PubMed Health⌕ Search

Biomedical subjects

K M Meek

Publications and source records attributed to K M Meek.

At least 37 records · Page 2Linked to original sources
PubMed1996

Ultrastructural study of the cornea in a bone marrow-transplanted Hurler syndrome patient.

This case report describes a 14-year-old girl with Hurler syndrome, who had received a successful bone marrow-transplant at the age of two. Corneal clouding was present at the time of transplant and has only partially cleared. A right penetrating keratoplasty was performed and the corneal specimen was examined by light microscopy, transmission electron microscopy with Cuprolinic blue staining for proteoglycans, and low-angle X-ray diffraction. The results show the corneal stroma to be disrupted by vacuolated stromal cells. There is abnormal accumulation of proteoglycans in the vacuolated stromal cells and nearby stroma. These proteoglycans mainly contain chondroitin/dermatan sulphate glycosaminoglycans since they are susceptible to chondroitinase ABC. There are a large range of fibril diameters (12.5-50.1 nm) and there is an abnormal distribution of the fibril diameters measured from micrographs. Both are confirmed by X-ray diffraction results (the mean collagen fibril diameters are in a range between 29.7 and > 51.1 nm). X-ray diffraction also shows that the mean centre-to-centre distance of the fibrils slightly increases. These findings suggest that proteoglycans play a role in modelling the stromal structure and can also explain the corneal clouding. Many long-spacing collagen structures with a mean periodicity of 91.8 nm are observed in the corneal stroma. The finding that the long-spacing collagen consists of fine collagen fibrils and that very few proteoglycans filaments bind to them suggests that some change in the interaction of proteoglycans and collagen is responsible for the formation of long-spacing collagen. To our knowledge, this is the first ultrastructural study of the cornea from a bone marrow-transplant patient with Hurler syndrome. The structural features documented here relate to a cornea incompletely corrected by bone marrow transplantation.

Adolescent↗
PubMed1996

Vitamins and analgesics in the prevention of collagen ageing.

The ageing of connective tissues involves modifications of collagen, which are currently generating much interest amongst protein researchers. Protein glycation, a non-enzymic reaction involving sugar, appears to play a role in the evolution of age-related physical changes and diabetic complications-retinopathy, neuropathy, renal failure and atherosclerosis. Our studies show that the glycation of human corneal and scleral collagen produces increases in the collagen intermolecular spacing-these increases are similar to those we previously reported on the ageing of collagen in these tissues. The present investigation employs X-ray diffraction to look at the structural effects of various substances that are believed in inhibit protein glycation. Aspirin-like compounds and certain vitamins successfully prevented the sugar-induced molecular changes from occurring in corneal and scleral collagen, suggesting that such compounds could have a useful role in this aspect of ageing.

Acetaminophen↗
PubMed1996

A histochemical and x-ray diffraction study of keratoconus epikeratoplasty. Report of two cases.

Two epikeratoplasty lenticules (Epi-L) with keratoconus host corneas (Epi-H) were removed by penetrating keratoplasty because of poor visual acuity although they were clinically clear. Light microscopy showed that the keratocytes were underpopulated and unevenly distributed in Epi-L. Transmission electron microscopy showed that abnormal sized proteoglycans (PGs) were unevenly accumulated in the stromas of both Epi-L and Epi-H. The low-angle x-ray diffraction results from case 1 showed lower collagen interfibrillar spacings, compared with those from one keratoconus cornea, seven cryolenticules, and four normal human corneas. The fibril diameters and the collagen axial periodicity D were similar in all tissues. Allowing selected samples to swell, the interfibrillar spacing increased by 30.6-45.5% in Epi-H and by 29.8% in the keratoconus control. However, it increased by only 7.6-9.7% in Epi-L and by 11.4% in a cryolenticule. The uneven distribution of PGs and the lower interfibrillar spacings in the swollen lenticules suggest that there is an uneven distribution of water in different parts of the Epi-L and Epi-H stromas. This may lead to a variation of the refractive index or optical properties in the stroma and the deterioration of the visual image, which may explain the poor visual results.

Adult↗
PubMed1994

The inhibition of sugar-induced structural alterations in collagen by aspirin and other compounds.

