During epithelial-mesenchymal interactions associated with mammalian tooth development, epithelially-derived and mesenchymally-derived extracellular matrix molecules form a discrete dentine-enamel junction. The developmental and molecular processes required to form this junction are not known. To address this problem we designed studies to test the hypothesis that ectodermally-derived epithelial cells synthesize and secrete enamel proteins which function to nucleate and regulate the growth of enamel calcium phosphate crystals. Initial enamel crystals were detected separate from the adjacent dentine. Electron-microprobe analyses revealed that early enamel crystals were octacalciumphosphate or tricalciumphosphate rather than hydroxyapatite. Thereafter, enamel crystals became confluent with the adjacent, albeit significantly smaller hydroxyapatite crystals associated with mineralized dentine. Therefore, we interpret our data to indicate that de novo enamel crystal nucleation and growth are independent from the mineralization processes characterized for dentine. We further argue that gene expression of enamel protein appears to have a constitutive function during early enamel formation and that supramolecular aggregates of amelogenin and enamelin provide the microenvironment for the nucleation and crystal growth of the initial enamel matrix.
Initiated at CERN in 1992, the "World Wide Web" (WWW, including the HTML language and the HTTP protocol) [ I ] provides a standard graphical interface to the Internet. " Point-and-click" clients The Mercury Project will be decommissioned in March 1995 to prepare for a new project. 0-7803-1965-6/95 $4.00 01995 IEEE be possible to use this medium to allow low cost public access to a teleoperated robot, in effect providing: desktop teleoperation. As illustrated in Figure 1, we set up a SCARA-type robot arm over a semi-annular workspace containing sand and buried artifacts. We attached a CCD camera to the end of the arm along with a nozzle to direct air bursts into the sand. IEEE lnternotlonal Conference on Robotics and AutomationWe then developed an interface so this hardware could be controlled via the WWW.Our primary criterion was that the system be reliable enough to operate 24 hours a day and survive user attempts at sabotage. A practical criterion was that the system be low in cost as we had a limited budget. It is worth noting that the manufacturing industry uses similar criteria, reliability and cost, to evaluate robots for production. Thus our experience with RISC robotics [2] proved helpful.Our secondary goal was to create an evolving W site that would encourage repeat visits by users. Towu-d this end, all of the buried artifacts were derived from an ucnamed 19th Century text. Users are challenged to identify this text and thereby collectively solve the "puzzle". After tach 5-minute operating session, users are prompted to describe their findings and hypotheses in an ongoing Operator's Log.As of 1 February 1995, although the Log includes over 1000 pages of entries, the puzzle has yet to be solved.
It has been suggested that many geometric illusions are caused by the application of depth or size constancy rules to an image which does not have sufficient cues to establish that the elements lie in a flat plane. Thus, converging lines are taken as depth cues, and the attributed depth provides the basis for adjusting the perceived size of stimulus elements. It this is the case, one should not see a distortion of relative size if the disparity cues provide for strong statification, i.e., localization in depth of the linear perspective cues. This expectation is challenged by demonstrations that show distortions of relative size using random-dot stereograms. In 1971 Julesz provided such examples but did not comment on the implications for theories of depth. Here we redemonstrate these distortion of length and size in autostereograms which contain the Ponzo and Corridor configurations. The illusory distortions can be seen in the cyclopean view even though the linear perspective elements are well stratified. We suggest that the processing of binocular disparity cues, as required for judgments of absolute distance, may involve the dorsal stream of vision, i.e., activity passing into and including the parietal lobe. Pictorial cues, on the other hand, are likely passed through the ventral stream into the temporal lobe. The analysis of depth by this system provides for size constancy and, possibly, the calibration of relative motion.
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