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Biomedical subjects

D W Leonard

Publications and source records attributed to D W Leonard.

At least 19 recordsLinked to original sources
PubMed2003

Transparency, swelling and scarring in the corneal stroma.

PURPOSE: This paper briefly reviews current explanations for corneal transparency and uses a well-developed model to try to explain the increased light scattering either accompanying corneal swelling or following phototherapeutic keratectomy (PTK). METHODS: The direct summation of fields (DSF) method was used to compute light transmission as a function of wavelength. The method requires input of a number of structural parameters. Some of these were obtained from electron micrographs and others were calculated from X-ray diffraction data. RESULTS: By swelling sections of stroma cut from different depths in the tissue, we have shown that fluid entering the cornea causes more swelling in the posterior lamellae than in the anterior lamellae. Furthermore, posterior lamellae can reach a higher final hydration than anterior lamellae. Collagen-free regions ('lakes') exist in corneas swollen in vitro and in Fuch's dystrophy corneas, many of which may be caused by the death of cells. The DSF method shows that local fibril disordering, increased refractive index mismatch, and increased corneal thickness together can account for a 20% increase in light scattering in a Fuch's dystrophy cornea at H=5.8 compared to the normal cornea. Additional scattering is probably caused by 'lakes'. The DSF method applied to PTK rabbit stroma with high levels of haze suggests that the newly deposited collagen is not the cause of the increased light scattering. CONCLUSIONS: Fluid is not uniformly distributed within the corneal stroma when the cornea swells. Increased hydration of posterior lamellae may be because of known differences in the glycosaminoglycans between the anterior and posterior stroma. Lamellar interweave in the anterior stroma probably limits the extent to which the constituent lamellae can swell. The DSF method can be used to account for increased light scattering in oedematous corneas but cannot account for haze following PTK.

Corneal Edema↗
PubMed2001

The Cytoscan model E-II in intraoperative parathyroid gland identification in a rabbit model.

BACKGROUND: Intraoperative parathyroid gland identification and preservation is often a challenge even in the hands of experienced surgeons as they could be indistinguishable from fat or thyroid tissue. OBJECTIVE: The goal of this study was to demonstrate the use of the Cytoscan Model E-II, which uses orthogonal polarization spectral (OPS) imaging technology, as an intravital microscope in identifying parathyroid glands intraoperatively and differentiating the parathyroid glands from fat and thyroid tissue in a rabbit model. METHODS: The necks of 4 New England white rabbits were explored with the animals under a general anesthesia. The Cytoscan was used to obtain images of the vasculature of tissue suspected to be parathyroid, fat, and thyroid tissue. These were confirmed by histologic evaluation. RESULTS: All tissues were correctly identified by the Cytoscan and confirmed by histologic analysis. There was an obvious difference in the images obtained of fatty tissue as compared with parathyroid tissues. There was also an appreciable difference between parathyroid and thyroid tissue based on the difference in vascularity. CONCLUSIONS: OPS imaging technology can be used in identifying parathyroid glands based on the difference in vascularity from fat and the pattern and density of vessels when compared with thyroid tissue in a rabbit model. SIGNIFICANCE: The Cytoscan may play a future role in real time intraoperative identification of human parathyroid glands. Future investigation is warranted.

Adipose Tissue↗
PubMed1998

Unusual septoplasty complication: Streptococcus viridans endocarditis.

Infection is an infrequently reported complication following septoplasty and septorhinoplasty. Among the recognized but rare infections are toxic shock syndrome, spinal osteomyelitis, meningitis, septic cavernous sinus thrombosis and endocarditis. A high index of suspicion is required to diagnose these infections early and thereby minimize morbidity and mortality. We present a case of endocarditis following septoplasty in a patient who had no identifiable preoperative risk factors but who experienced recurrent fever and chills postoperatively.

Adult↗
PubMed1997

Refractive indices of the collagen fibrils and extrafibrillar material of the corneal stroma.

Ultrastructural data from x-ray diffraction studies of the cornea were used to estimate the refractive indices of the collagen fibrils and extrafibrillar material of human, ox, trout, and rabbit corneas. X-ray diffraction measurements of the size and spacing of the collagen fibrils and the separation between the constituent molecules of the fibrils were taken from a previous species study. The tissue volume fractions occupied by the stromal components were estimated and their refractive indices were calculated using the Gladstone-Dale law of mixtures. For the fibrils and extrafibrillar material, the refractive indices in the human cornea were 1.411 and 1.365; for the ox 1.413 and 1.357; for the rabbit 1.416 and 1.357; and for the trout 1.418 and 1.364, respectively. An alternative estimate based on the physical properties and chemical composition of bovine cornea, accounting for interfibrillar type VI collagen and cellular water, produced similar estimates of 1.416 and 1.356 for the fibrils and extrafibrillar material, respectively.

Animals↗
PubMed1997

X-ray diffraction and transmission electron microscopy of Morquio syndrome type A cornea: a structural analysis.

PURPOSE: This case report describes the structural characterization of the corneal stroma from a patient with Morquio syndrome type A. METHODS: A left penetrating keratoplasty was performed, and the cornea was examined using transmission electron microscopy and synchrotron x-ray diffraction. The interfibrillar proteoglycans were visualized in the electron microscope by using cuprolinic blue. RESULTS: Stromal collagen fibrils showed a bimodal distribution of diameters: 70% had a distribution comparable to that in normal tissue (20-30 nm) and 30% contained larger fibrils (30-42 nm) as seen by electron microscopy. Both electron microscopy and x-ray diffraction showed that the bulk numeric density of fibrils per unit area in cross-section (number density) was higher than normal in the Morquio syndrome cornea. The arrangement of proteoglycans throughout most of the Morquio syndrome cornea appeared normal, but many of the filaments were twice their normal length. In the anterior stroma, very large proteoglycan filaments (< or = 400 nm long) were found. Other ultrastructural differences also were noted, including abnormal keratocytes and long spacing collagen. CONCLUSION: The variation in fibril diameter and number density were modeled to account for only a 5% decrease in light scattering compared with the normal cornea. The extensive corneal clouding seen in the Morquio syndrome cornea cannot therefore be attributed to the variation in fibril diameters; collagen-free areas and expanded cells seem to be the most likely cause.

Adult↗
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

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↗
PubMed1975

Partial reinforcement effects in classical aversive conditioning in rabbits and human beings.

The partial reinforcement extinction effect (the PREE) in classical aversive conditioning was investigated in 2 experiments. In the first, the nictitating membrane responses of 120 rabbits were conditioned at a 250-msec. interstimulus interval (ISI) under continuous reinforcement, partial reinforcement with the unconditioned stimulus (US) omitted (Group PO), or partial reinforcement with the US delayed to 1,500 msec. (Group PD). These 3 groups were factorially extinguished under US-Omitted, US-Unpaired, or US-Delayed extinction regimens. A significant PREE was obtained, but only for PO training and US-Omitted extinction. The second experiment, employing human subjects in a masked eye blink conditioning task, produced parallel results. A general discrimination view of the classical PREE seems applicable, but one in which neither cognitive factors nor intertrial conditioning of reinforcement aftereffects play a significant role.

Acoustic Stimulation↗