With age human collagen demonstrates, amongst other changes, reductions in solubility, elasticity and permeability. Many of these changes have been attributed to non-enzymic glycosylation (glycation)-a spontaneous addition of sugar molecules to any protein with free amino groups. The resulting formation and accumulation of Advanced Glycation End-products, some of which may be cross-links, has been shown in both long- and short-lived proteins. We have shown that glycation of human corneal and scleral collagen increases with age and that this is accompanied by increases in cross-linking and collagen intermolecular spacing. We have now investigated several compounds that have been used to inhibit glycation, including aspirin, and have shown that all the inhibitors also prevent the increase in intermolecular spacing caused by glycation.

Aspirin↗
PubMed1994

An ultrastructural, time-resolved study of freezing in the corneal stroma.

Synchrotron X-ray diffraction was used to monitor the changes occurring in the extracellular matrix of the corneal stroma as a result of freezing and thawing. The parameters monitored were the lateral centre-to-centre spacing between the collagen molecules within the fibrils (intermolecular spacing) and the centre-to-centre spacing between the collagen fibrils (interfibrillar spacing). Our findings suggest that, while frozen, the fibrils are reduced in diameter and are forced into close association with each other. The data also suggest that the extrafibrillar components of the cornea may become concentrated around the fibrils during freezing. However, X-ray patterns of thawed corneas show normal interfibrillar and intermolecular spacings. Time-resolved data show that, as thawing takes place, the fibrils gradually separate and regain their normal spacing while at the same time regaining their normal diameter. It seems probable that the mechanism which allows the fibrils to regain their normal arrangement after thawing involves charge interactions between the proteoglycans associated with the fibrils. However, unlike corneas at physiological hydration, certain regions of the stroma of swollen corneas do suffer irreversible damage as a result of freezing. It is possible that this ice damage may occur in regions of abnormal fibril arrangement called "lakes", which are reported to occur in swollen cornea.

Animals↗
PubMed1994

Structure of corneal scar tissue: an X-ray diffraction study.

Full-thickness corneal wounds (2 mm diameter) were produced in rabbits at the Schepens Eye Research Institute, Boston. These wounds were allowed to heal for periods ranging from 3 weeks to 21 months. The scar tissue was examined using low- and wide-angle x-ray diffraction from which average values were calculated for 1) the center-to-center collagen fibril spacing, 2) the fibril diameter, 3) the collagen axial periodicity D, and 4) the intermolecular spacing within the collagen fibrils. Selected samples were processed for transmission electron microscopy. The results showed that the average spacing between collagen fibrils within the healing tissue remained slightly elevated after 21 months and there was a small increase in the fibril diameter. The collagen D-periodicity was unchanged. There was a significant drop in the intermolecular spacing in the scar tissues up to 6 weeks, but thereafter the spacing returned to normal. The first-order equatorial reflection in the low-angle pattern was visible after 3 weeks and became sharper and more intense with time, suggesting that, as healing progressed, the number of nearest neighbor fibrils increased and the distribution of nearest neighbor spacings reduced. This corresponded to the fibrils becoming more ordered although, even after 21 months, normal packing was not achieved. Ultrastructural changes in collagen fibril density measured from electron micrographs were consistent with the increased order of fibril packing measured by x-ray diffraction. The results suggest that collagen molecules have a normal axial and lateral arrangement within the fibrils of scar tissue. The gradual reduction in the spread of interfibrillar spacings may be related to the progressive decrease in the light scattered from the tissue as the wound heals.

Animals↗
PubMed1994

Remodelling of the corneal stroma after lamellar keratoplasty. A synchrotron x-ray diffraction study.

