Biomedical subjects
Publications and source records attributed to L S Kwok.
Junior researchers can be abused and bullied by unscrupulous senior collaborators. This article describes the profile of a type of serial abuser, the White Bull, who uses his academic seniority to distort authorship credit and who disguises his parasitism with carefully premeditated deception. Further research into the personality traits of such perpetrators is warranted.
This study presents a computational model of the growth of the normal human lens and the induction of spoke-like cortical cataract in the aging lens. The anterior lens is modelled as a 2-D disk with a circumferential germinative zone. Lens cortical fibre cells in the same generation cover the surface in three identical 120 deg growth wedge-shaped sectors, with centre cardinal fibres at the 90, 210 and 330 deg meridians. In the foetal lens all primary fibre cells begin to elongate simultaneously. Anterior migration is spatially asynchronous, where the centre fibre begins to move towards the anterior pole first. The fibres at the end of the sector move last in the anterior direction. Fibre elongation advanced at constant speed until the boundary of the sector is reached. Spatio-temporal asynchrony and random fluctuations were increased for the adult lens. The model foetal lens evolved Y-shaped sutures anteriorly, and an inverted Y-shaped posteriorly. Fibre length varied periodically with meridional angle. The adult lens displayed irregular growth. If clusters of germinative cells are caused to opacify the resultant opacities are predominantly spoke-shaped. The model mimics crystalline lens fibre growth to the extent of successfully evolving lens sutures. Fluctuations in lens mass are consistent with an ordered pattern of growth. Lens senescence includes a progressive loss of spatio-temporal synchrony in fibre migration from the germinative zone. Peripheral light focusing by the anterior eye is a possible explanation for the nasal predilection and cuneiform shape of age-related cortical cataract.
The formation of a pterygium is modelled using a population balance model of corneal and limbal epithelial production. Scattered light incident at the temporal limbus is focused at the nasal limbus with a peak intensity > or = 20 times. We hypothesize that this causes a focal alteration of corneal epithelial stem cells (which play a role in maintaining a barrier between corneal and conjunctival epithelia). Normative rabbit corneal data were used. Light-induced conjunctival epithelial proliferation was implemented by amplifying normal epithelial cell production along a nasal sector of the limbal circumference. A ten-fold normal peak in stimulation was assumed at the nasal zero azimuth (3 o'clock position in the right eye) with a quadratic attenuation to zero stimulation (normal) at the circumferential limits. Adjacent points on the outer limbus were assumed to be independent generators of epithelial cells that migrated in centripetal streams into the cornea. Normalization of the attrition rate for a net surface accumulation of the proliferating cells was embedded into the computational algorithm. The localized conjunctival mass was allowed to travel the predicted distance along the surface before the final shape was computed. The result shows a wing-shaped mass with a curved leading edge (corneal side). It is proposed that the initial biologic event in pterygium pathogenesis is an alteration of limbal stem cells due to chronic ultraviolet light exposure. The concomitant breakdown of the limbal barrier and subsequent conjunctivalization of the cornea explain the shape and formation of a primary pterygium.
Adherence of a rigid gas permeable (RGP) contact lens to the cornea is an unsolved clinical problem in extended wear. This communication proposes that intraocular pressure (IOP) is a cofactor of this phenomenon. Intradiem fluctuations in the IOP can additionally stress the corneal epithelium and affect its biomechanical properties. The corneal surface is then susceptible to lens imprinting and eventual lens immobilization. The prevalence of lens adherence in the morning correlates with the early morning peak in IOP that is known to occur in many normotensive patients. The possibility arises that lens immobilization during sleep is causally related to the peak in IOP.
Rabbit corneas were mounted atraumatically into an automatic voltage clamp apparatus for the determination of open circuit potential difference and short circuit current. The corneal tissue was bathed on both sides with a Ringer solution, continuously stirred and aerated. After a minimum 1 hour equilibration, a single dose of tert-butyl hydroperoxide (t-BHP) (0.1 mM to 2 mM) was administered to the tear-side solution. This led to an immediate fall in epithelial potential difference and short circuit current. The electrical conductance tended to be subnormal during the peak response. The initial decreases were partially reversed, but even after several hours the majority of corneas sustained abnormal levels of potential difference and conductance. Administration of 2 mM t-BHP to the tearside solution led to an immediate fall in epithelial thickness to -25% of baseline. This was followed by large-amplitude oscillations in corneal epithelial thickness which were sustained over several hours. We conclude that prolonged exposure of the perfused rabbit cornea to small tearside amounts of t-BHP are capable of modulating the electrophysiologic integrity of the corneal epithelium. T-BHP exceeding 1 mM in the tears could compromise the non-catalase dependent antioxidant defense mechanisms in rabbit corneal epithelium.
The kinematics of wound closure in the healing rabbit corneal epithelium are modelled. The analysis is able to model the movement of the wound edge on the corneal surface. A unified formalism is described which embeds the surface velocity of the wound margin within the computational algorithm to enable calculation of the surface velocity of the wound margin from the time course of (1) planar wound area, or (2) wound diameter. The procedure can be implemented with the standard least-squares estimation procedure. A polynomial velocity function is discussed, but the model allows for any appropriate velocity function to be used to suit particular conditions of wound closure. Corneal curvature effects are incorporated to avoid the errors associated with a planar representation of wound closure on a curved corneal surface. The approach offers greater flexibility by avoiding the theoretical flaws of previous models which are often limited in scope and nonbiological in nature.
Explore the source record for details and available documents.
