The internalization of posterior subcapsular cataracts (PSCs) in Royal College of Surgeons (RCS) rats. II. The inter-relationship of optical quality and structure as a function of age

PURPOSE

The Royal College of Surgeons (RCS) rat is an animal model for human retinal degenerative disease and posterior subcapsular cataracts (PSCs). The purpose of this study was to correlate the structure and optical quality of RCS lenses with PSCs as a function of their internalization, with normal, non-cataractous, age-matched control lenses.

METHODS

Correlative light (LM), scanning electron microscopic (SEM), three-dimensional computer assisted drawings (3D-CADs) and low power helium-neon laser scan analysis were used to examine the structure and function of lenses.

RESULTS

The optical properties (average focal length variability; sharpness of focus) of RCS rat lenses are quantitatively compromised by PSCs. Correlative LM and SEM analysis of RCS lenses at various stages of PSC internalization (1.5, 3, 6, 9, 12 and 15 months of age), revealed that the sutures formed by additional fiber growth were progressively more abnormal. During PSC internalization, two to nine small suture branches were formed and arranged in modified line to multiple y configurations rather than the normal three branch y sutures. These temporal changes were also chronicled in animated 3D-CAD videos derived from lens reconstructions based on LM and SEM micrographs from the selected time points stated above. However, laser scan analysis also revealed that as the PSCs of RCS rat lenses were progressively internalized, there was a steady improvement in total sharpness of focus that reached normal levels by 12 months of age. The correlation of laser scan and structural data from specific regions of lenses revealed the following: 1. The abnormal posterior sutures of RCS rats with internalized PSCs effect a greater reduction in optical quality than normal posterior sutures of age-matched controls; 2. However, the resulting abnormal suture plane area was cumulatively similar to that of age-matched controls; 3. Thus, total optical quality was similar between RCS lenses with internalized PSCs and age-matched controls by 12 months of age.

CONCLUSIONS

The results of this study show that RCS lenses with internalized PSCs can appear grossly, and indeed optically perform, at levels comparable to aged lenses. These findings are consistent with clinical observations of spontaneous recovery from PSC. The results suggest that human PSCs that occur as a consequence of retinal degenerative disease could also be the result of abnormal posterior suture growth. If this is proven to be the case, such PSCs may have some capacity for repair or recovery thereby obviating their surgical removal.

The structure of posterior subcapsular cataracts in the Royal College of Surgeons (RCS) rats

The Royal College of Surgeons (RCS) rat is an animal model for human autosomal recessive retinitis pigmentosa. As the retinas of these animals degenerate from two to six weeks after birth, posterior subcapsular cataracts (PSCs) develop, presumably in response to toxic lipid peroxides formed by degenerating rod outer segments. Morphologically, these PSCs are thought to be characterized by a proliferation of dysplastic bladder-like fibers, or Wedl cells, in the meridional region of the lens, that subsequently migrate to, and aggregate at, the posterior pole as the PSC. This report presents the results of correlative scanning (SEM) and transmission (TEM) electron microscopic as well as light microscopic (LM) analysis of the ultrastructure of RCS PSCs. SEM analysis of two, four and six week old lenses (n=6-10 specimens per age group) demonstrated that the PSCs of RCS rats resulted from a growth malformation of the posterior fiber ends from four to six weeks. The PSC is composed of markedly enlarged and irregular posterior fiber ends aberrantly curved away from the polar axis toward the vitreous rather than overlapping and abutting to form suture branches within and between concentric growth shells. LM analysis revealed evidence of progressively more numerous, enlarged, and irregular, ovate cellular profiles at the posterior pole from four to six weeks. However, there was no evidence of Wedl cells either within the meridional row region or along a migratory path from the equator to the posterior pole at any age. TEM analysis confirmed that the size and abnormal shapes of cellular profiles were consistent with SEM analysis and that nuclei were never observed within the plaque. In addition, there was considerable variation in cytoplasmic densities between cells. Also, dense deposits were frequently noted between cells and beneath the capsule. The orientation of posterior fiber end profiles to the posterior capsule was 45, 70 and 90 degrees at respectively two, four and six weeks of age. These results show that RCS PSCs are a consequence of abnormal posterior fiber end growth culminating in a posterior opacity.