Patients healing from lamellar keratorefractive surgeries invariably experience postoperative corneal haze. In our lamellar keratoplasty (LKP) rabbit model, visual recovery is concomitant with the invasion of the grafted tissue by viable keratocytes from adjacent host tissue, and the return of corneal clarity is attributed to the remodelling of the stromal tissue by these keratocytes. We used synchrotron x-ray diffraction techniques to elucidate the ultrastructure of the stromal collagen fibrils in rabbit corneas at various time points in the 3 months after LKP surgery. Lenticules were frozen in 50% of the cases. The average spacing of the collagen molecules, which constitute the stromal fibrils, remains unchanged by LKP in both frozen (1.69-1.73 nm) and nonfrozen (1.58-1.75 nm) cases. In the nonfrozen case, the collagen fibril diameters are initially slightly larger than normal (38.3-41.9 nm) but have receded by 2 months postoperatively [similar to the frozen cases (37.7-41.9 nm)]. The post-LKP spacing of the collagen fibrils in the nonfrozen corneas is unremarkable (56.2-69.2 nm). In contrast, the increased collagen interfibrillar spacing in the frozen case is considerable (56.7 to > 94.6 nm) and variable up to 21 days postoperatively. Because the changes in interfibrillar spacing did not always mirror pachymetry changes in the frozen cases, we suspect occasional graft malapposition or the formation of intrastromal, fluid-filled, collagen-free "lakes."

Animals↗
PubMed1993

Synchrotron X-ray diffraction and histochemical studies of normal and myopic chick eyes.

Synchrotron X-ray diffraction patterns were obtained from the cornea and posterior sclera of control and myopic chicks. No significant differences was found in the interfibrillar or in the intermolecular spacing of the collagen fibrils from the corneas of control and myopic chicks. The intermolecular spacing of myopic sclera was shown to be significantly (p < 0.01) higher than in controls. Sclera and cornea from normal and myopic chicks were stained for proteoglycans using the 'critical electrolyte' method of Scott and Orford (1981). In the sclera, two morphologically distinct types of proteoglycans could be distinguished; one small and usually elongated (approximately 20 nm), the other larger and irregularly shaped. The small proteoglycans were seen binding preferentially to the 'd' and 'e' bands of the collagen fibrils. Small proteoglycans were also present within the fibrils, these were usually approximately 10 nm in diameter although sizes up to 30 nm were observed. Longitudinal sections of fibrils revealed that these intrafibrillar proteoglycans were chiefly orientated parallel to the axis of the collagen fibrils, and preferentially located along the gap region of the fibrils. No difference was observed in the binding sites of the proteoglycans between normal and myopic sclera. The larger proteoglycans were also seen aggregated into clumps, which were usually associated with spaces between collagen bundles. The differences between control and myopic sclera appear to be quantitative rather than qualitative suggesting that the scleral expansion in myopia is an enhanced form of normal scleral growth.

Animals↗
PubMed1993

Ultrastructure of the corneal stroma: a comparative study.

Using a high intensity synchrotron x-ray source, we have recorded diffraction over a range of angles from the corneas of a wide variety of species. The results show that the interfibrillar Bragg spacing varies from 39 nm to 67 nm, the fibril diameter varies from 24 nm to 43 nm, but in the species studied intermolecular Bragg spacing is constant (1.58 +/- 0.03 nm). Using these data, a number of other structural parameters were calculated including the interfibrillar volume, V, and the surface-to-surface fibril separation, S. Large variations were found, particularly between aquatic and terrestrial animals. We found that the parameter which appears to be most constant throughout the species was the volume fraction, that is, the proportion of the tissue occupied by the hydrated fibrils. Ignoring the volume of the stroma occupied by cells, the tissue fibril volume fraction was (28 +/- 3)% for both aquatic and land animals. The observation of a constant volume fraction led us to propose a simple model in which collagen molecules and interfibrillar glycosaminoglycans occur in a fixed ratio in all the species--thus species with narrow fibrils have fewer interfibrillar glycosaminoglycans and the fibrils are thus more closely spaced, and vice versa. This model agrees with many of the experimental data on corneal composition and on the physical properties of the tissue reported in the literature.

Animals↗
PubMed1993

Synchrotron X-ray diffraction in atypical macular dystrophy.

