorstellungen betrieblicher Zusatzleistungen durchzuführen, in dem eine geldwerte Komponente in die Messung mit aufgenommen wird. (Autor/inn/enref.)