Stromal swelling in human, cat, and rabbit cornea is biphasic, interpretable as an elementary cusp catastrophe proposed by Thom, with t* = log t and Q* = log Q (stromal charge Q, time t) as control parameters, and H0.5 (hydration H) as the state variable. A thermodynamic potential with two attractor regions, each with a local minimum, governs corneal stromal swelling. Transitions follow a 'saturation convention' whereby the second minimum is preferred upon availability. Corneal swelling is an example of a space-equivalent unfolding, where the transition plane moves in time. It is proposed that the transition plane coincides with the uncoupling of interfibrillary linkages or 'springs' in the corneal stroma, and is associated with a critical hydration of ca. 10 kg H2O per kilogram dry mass, and stromal charge ca. 1 x 10(-7) mol electrons.
In the rabbit corneal stroma, the swelling pressure, P, has been reported to have an anomalous (negative) temperature coefficient, alpha P, contradicting traditional Donnan swelling theory. A parallel-plate, diffuse double layer Gouy-Chapman model was used to resolve this discrepancy. The present model incorporates the possibility that surface charge, sigma, is temperature dependent. It is shown that negative, zero, or positive coefficients of swelling pressure change with temperature are not mutually exclusive conditions, but can be attributed to the same underlying mechanism. For likely values of alpha P(range -7 x 10(-3) K-1 to +3.2 x 10(-3)K-1), the effective stromal charge has a negative temperature dependency, or dln sigma/dT less than 0. The present formalism is robust against variation in assumed alpha P, and is able to simultaneously satisfy the known values of swelling pressure, its thermal dependency, and stromal charge. These results implicate significant coulombic forces behind P. Predicted stromal surface charge is approximately 0.01 Cm-2. The predictions were confirmed with macrocontinuum Donnan swelling theory, suggesting that Donnan osmotic swelling is the principal macroscopic component of P.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
The wound healing response of the cat corneal epithelium was examined to provide information on the effect of extended wear contact lenses on epithelial integrity. During the wounding procedure, a loss of epithelial adhesion in the lens wearing eye relative to the control eye was noted. This indicates a loss of epithelium-to-basement membrane adhesion which may be a result of contact-lens induced hypoxia.
Polarographic oxygen sensors placed on the postsurgical human cornea indicate subnormal oxygen uptake rates. Given the nature of the method of measurement (due to Hill and Fatt), this implies that epithelial oxygen consumption rate is reduced post-surgically. However, other studies using the Rasson-Fatt method report no difference in corneal oxygen uptake through a thick soft contact lens after unilateral surgery. Application of a recently described three-layer computer model of corneal oxygen distribution suggests that the difference between normal and postsurgical corneal oxygen flux is greater at high precorneal oxygen tensions (the measurement area of the Hill-Fatt method). In contrast, the difference is small at low precorneal oxygen tensions, which would tend to confound efforts to detect a difference in postsurgical corneal oxygen uptake using the Rasson-Fatt method. This probably explains the null results reported previously.
A two-membrane equivalent electrical model was applied to the rabbit corneal epithelium. Single parameters were varied (with the other parameters kept constant) to generate equations predicting the electrical behavior of the corneal epithelium. Due to electrical feedback through the epithelial shunt, each transmembrane voltage is not solely a function of its own membrane parameters. The equations were evaluated with published data and the model successfully approximated the known electrical characteristics of the rabbit corneal epithelium. The principal site of voltage variation in the corneal epithelium is inferred from the slopes calculated from voltage scattergrams. Reduced shunt resistance results from injury to the corneal epithelium in vitro, or from hard contact lens wear in vivo.
The aerobic response of epithelial metabolism to cellular injury or disease was studied with a three-layer model of the cornea. Nonclassical (Michaelis-Menten) respiratory kinetics were assumed, and predicted fluxes validated by reference to known estimates. The model indicates that epithelial oxygen flux is linearly related to epithelial oxygen consumption, and is therefore a direct index of epithelial aerobic metabolism. Injury leading to de-epithelialization decreases total anterior corneal oxygen flux. The decreases reported experimentally with a polarographic oxygen sensor on the denuded stroma in vivo are greater than theoretically predicted. This effect is due to a limited capture depth of the technique, which "sees" only the anterior stroma.
The macroscopic anterior surface area was calculated for three models of the average human cornea. Two models, a general ellipsoid and a rotational ellipse (rotationally symmetric ellipsoid) gave a surface area of 132 mm2, while a spherical model gave 126 mm2. A general ellipsoidal model having the maximum radius horizontal (with-the-rule corneal astigmatism) has less surface area than a rotational ellipse with the same horizontal radius. For a corneal sagittal height of 2.59 mm, the surface area of an ellipsoidal cornea equals -19.2Q + 16.3R -0.476 which specifies a rotational ellipse (radius R, asphericity Q) of equal surface area. In a cornea with the maximum radius vertical (against-the-rule corneal astigmatism), the ellipsoid has slightly more surface area than a rotational ellipse with the same horizontal radius of curvature. For a given horizontal radius of curvature, the sphere has the lowest surface area. For a corneal sagittal height s of 2.59 mm, the sphere underestimates by 8% the surface area of a rotational ellipse with asphericity -0.5. The anterior corneal surface area of a rotational ellipse model, radius R, asphericity Q is given by 2 pi Rs- 19.2Q. In all three models, the surface area increases with horizontal radius of curvature. In the rotational ellipse model, the rate of increase (slope) is independent of asphericity, and the slope found in with-the-rule astigmatism is less than the slope found with against-the-rule astigmatism. The calculated surface area predicts a precorneal tear volume of 0.86 microliter for a 6.5 micron tear thickness. The apparent, or plane projected are of an epithelial lesion underestimates the curved surface area with a percentage error that increases rapidly with lesion diameter. For a 12 mm diameter lesion on a rotational ellipse model, the apparent area underestimates the surface area by 18%. The average posterior corneal surface in human is not spherical but imitates the anterior surface, and has an area of 137 mm2 or 3.8% greater than the anterior area.