This report documents the first case of X-ray diffraction techniques aiding the diagnosis of a corneal dystrophy with a clinically ambiguous presentation. Post-operatively, a high-angle synchrotron X-ray diffraction pattern was obtained from an in vitro portion of a pathological cornea. This pattern displayed two X-ray reflections which we recently demonstrated to be unique to the high-angle X-ray diffraction patterns of both type I and type II macular dystrophy corneas; on the basis of this evidence we were able to offer a post-operative diagnosis of macular corneal dystrophy. An electron microscopical evaluation of the cornea revealed stromal lacunae at all levels and an extensive layer of vacuoles, predominantly between Bowman's layer and the anterior stroma. These vacuoles were often associated with large proteoglycan filaments, as identified by Cuprolinic blue staining. Abnormally large collagen fibrils were documented, for the first time, in a macular dystrophy cornea; they existed in localised regions, frequently adjacent to the vacuoles and abnormal proteoglycans, and could well have implications for corneal transparency. We propose that the dystrophy is an atypical variant of macular corneal dystrophy which is encompassed by the heterogeneous nature of the condition.

Aged↗
PubMed1993

A synchroton X-ray study of the changes occurring in the corneal stroma during processing for electron microscopy.

Using a high-intensity synchroton X-ray source, the structural changes occurring in the corneal stroma were monitored during each stage of several different processing runs for the transmission electron microscope (TEM) and scanning electron microscope (SEM). The parameters studied were interfibrillar spacing, intermolecular spacing, D-periodicity and fibril diameter. The processing schedule that produced the least changes in spacings for TEM specimens involved extended fixation in glutaraldehyde followed by low-temperature embedding in Lowicryl K4M resin. However, interfibrillar material was better preserved after embedding in LR White resin or Nanoplast. Almost every processing stage for electron microscopy produced significant changes in one or more structural parameters in the cornea. Glutaraldehyde fixation significantly increased the intermolecular spacings, while resin infiltration and resin polymerization each resulted in shrinkage of all the spacings monitored. Critical-point drying for SEM specimens resulted in considerable shrinkage in all three spacings, but was still preferable to air drying, which caused reduction in the order of the fibril packing, resulting in loss of the interfibrillar X-ray pattern. Perhaps the most drastic effect was caused by post-fixation in osmium tetroxide, which resulted in loss of the intermolecular pattern, and also increased the amount of shrinkage in the interfibrillar spacings and the D-periodicity which occurred during later stages of processing.

Animals↗
PubMed1993

Scheie's syndrome: the architecture of corneal collagen and distribution of corneal proteoglycans.

Processes that modulate the regular architecture and, hence, transparency of the cornea are poorly understood, although proteoglycans are thought to be involved. Scheie's syndrome displays corneal opacification and systemic accumulation of glycosaminoglycans. The manifestations of these two occurrences were examined in relation to the corneal stroma. Collagen architecture was investigated by transmission electron microscopy and synchroton x-ray diffraction. Cuprolinic blue staining located sulfated glycosaminoglycan deposits that disrupted the extracellular matrix. Unlike normal cornea, which contained collagen fibrils of remarkably uniform diameter (26.0 +/- 2.4 nm), there was a large range of fibril sizes in the Scheie's syndrome stroma (19.9 to 52.0 nm). Moreover, the distribution of fibril diameters appeared bimodal. X-ray diffraction confirmed the discovery of abnormally large stromal collagen. The results suggest a link in Scheie's syndrome between proteoglycan content/distribution and stromal disruption, and between stromal disruption and corneal opacification.

Collagen↗
PubMed1992

Ageing of the human corneal stroma: structural and biochemical changes.

High and low angle X-ray diffraction patterns from the corneal stroma give information about the mean intermolecular spacing of the collagen molecules and the mean interfibrillar spacing of the collagen fibrils, respectively. X-ray data were collected, using a high intensity synchrotron source, from human corneas and sclera at approximately physiological hydration. The spacings were measured as a function of tissue age. Between birth and 90 years there is an increase in the cross-sectional area associated with each molecule in corneal collagen from approx. 3.04 nm2 to 3.46 nm2, and an increase in scleral collagen from approx. 2.65 nm2 to 3.19 nm2. These changes may be due to an increase in the extent of non-enzymic cross-linking between collagen molecules over the age range. We have investigated this possibility by measuring collagen glycation using the thiobarbituric acid assay and the subsequent advanced glycation end-products (AGEs) using fluorescence emission. The results obtained have shown an age-related increase in glycation and AGEs in both tissues. We have also demonstrated a decrease in the interfibrillar spacing of corneal collagen with increasing age which may be related to changes in the proteoglycan composition of the interfibrillar matrix.

