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We dissect genetically a gene regulatory network that involves the transcription factors Tbx4, Pitx1 and Isl1 acting cooperatively to establish the hindlimb bud, and identify key differences in the pathways that initiate formation of the hindlimb and forelimb. Using live image analysis of murine limb mesenchyme cells undergoing chondrogenesis in micromass culture, we distinguish a series of changes in cellular behaviours and cohesiveness that are required for chondrogenic precursors to undergo differentiation. Furthermore, we provide evidence that the proximal hindlimb defects observed in Tbx4 mutant mice result from a failure in the early differentiation step of chondroprogenitors into chondrocytes, providing an explanation for the origins of proximally biased limb defects. [ABSTRACT FROM AUTHOR]

The interpretation of the segmentation and tagmosis in ostracod crustaceans has remained uncertain. Most ostracods have only seven pairs of anterior limbs, including the cephalic ones. The posterior, limbless trunk region usually has no segmental structures, although there are a few exceptions, such as the platycopid ostracods. Therefore, ostracods are regarded, at least morphologically, as the most oligosegmented crustaceans. The engrailed gene has been known as a segmentation marker in arthropods and their relatives. The expression of two engrailed genes, Vh-en-a and Vh-en-b, was examined in the embryos of the myodocopid ostracod Vargula hilgendorfii (Müller, 1890). Both genes were expressed in the five pairs of anterior embryonic limb buds and segments and in the prospective sixth and seventh limb segments. Vh-en-b expression was also observed in the posterior limbless trunk as two stripes, which should be considered vestiges of trunk segmentation. [ABSTRACT FROM AUTHOR]

The post-embryonic development of a species of the enigmatic crustacean group Remipedia is described in detail for the first time under various aspects. Applying a molecular approach, we can clearly prove the species identity of the larvae as belonging to Pleomothra apletocheles. We document the cellular level of several larval stages and the differentiation of segments, limbs, and the general body morphology applying the techniques of confocal laser scanning microscopy and scanning electron microscopy. In addition, we document the swimming behavior and the peculiar movements of the naupliar appendages. A comparison of our results with published data on other Crustacea and their larval development tentatively supports ideas about phylogenetic affinities of the Remipedia to the Malacostraca. [ABSTRACT FROM AUTHOR]

Seventy trees from seven stands 50-650 years old were selected for this investigation of crown structural development in Pseudotsuga menziesii. All branches, limbs, and trunks were nondestructively measured for size, structure, and location while climbing the trees with ropes. These data were used to generate a computer model of each tree's crown that was error-checked trigonometrically. Leaves, bark, cambium, and wood were quantified by using limited destructive sampling to develop predictive equations that were applied to the complete inventory of structures in each tree's crown. Summations of these values yielded whole-tree estimates of several structural variables. A second set of equations was then developed to predict these whole-tree parameters from simple, ground-based measurements. Principal components analysis of 24 tree-level variables revealed two orthogonal dimensions of structure that accounted for 71.3% and 12.4% of total variation in the 70 trees. The first dimension represented a gradient of overall tree size and structural complexity that was positively correlated with tree age (R2 = 0.89). The second dimension represented a gradient of incremental growth potential that was positively correlated with measured growth of main trunks at breast height during the five-year period after trees were mapped (R2 = 0.39). The strongest determinant of incremental growth potential was the ratio of a tree's total cambium surface area to projected leaf area. A tree's incremental growth potential may thus be unrelated to either size or structural complexity and depend instead on factors related to whole-tree carbon balance (e.g., how much the tree invests in fighting and feeding fungi). Our intensive analyses of Pseudotsuga menziesii and extensive observations of other species were used to develop a conceptual model of crown development in tall conifers. Trees respond to disturbances by growth of replacement structures whose form depends on disturbance intensity and age. Young trees rebuild damaged crowns through epicormic shoot production on original branches, whereas older trees also recover by producing new, epicormic branches. Damage to main trunks and top dieback stimulate production of reiterated trunks. Repeated damage leads to multiple hierarchical levels of trunk reiteration as well as to limb formation and increasing tree individuality. [ABSTRACT FROM AUTHOR]

Members of the CD44 protein family are engaged in various physiological and pathological processes such as lymphocyte activation, lymphocyte homing, wound healing, delayed type hypersensitivity, apoptosis, tumor progression and limb development. Here we summarize observations which lead to a molecular understanding of the function of CD44 in some of these physiological processes. The most striking function for a CD44 variant protein is its presentation of growth factors to high affinity receptors on mesenchymal cells in the developing limb. Furthermore we summarize the experiments that allowed the identification of CD44 variant exon encoded sequences which account for differentiation specific and inducible splice regulation. [ABSTRACT FROM AUTHOR]