An ultrastructural analysis of the epithelial-fiber interface (EFI) in primate lenses

The purpose of this study was to conduct a comprehensive ultrastructural analysis of the epithelial-fiber interface (EFI) in normal adult primate (Macaque nemestrina and fascicularis; 6-9 years old, n = 10) lenses. Scanning electron microscopy (SEM) was used to initially characterize the gross size, shape and three-dimensional organization of central zone (cz) epithelial cells and the anterior ends of elongating fibers beneath these cells. This fiducial information was essential to properly orient lens pieces in freeze fracture specimen carriers for the production of replicas with unambiguously identifiable EFI. Transmission electron microscopy (TEM) of replicas and thin-sectioned material were used to ultrastructurally analyse the cz EFI. TEM thin-sectioned material was also used to ultrastructurally analyse the pregerminative (pgz), germinative (gz) and transitional zone (tz) EFI. Correlative SEM and TEM of cz EFI components revealed that the apical membrane of both epithelial and elongating fiber cells were irregularly polygonal in shape, and aligned in parallel as smooth, concave-convex surfaces. However, whereas epithelial cell apical surfaces had minimal size variation, elongating fibers were larger and considerably variable in size. Quantitative analysis of > 10000 micron2 cz elongating fiber apical surfaces failed to detect any gap junctions defined in freeze fracture replicas as complementary aggregates of transmembrane proteins (connexons) conjoined across a narrowed extracellular space. However, a comparable frequency of vesicular events was noted in this region as quantified previously in adult and embryonic chick lens. Correlative TEM analysis > 1500 linear micrometers of thin-sectioned EFI from this region confirmed the presence of epithelial-epithelial gap junctions, elongating fiber-elongating fiber gap junctions, and an extreme paucity of epithelial-elongating fiber gap junctions. In contrast, TEM analysis of > 1000 linear micrometers of thin-sectioned pgz, gz and tz EFI, confirmed the presence of epithelial-epithelial gap junctions, elongating fiber-elongating fiber gap junctions, numerous epithelial-elongating fiber adherens junctions and a few epithelial-elongating fiber gap junctions. Thus, the results of this and previous quantitative morphological and physiological studies (electronic and dye coupling) demonstrate that there is limited coupling between cz epithelial cells and underlying elongating fibers. Furthermore, the absence of gap junctional plaques in cz EFI freeze-fracture replicas and either pentalaminar or septalaminar profiles in correlative thin-sections, suggests that this limited coupling could be mediated via isolated gap junction channels. However, the results of this and previous quantitative studies further show that a greater degree of coupling exists across the pgz, gz and tz regions of the EFI and that this coupling is likely to be mediated by gap junction plaques. Finally, this and other studies continue to demonstrate that transcytotic processes play a role in lens physiology at the EFI.

Electrophoretic and Immunological Comparisons of Developmentally Regulated Proteins in Members of the Sclerotiniaceae and Other Sclerotial Fungi

The fungal stroma is a distinct developmental stage, a compact mass of hyphal cells enveloped by a melanized layer of rind cells which is produced from vegetative mycelium. Two types of stromata that are characteristic of members of the Sclerotiniaceae but are also produced in a wide range of other fungi, i.e., the determinate tuberlike sclerotium and the indeterminate platelike substratal stroma, were compared in these studies. Developmental proteins found in determinate sclerotial and indeterminate substratal stromata, but not in mycelia, were characterized and compared in 52 isolates of fungi, both ascomycetes (including 18 species in the Sclerotiniaceae and 5 species of Aspergillus ) and the basidiomycete Sclerotium rolfsii . One-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis of mycelial, stromatal initial, and stromatal extracts demonstrated that all members of the Sclerotiniaceae produced proteins unique to stromatal extracts within a molecular weight range of 31,000 to 39,500 which composed 13 to 58% of the total protein in stromata. Proteins unique to the sclerotial stage were also produced in Sclerotium rolfsii and the Aspergillus species but within a generally lower-molecular-weight range of 11,000 to 30,000. The proteins were then characterized by two-dimensional electrophoresis to determine the number and isoelectric point of polypeptides composing each protein. Polyclonal antibodies were raised to the major 36-kDa sclerotial protein of Sclerotinia sclerotiorum (Ssp). Immunoblots demonstrated that all sclerotial proteins from species in the Sclerotiniaceae cross-reacted with anti-Ssp antibodies, while no cross-reaction was observed with proteins from substratal stromatal species in the Sclerotiniaceae , sclerotial species of Aspergillus , or Sclerotium rolfsii . Results of discriminant analysis of the data from competitive inhibition enzyme-linked immunosorbent assays were consistent with the results of immunoblotting. Three groupings, sclerotial species in the Sclerotiniaceae , substratal stromatal species in the family, and sclerotial species outside the family, were delimited on the basis of relative decreasing ability to compete for anti-Ssp antibody. These data demonstrate that stromatal proteins differ among different taxonomic groups of fungi and suggest that the Sclerotiniaceae may include two distinct lineages of genera.