Adolescent↗
PubMed1992

Proteoglycan and collagen morphology in superficially scarred rabbit cornea.

We have examined the changes in collagen and proteoglycan morphology in superficial lamellar keratectomy wounds produced in rabbit corneas. The ultrastructural location within the tissue of keratan sulphate and chondroitin sulphate proteoglycans was demonstrated using the cationic dye Cuprolinic Blue under critical electrolyte conditions. Large proteoglycan filaments (up to 500 nm long) appeared in the early stages of wound healing; these were most common after two weeks' wound healing, after which they decreased both in number and size. At these early stages of scar formation, spaces containing proteoglycans were present amongst bundles of collagen fibrils. As proteoglycans play an important role in controlling corneal hydration, the presence of the large proteoglycan-filled spaces would result in an abnormally high water content which is found in early scar tissue.

Animals↗
PubMed1992

A morphological study of rabbit corneas after laser keratectomy.

We have examined the morphology of the collagen and proteoglycans in rabbit corneas that have undergone excimer laser photorefractive keratectomy using a clinical, 193 nm excimer laser. The photoablation was carried out to a stromal depth of 100 microns and a diameter of 6 mm. All ablated corneas developed a haze that was most intense between week 4 and week 8 and which showed no improvement after week 16. The corneas were stained with the cationic dye cuprolinic blue to visualise proteoglycans and were then processed for transmission electron microscopy. The ultrastructural location of proteoglycans (keratan sulphate and dermatan sulphate) was observed in the corneal wounds at different time intervals. Corneas that had undergone steroid treatment post-operatively were also examined. In the healing tissue proteoglycan filaments of abnormal size were observed, which became most prominent after 2 weeks. As healing progressed these abnormal filaments decreased but after 45 weeks some were still present, indicating that the proteoglycan content had not returned to normal.

Animals↗
PubMed1992

Synchrotron x-ray diffraction studies of keratoconus corneal stroma.

The aim of this study was to determine any differences in the collagen molecular and fibrillar packing, or the arrangement of the proteoglycans along the fibril axis, in the stroma of keratoconus and control corneas. High and low-angle x-ray diffraction patterns from the fibrillar and molecular packing of collagen in keratoconus and control corneas were obtained using a synchrotron radiation source. The results indicate no difference in interfibrillar spacing between keratoconus and control corneas at normal physiological hydration, or over a range of hydrations (H = 1-11). This unambiguously demonstrates that the thinning of the stroma that occurs in keratoconus is not a result of closer packing of the collagen fibrils in the stroma. Intermolecular spacings were shown to be significantly (P less than 0.001) lower in keratoconus corneas at normal physiological hydration and over a range of hydrations (H = 1-11). Meridional patterns from the axial distribution of electron density along the collagen fibrils were obtained from untreated control and keratoconus corneas and from the corneas after their proteoglycans were stained with cupromeronic blue. Analysis of the integrated intensities of the first nine orders of these reflections show there is a difference in the staining behavior of collagen-associated proteoglycans in control and keratoconus corneas. Determination of the electron density vectors along the collagen fibrils of cupromeronic blue-stained corneas by the use of Patterson functions indicates that the keratoconus corneal stroma has a specific, ordered proteoglycan that is present in lower numbers along the collagen fibrils, and that it stains less with cupromeronic blue or is in a more disordered arrangement than in the controls.

Adult↗
PubMed1992

Analysis of high-angle synchrotron x-ray diffraction patterns obtained from macular dystrophy corneas.

Six macular dystrophy corneas from six individuals were investigated by synchrotron x-ray diffraction. Immunochemical evidence demonstrated that five of the patients had type I macular corneal dystrophy and that one had type II. Analysis of the high-angle x-ray diffraction patterns showed that the intermolecular spacing of macular dystrophy corneal collagen increases with hydration from the dry state in a fashion similar to the intermolecular spacing of normal human corneal collagen. The high-angle x-ray diffraction pattern from all six corneas contained two "extra reflections" not obtained from other human corneas, normal or pathologic. The reflections, which arise from 4.61 and 9.62 A periodic structures, are independent of the type of macular dystrophy. Furthermore, their position is independent of corneal hydration. We propose that a glycosaminoglycan structure is the origin of the unique extra reflections.

Collagen↗