The adhesion of cells to the extracellular matrix is a dynamic process, mediated by a series of cell-surface and matrix-associated molecules that interact with each other in a spatially and temporally regulated manner. These interactions play a major role in tissue formation, cellular migration and the induction of adhesion-mediated transmembrane signals. In this paper, we show that the formation of matrix adhesions is a hierarchical process, consisting of several sequential molecular events. One of the earliest steps in surface recognition is mediated, in some cells, by a 1 microm-thick cell-surface hyaluronan coat, which precedes the establishment of stable, cytoskeleton-associated adhesions. The earliest forms of these integrin-mediated contacts are dot-shaped FXs (focal complexes), which are formed under the protrusive lamellipodium of migrating cells. These adhesions recruit, sequentially, different anchor proteins that are involved in binding the actin cytoskeleton to the membrane. Conspicuous in its absence from FXs is zyxin, which is recruited to these sites only on retraction of the leading edge and the transformation of the FXs into a focal adhesion. Continuing application of force to focal adhesions results in the formation of fibrillar adhesions and reorganization of the extracellular matrix. The formation of these adhesions depends on actomyosin contractility and matrix pliability.

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Some crystalline salicylideneanilines (anils) undergo reversible colour change when irradiated with ultraviolet light (photochromy) . This phenomenon is largely topochemically dominated since : (i) no correlation exists between photosensitivity and the chemical nature of the ring-substituents, and (ii) polymorphic forms of a given anil differ markedly in light-sensitivity.Absorption spectra measured in solid films of several (monomorphic and dimorphic) anils in temperature ranges from -180" to near the melting points suggest the following classification : a-type ; photochromic, pale yellow a t all temperatures, not thermochromic : P-type ; not photochromic, colour varies with temperature, thermochromic.Further experimental data include the crystallographic constants of 15 anils in their various polymorphic forms, the fluorescence spectra of thermochromic and photochromic crystals, and the thermal fading rates of several photo-coloured materials, including O-deuterated anils.HAVING discussed in previous communications of this series the geometry of irreversible reactions in the solid state (dimerisation, isomerisation) we proceed to analyse in this and subsequent Papers the mechanism of reversible reactions. As example of the latter type of system we have chosen the photochemistry of anils of o-hydroxybenzaldehydes, partly because of the large amount of information available on the polymorphism and photo-behaviour of these compounds (see early summaries 1*2), and partly because of the ease with which they can be prepared. Preliminary accounts of our work on this system have been pre~ented.~" Photochromy.-Salicylideneanilines (I) (referred to as '' anils " in this Paper; and for convenience of discussion locants in the aniline ring are unprimed, those in the salicylidene ring are primed, when the term " a n i l " is used) are pale yellow to dark orange when freshly prepared. Some of the pale yellow anils are sensitive to ultraviolet light (eg., R' * N a 3' OH3655 A) in which their colour deepens to red or brown; certain anils exist in more than one crystal modification,not all of which are necessarily photochromic. Previous work6" has shown, and we have confirmed, that the colour change in photochromic crystals is not accompanied by any observable changes in the X-ray diffraction pattern and infrared (i.r.) spectrum of the crystal. Many previous attempts have been made to correlate photochemical activity with other properties of the members of this family of compounds; no definite correlation has as yet been established.The photo-colour can be "eradicated" by heat or irradiation with visible light (e.g., 4358 A). Fading rates are characteristic for each compound and, wherever polymorphism occurs, for each (photochromic) crystal modification. Stobbe had found that certain systems (fulgides, hydrazones, and stilbene derivatives) were photochromic 5' 2 R2* Part V, preceding paper.

During the years that he was at the Weizmann Institute of Science the late G. M. J. Schmidt initiated and developed a very vital research program on the relationship between the structures of organic crystals and the reactions which occur in these crystals. As the program developed the “solid‐state group” became larger and more diversified—X‐ray crystallographers, organic chemists, spectroscopists, and even theoreticans had their contributions to make. Professor Schmidt died in 1971. As one of the members of this group, I here present a review of recent developments in parts of the field of interest to me. Of the reactions of organic crystals which have been studied thus far a large number are induced by light and other radiation. This is because radiation, unlike most chemical reactants, can readily penetrate the surface of the crystal, and thus reach the sites one wishes to activate. It is my aim in this article to point out some of the principles operative in the photochemistry of organic crystals. I shall choose examples to illustrate these principles and will not attempt to cover all the various types of reaction and systems which have been studied. It is appropriate at this stage to pay tribute to the memory of another great scientist who recently passed away—Theodorr Föster. Many of the ideas which I develop here are extensions of Professor Förster's earlier work, as are so many of the recent developments in photochemistry.

Heparanase is an endo-beta-D-glucuronidase that cleaves heparan sulfate and is implicated in diverse physiological and pathological processes. In this study we report on a novel direct involvement of heparanase in cell adhesion. We demonstrate that expression of heparanase in nonadherent lymphoma cells induces early stages of cell adhesion, provided that the enzyme is expressed on the cell surface. Heparanase-mediated cell adhesion to extracellular matrix (ECM) results in integrin-dependent cell spreading, tyrosine phosphorylation of paxillin, and reorganization of the actin cytoskeleton. The surface-bound enzyme also augments cell invasion through a reconstituted basement membrane. Cell adhesion was augmented by cell surface heparanase regardless of whether the cells were transfected with active or point mutated inactive enzyme, indicating that heparanase functions as an adhesion molecule independent of its endoglycosidase activity. The combined feature of heparanase as an ECM-degrading enzyme and a cell adhesion molecule emphasizes its significance in processes involving cell adhesion, migration, and invasion, including embryonic development, neovascularization, and cancer metastasis.