Ruth S. Anderson

Review of selenite cataract.

Recent advances in understanding the mechanism of selenite cataract have resulted from locating the cleavage sites on proteolyzed beta-crystallins from the cataract, mimicking the insolubilization of crystallins found in the cataract in an in vitro system, studying cataract produced in lenses cultured in selenite, and permanently or temporarily reducing the rate formation of selenite cataract by use of various inhibitors. The present review discusses the selenite cataract as a useful model for understanding the role calcium-induced proteolysis in cataract formation.

Selenite cataract: A review

Selenite cataract is a fairly recently described, experimental animal model for cataract (1). Selenite cataract has been extensively characterized histologically (2) and biochemically (3,4). The model has been particularly useful for studies on the roles of calcium accumulation and lens proteolysis in cataract formation (4). This review describes current knowledge of the biochemical mechanism for selenite cataract and indicates how the model may be used for further understanding of cataractogenesis in general.

Selenite-induced epithelial damage and cortical cataract

The purposes of these experiments were 1) to measure microscopic changes in the epithelium associated with selenite cataract, and 2) to describe the formation and subsequent clearing of selenite cortical cataract. Fourteen-day old suckling rat pups received a single subcutaneous injection of an overdose of sodium selenite at 2.25 mg Se/kg b.w. Development of cortical cataract was observed by biomicroscopy, and changes in epithelium were studied by light microscopy of flat-mounted lens epithelia. Selenite administration caused cortical cataract 15-30 days after injection in addition to previously characterized nuclear cataract. The cortical cataract progressed through equatorial vacuolization, opacity, and finally clearing of the cataract. Mitosis was suppressed and karyorrhexis was observed in the germinative zone of the epithelium 5 hours after selenite injection. Pathological disorganization of the epithelium followed. Changes included vacuolization, loss of meridional rows, and defective fiber formation. Restoration of epithelial morphology was associated with clearing of cortical opacity. Epithelial damage at 5 hours was the earliest change yet recorded for selenite cataract, and these data are consistent with our working hypothesis that the initial site of attack of selenium in both cortical and nuclear cataract is the lens epithelium.

Proteolytic changes in main intrinsic polypeptide (MIP26) from membranes in selenite cataract

Experimental nuclear cataract produced by an overdose of sodium selenite exhibited limited proteolysis, including breakdown of main intrinsic polypeptide (MIP26) to 24 and 22 kD fragments. Micro-sequencing and site specific immunologic probes were used in the present study to determine regions of cleavage in MIP26 during selenite cataractogenesis. Data suggested that proteolysis occurred in the C-terminus of MIP26. This may have lead to exposure of normally hidden amino acid residues on the C-terminal extension of MIP26. Loss of antigenicity of the N-terminus occurred. These significant changes to the MIP26 molecule might adversely affect communication between lens fiber cells and contribute to selenite cataract.

Histologic changes during selenite cataractogenesis: A light microscopy study

The purpose of this research was to study the early histologic changes occurring in the lens during the formation of selenite overdose cataract. Fourteen-day-old rat pups received a single injection of 2.25 mg Se (kg body wt)-1. Lenses at each of three stages of cataract formation were observed biomicroscopically, and then 1-2 micron sections of methacrylate-embedded lenses were evaluated with the light microscope. The first observable change was vacuolization, which started at the lens bow and later spread throughout the posterior subcapsular region (Stage 1). This was followed by an abrupt increase in basophilia at the interface between the peripheral nucleus and cortex (Stage 2). Subsequent formation of nuclear opacity was characterized by the presence of opaque particles, abnormal basophilic structures, and lens fiber damage (Stage 3). The data were consistent with the hypothesis that an early site of attack of selenite is at or near the lens bow, and this leads to biochemical changes, fiber cell damage, impaired nutrient circulation, and formation of nuclear cataract.

Leg proprioceptors of the tobacco hornworm, Manduca sexta: organization of central projections at larval and adult stages.

Organization of the central neuropil of the insect ganglion is characterized in part by a modality‐specific layering of afferent projections. This organization has been particularly well described for the central projections of thoracic leg sensory neurons of adult locusts, crickets, and flies. Tactile sensory neurons project into a ventral layer of neuropil, while proprioceptive sensory neurons project into an intermediate layer of neuropil. In order to determine whether a modality‐specific layering exists in the CNS of larval Manduca sexta, we have examined the projections of sensory neurons innervating one class of putative proprioceptors, the campaniform sensilla, of the larval metathoracic legs. We find that campaniform sensory neurons of the larval legs have central projection patterns that generally distinguish them from each other and from the tactile sensory neurons. The campaniform projections, however, are not completely segregated from tactile projections in ventral layers of neuropil, as has been described in other insects. By contrast, the projections of campaniform sensory neurons from the adult legs are more extensive and elaborate than their larval counterparts and dramatically different from projections of nearby adult tactile hairs, having extensive arborizations in more dorsal regions of neuropil while those of tactile sensory neurons are restricted to very ventral layers of neuropil. This difference in organization of the afferent projections in larval and adult ganglia may reflect different functions of the leg sensilla and different functions of the legs at the two stages.

Selenite nuclear cataractogenesis: a scanning electron microscope study

The sequential changes during selenite nuclear cataractogenesis were examined with a scanning electron microscope (SEM) and correlated with slit lamp observations. A posterior opacity, visible with the slit lamp 1-2 days after injection of sodium selenite, was found to consist of masses of vacuoles in the superficial posterior cortex by SEM. 2-3 days post injection, a biomicroscopic refractile ring around the nucleus was represented by SEM abnormalities suggesting membrane damage and possible loss of cytosol in the perinuclear region. All normal structure in this region was lost by 5 days after injection when the central nucleus had become opaque. SEM also showed evidence for damage in areas which were still clear by slit lamp examination. Changes, characteristic of aging, were found near selenite induced damage in peripheral (younger) fibers.

Glycol Methacrylate Sections of the Crystalline Lens

Sections of the crystalline lens are difficult to prepare because of the hardness of the fixed lens. After paraffin procedures the lens shatters and cracks when cut because the reagents and high temperatures used for infiltration further harden it. Plastic has been successfully used as an embedding medium for other difficult tissues. It allows prolonged infiltration times at room temperature, and provides a firm matrix for tissues containing areas of varying density. However, standard procedures for embedding tissue in plastic do not allow for complete infiltration of the crystalline lens. The purpose of this report is to describe a modification of the glycol methacrylate embedding technique which ensures complete infiltration of the lens. The following protocol was found to produce consistently good 1–5 μm sections of lenses from 10–2O-day-old rats.