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A brief review of Becke's (1908) replacement and Schwantke's (1909) exsolution models for myrmekite genesis and further consideration of the morphology and spatial distribution of myrmekite as outlined by Phillips (1974) lead to the conclusion that both hypotheses have a place in explaining the origin of myrmekite. The Schwantke model is generally best applied, for example, to high-level undeformed massive granitoids and the Becke model to deformed metamorphic rocks. The latter hypothesis and a third model involving interaction between exsolution and replacement (Ashworth, 1972) may be used especially to explain the association of muscovite and myrmekite in rocks whose mineral assemblages have undergone retrograde modification, involving either redistribution of Na, Ca and K by local metasomatic reactions among the mineral grains, or metasomatic change of bulk rock composition.

Biomimetic proteoglycan (BPG) diffusion into articular cartilage has the potential to restore the lost proteoglycan content in osteoarthritic cartilage given these molecules mimic the structure and properties of natural proteoglycans. We examined the diffusion characteristics of BPGs through cartilage with the use of a custom‐made in vitro cartilage diffusion model in both normal bovine and human osteoarthritic cartilage explants. BPGs were introduced into the cartilage through essentially one‐dimensional diffusion using osteochondral plugs. The molecular diffusion was shown to be size and concentration dependent. Diffusion profiles were found over different diffusion time intervals and the profiles were fit to a nonlinear Fickian diffusion model. Steady state 011012‐7diffusion coefficients for BPGs were found to be 4.01 and 3.53 μm2/s for 180 and 1600 kDa BPGs, respectfully, and these values are similar to other large molecule diffusion in cartilage. In both bovine and osteoarthritic human cartilage, BPGs were found localized around the chondrocytes. BPG localization was examined by labeling collagen type VI and soaking 5 μm thick sections of cartilage with BPG solutions demonstrating that the BPGs diffused into the cartilage and preferentially localized alongside collagen type VI in the pericellular matrix.

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Molecular engineering of biological tissues using synthetic mimics of native matrix molecules can modulate the mechanical properties of the cellular microenvironment through physical interactions with existing matrix molecules, and in turn, mediate the corresponding cell mechanobiology. In articular cartilage, the pericellular matrix (PCM) is the immediate microniche that regulates cell fate, signaling, and metabolism. The negatively charged osmo-environment, as endowed by PCM proteoglycans, is a key biophysical cue for cell mechanosensing. This study demonstrated that biomimetic proteoglycans (BPGs), which mimic the ultrastructure and polyanionic nature of native proteoglycans, can be used to molecularly engineer PCM micromechanics and cell mechanotransduction in cartilage. Upon infiltration into bovine cartilage explant, we showed that localization of BPGs in the PCM leads to increased PCM micromodulus and enhanced chondrocyte intracellular calcium signaling. Applying molecular force spectroscopy, we revealed that BPGs integrate with native PCM through augmenting the molecular adhesion of aggrecan, the major PCM proteoglycan, at the nanoscale. These interactions are enabled by the biomimetic “bottle-brush” ultrastructure of BPGs and facilitate the integration of BPGs within the PCM. Thus, this class of biomimetic molecules can be used for modulating molecular interactions of pericellular proteoglycans and harnessing cell mechanosensing. Because the PCM is a prevalent feature of various cell types, BPGs hold promising potential for improving regeneration and disease modification for not only cartilage-related healthcare but many other tissues and diseases.

Biomimetic proteoglycans (BPGs) have the potential to treat osteoarthritis (OA) given that these molecules mimic the structure and properties of natural proteoglycans, which are significantly reduced in OA. We examined the effects of BPGs injected into the intra-articular space in an in vivo OA rabbit knee model and evaluated the effect on histological response, joint friction, and BPG distribution and retention. Rabbits underwent ACL transection to create an arthritic state after 5 weeks. OA rabbits were treated with BPGs or Euflexxa 1 (hyaluronic acid) intra-articular injections. Non-OA rabbits were injected similarly with BPGs; contralateral joints served as controls. The progression of OA and response to injections were evaluated using Mankin and gross grading systems indicating that mild OA was achieved in operated joints. The coefficient of friction (COF) of the intact knee joints were measured using a custom pendulum friction apparatus, showing that OA joints and OA þ Euflexxa 1 joints demonstrated increased COF than nonoperated controls, while BPG-injected non-OA and OA þ BPGs were not significantly different from non-OA controls. Injected fluorescently labeled BPGs demonstrated that BPGs diffused into cartilage with localization in the pericellular region. ß

SUMMARY. Retrograde metamorphism of gneisses and pegmatites leads in part to the destruction of feldspar and its replacement by late-stage lobate myrmekite and muscovite. Reactions promoted by retrogression suggest a range in volume of quartz production that may supplement that developed by exsolution and lead to deviations from the strict proportionality relationship suggested by previous workers. There is no need, however, to propose that quartz in myrmekite originates by constriction of pre-existing quartz within exsolved albite.RECENT papers by Phillips and Ransom (I97O) and Shelley (197o, have drawn attention to the co-existence of muscovite and myrmekite in certain metamorphic rocks. Concomitant with this association in some gneisses is the apparent paucity of potash feldspar. This stands in contrast to many other occurrences, particularly in igneous rocks, where rim and intergranular myrmekite, usually without muscovite but commonly with secondary albite, are associated with well-developed perthite (e.g. Phillips, I964).Preliminary petrographic investigations of the Potosi gneiss at Broken Hill revealed the development of a lobate myrmekite associated with fine-grained muscovite and a corresponding low potash feldspar content (Phillips and Ransom, x97o, P. 729). It was suggested that such an association was connected with retrograde metamorphism affecting some of the Broken Hill gneisses (Hobbs et al., 2968, p. 296; Vernon, 1969; Vernon and Ransom, 1971). Additional work has shown that a similar myrmekitemuscovite association occurs in some deformed pegmatites of the Broken Hill region. This paper presents descriptions of the myrmekite-muscovite intergrowths occurring in both the pegmatites and the gneisses and discusses their mode of origin.Myrmekite and muscovite in the pegmatites. Coarse-grained pegmatites occur in many of the high-grade metamorphic rocks of the Broken Hill area (Vernon, 1969, PP. 33-4). Where intersected by retrograde schist-zones they have been extensively deformed and partly recrystallized. Deformed pegmatites have been examined in detail in the area 9

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