Change with age of the refractive index gradient of the human ocular lens.

PURPOSE: To study age-related changes in the refractive index distribution of the human ocular lens. METHODS: Biometric data collected on 48 eyes in subjects ranging in age from 19 to 31 years and 48 eyes in subjects ranging in age from 49 to 61 years allowed estimation of a single parameter related to the refractive index distribution of the crystalline lens. The authors selected a gradient index model of the lens characterized by a fixed index at the lens center, a somewhat lower fixed index at the surface, and a continuum of index values between center and surface depending on a single parameter, beta. This parameter was evaluated for each of the two age groups. RESULTS: The distributions of the gradient index parameter beta for the two age groups were found to be statistically well separated. On average, the older group was found to have an index gradient that was flatter near the lens center and steeper near the surface, implying a lower refractive power of about 2 D for representative lens surface curvatures. CONCLUSIONS: It has been observed that surface curvatures and thicknesses of the ocular lens increase with age, whereas other ocular dimensions apparently do not change, implying a trend toward myopia. This trend has not been observed. The authors' results are consistent with and strongly in support of the hypothesis that subtle index changes in the aging lens compensate to a large extent for changes in surface curvatures.

Mechanisms of emmetropization in the aging eye.

Refraction and its components were measured on 96 subjects consisting of 48 Young Adults (19 to 31 years old) and 48 Mature Adults (49 to 61 years old) having refractive errors in the range from +2.50 to -2.50 D. Subjects for the two groups were matched on the basis of sex and refractive error. A quota sampling technique was used to obtain subjects for myopic, emmetropic, and hyperopic refractive-error categories. An autorefractor was used to measure refractive error, an autokeratometer was used to measure corneal radius of curvature, and an A-scan ultrasonometer was used to determine the axial dimensions. Crystalline lens radii were determined by submitting Purkinje image data to a ray-tracing program, with the assumption that the index of refraction of the ocular media is constant throughout life. By comparing the results for the two groups of subjects, it was found that there were no significant age-related differences in mean corneal radius of curvature or in mean axial length. However, the following significant age-related differences were found: the Mature Adult eyes had shallower anterior chambers, thicker lenses, shorter vitreous chambers, steeper anterior and posterior lens surfaces (shorter radii of curvature), more highly powered lenses, and higher ocular refracting power than the Young Adult eyes. It was concluded that a decrease in the gradient-index of the lens occurs with increasing age, acting as an emmetropizing mechanism by compensating for the steepening of both the front and back surfaces of the lens.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic variation in the malaria mosquito vector Anopheles maculatus from Peninsular Malaysia.

Starch-gel electrophoretic studies on nine gene-enzyme systems comprising 14 loci revealed a fair level of genetic variation in two population samples of Anopheles maculatus from Peninsular Malaysia. The proportion of polymorphic loci was 0.36 for the Fort Bertau sample and 0.29 for the Gua Musang sample, while the mean heterozygosity value was 0.09 for Fort Bertau and 0.07 for Gua Musang. The values of genetic similarity (I = 0.98) and genetic distance (D = 0.02) were of the rank of geographical populations.

Enzyme polymorphism in Mansonia uniformis, a mosquito vector of brugian filariasis.

Three temporal samples of a wild population of Mansonia uniformis were analysed for genetic variation at six gene-enzyme systems. Adenylate kinase, hexokinase (3 loci) and cathodal malate dehydrogenase were monomorphic. Phosphoglucomutase, glucose phosphate isomerase, isocitrate dehydrogenase and anodal malate dehydrogenase were polymorphic. Each of the polymorphic loci was represented by three alleles. The average heterozygosity or gene diversity was 0.0437.

General and specific combining abilities for different economic traits in broiler chickens.

A complete diallel cross of four broiler breeds was made to investigate whether there are breed differences in the combining abilities for the traits, body weight, weight gain and feed efficiency, measured during the growing period from 4 to 12 weeks of age. Data collected from male and female birds were analysed separately. General combining ability (GCA) was found to be the largest and most significant source of variation contributing to differences between crosses for all the traits, in both male and female birds. Specific combining ability (SCA) was important for body weight in both sexes and for weight gain in females. Feed efficiency in both sexes did not appear to be influenced by SCA effects. Reciprocal effects (RE) were generally absent in both sexes for all the traits, except possibly for feed efficiency.