993). 6. Crystal faces are described by a set of indices (hkl) that unequivocally define the orientation of the face relative to the crystallographic axes a, b, and c of the structure. The notation {h,k,l}, such as {OI I } , includes a set of faces-(OIT), (OTI), and (077)-that are identical to (01 1) because they are related to it by the symmetry of these crystals. 7. G. K. Ambady, Acta Crystallogr. Sect. 824, 1548 (1 968). 8. D. Hanein, B. Geiger, L. Addadi, Langmuir 9, 1058 (1 993). 9.' Many

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... dhesive proteins present in the extracellular matrices and in the blood contain the sequence RGD as their major cell recognition site. The RGD sequences of each of the adhesive proteins are recognized by at least one member of structurally related receptors of the integrin superfamily. The synthetic peptides, containing the RGD sequence, inhibit extracellular matrix-mediated cell adhesion. 10. Routinely, cell growth was performed in the presence of 10% fetal bovine serum, saturated with respect to the particular crystal used (saturated complete medium) 11. A6 cells were seeded on a mixture of calcium (S,S)-and (R,R)-tartrate tetrahydrate crystals in 5.5-cm Falcon (Plymouth, United Kingdom) culture dishes. The crystals were homogeneous in size, and the starting weight of each fraction was -60.0 mg. After 10 min, the cells were fixed with 2% glutaraldehyde in 0.1 M cacodylate buffer, pH 7.2, containing 5 mM CaCI,. The dishes were rinsed four times, postfixed with 1% osmium tetroxide in the same buffer, and dried. The fixation kept the cells in situ and enhanced contrast. The crystals on which cells were obsewed were manually separated from others under a Zeiss (Oberkochen, Germany) stereomicroscope, 80x magnification. The selected cellcoated crystals (16.2 mg) were dissolved in 1 ml of 1 M HCI, and the optical activity of the solution was measured with a Perkin-Elmer 141 polarimeter. The optical activity of the solution was a = +0.104, [a], = +6.30, corresponding to (R,R) Evidence of structural inhomogeneities in twopigh-transition-temperature superconductors, YBa2Cu,0,-, and Nd2-xCexCu0" is presented. When samples were illuminated by highly collimated x-rays produced on a synchrotron wiggler, small changes in the lattice were detected over a spatial scale of 10 micrometers. These changes are interpreted as evidence of variations in the oxygen content in one case and in the cerium content in the other; both affect the superconducting properties. The existence of such structural inhomogeneities brings into question whether exotic experimental results obtained from superconducting materials with high transition temperatures actually reflect intrinsic properties. High-temperature superconductors (HTS) still present many mysteries in both their chemical properties and the mechanism of superconductivity. One of the few characteristics that is universally accepted is that their coherence lengths, 5, are short (on the order of tens of angstroms). Because 5 is the decay distance of the superconducting order parameter, sample inhomogeneities, even in single crystals, strongly affect the