Old Theories and New Ideas on Ocular Function

Can the Choroid Affect Primate Eye Size?

Earlier studies with chicks have shown that choroid thickness is actively regulated in response to retinal defocus and may be involved in the regulation of eye growth. In two articles in this issue, Troilo et al. (p. 1249) and Hung et al. (p. 1259) independently show that the primate choroid also undergoes changes in thickness associated with visual experience and experimentally induced changes in eye size and refraction. There is some evidence that these changes, although much smaller than those seen in chick, might also partially compensate for imposed retinal defocus. 

Is There Conjunctiva-Associated Lymphoid Tissue in the Human?

Although the mucosal immune system is of increasing relevance, there is still little evidence for a respective tissue in the eye, especially in the human. Knop and Knop (p. 1270), in a study on a number of human autopsy tissues, provide evidence for the existence of a conjunctiva-associated lymphoid tissue (CALT) in normal individuals, its topographic anatomy, its continuation in the lacrimal drainage system, and its contribution to the secretory immune system. These findings characterize the basis for immunologic phenomena on the ocular surface and may enhance our understanding of physiologic homeostasis and pathologic alterations of the ocular surface. 

Extraocular Muscles Do More than Turn the Eye

Extraocular muscles consist of two layers: orbital and global. Demer et al. (p. 1280) used magnetic resonance imaging in living people, and histological examination in cadavers, to demonstrate that only the global layer of each rectus extraocular muscle inserts on and rotates the eye. The orbital layer instead inserts on that muscle’s connective tissue pulley, translating it along the muscle axis. This finding implies that pulleys, which act as functional origins of the extraocular muscles, are under powerful active control. 

Is ‘Visual Acuity’ the Best Measure of Visual Function?

Is visual acuity the most appropriate visual function to measure in determining the difficulties that patients have with their vision? For patients with macular disease, Hazel et al. (p. 1309) find that low-contrast tests explain most of the variance in self-reported problems with reading, and that text-reading speed correlates highly with overall concern about vision. High-contrast distance visual acuity is not the most relevant outcome measure for assessment of macular degeneration patients. Therefore, the assumption that visual acuity is the most relevant measure of visual function in other disease groups is questionable. 

Abnormal Telomerase Activity in Pterygia

Telomerase is an enzyme that extends the life span of cells by elongating telomeres at the chromosome ends. Telomerase activity is often detected in neoplastic cells, but is very rare in somatic cells in adults. By using a technique called the telomeric repeat amplification protocol (TRAP), Shimmura et al. (p. 1364) demonstrate the presence of telomerase activity in the epithelium of pterygia, but not in normal conjunctiva. This indicates that although a pterygium is not a neoplastic disease, abnormal telomerase activity in epithelial cells is associated with its hyperproliferative property. 

Is Glaucoma Only a Disease of the Eye?

Glaucoma is a disease of the optic nerve that also results in the degeneration of ganglion cells within the retina. While glaucomatous changes within the retina and optic nerve are well-described, little is known about the degeneration that occurs postsynaptically within the LGN, a primary site of visual integration. This study by Weber et al. (p. 1370) shows that glaucoma has a significant effect on the size, density, and number of neurons in the LGN, as well as volume of the nucleus itself. These data indicate that strategies aimed at mitigating glaucomatous neuropathy need to consider treatment of the entire central visual pathway, and not the eye alone. 

Is Lens Yellowing a Function of the Age?

There is much discussion in the current literature concerning the regions of the ultraviolet spectrum that are most damaging to the human lens. Of fundamental importance to addressing this issue is an accurate knowledge of the absorption characteristics of the lens as a function of age. Gaillard et al. (p. 1454) report a novel method, wherein whole lenses are sectioned into thin slices and the absorption spectra of individual slices are added to yield cumulative spectra along the visual axis. The spectral changes observed for a series of primate lenses of different ages indicate that the age-related yellowing of lens protein is the result of a chronological process (such as chemical or photochemical modification) rather than biological aging (senescence). 

Cataract: The Osmotic Theory Revisited

Osmotic stress, due to large accumulation of sorbitol and galactitol in the lens, is thought to be the cause of diabetic and galactosemic cataract, respectively. However, it remains controversial as to what extent cataract in these models is due to a disruption of signal transduction pathways by myo-inositol, in addition to altered osmolarity. This paper by Jiang et al. (p. 1467) shows that increase in myo-inositol, as a consequence of overexpression of a sodium-dependent myo-inositol co-transporter in the lens, also leads to cataract. Further, sorbitol and myo-inositol have an additive effect on cataract development. These results are consistent with an osmotic theory of cataract. 

Copper, CML, and Cataract

The crystallins of the aging human lens become progressively pigmented, cross-linked, oxidized, and fragmented with age, and it is generally thought that these modifications predispose the lens toward formation of senile cataract. Saxena et al. (p. 1473) have tested the hypothesis that N^ε-carboxymethyl-l-lysine (CML), a modification occurring in aging lens crystallins, can bind redox active copper due to its EDTA-like structure. The authors find that copper content and copper binding by CML-rich lens crystallin extracts from senile cataracts are increased, and that the crystallin copper complexes can oxidize ascorbic acid. When crystallins are modified by ascorbic acid degradation products, they form CML and bind redox active copper. The occurrence of this process in the aging human lens may lead to a vicious cycle consisting of glycoxidation, lipoxidation, and metal binding. 

A Cone Influence on Subretinal Fibrosis?

Following retinal detachment, retinal Müller cells often extend processes into the subretinal space. In this condition, known as subretinal fibrosis, the growth of Müller cell processes into the subretinal space can completely inhibit the regeneration of photoreceptor outer segments following retinal reattachment surgery. Lewis and Fisher (p. 1542) have found that when Müller cells grow into the subretinal space they are always adjacent to cone photoreceptors, suggesting that cones may produce factors that initiate the Müller cell outgrowths. Identification here of a relationship between cones and Müller cells may aid in choosing strategies for inhibiting this response. 

Is mERG and mVEP Useful in Glaucoma?

Because significant damage to ganglion cells can occur before changes are detected in visual fields, alternative measures are being sought. Hood et al. (p. 1570) show that although the multifocal ERG (mERG) is altered in some patients with glaucoma, other patients with clearly altered visual fields can have mERG responses that appear normal. Further, even in patients who exhibit altered mERG responses, local waveforms do not appear to correlate with local changes in the visual field. The authors are not optimistic that a measure of the mERG will prove useful in detecting either local or early glaucomatous damage. The multifocal VEP (mVEP) has also been suggested as an objective alternative to visual field perimetry. Hood et al. (p. 1580) argue that intersubject variability is too great for the standard mVEP to be useful as a quantitative measure of visual field defects. However, the authors show that local monocular damage to the ganglion cells can be quantitatively measured by comparing the mVEP responses obtained by stimulating each eye separately. These interocular comparisons may provide an alternative to standard perimetry. 

Do Rods Determine Their Visual Sensitivity?

What determines infants’ low scotopic visual sensitivity, the immature rods or postreceptoral processing? Fulton and Hansen (p. 1588) used electroretinographic and psychophysical procedures to study the course of development of rod photoreceptor sensitivity and dark-adapted visual sensitivity. The shape of the logistic growth curve and the age at which sensitivity reaches 50% of the adult value is similar (10 to 13 weeks) for the rods and peripheral visual sensitivity. A parsimonious explanation is that, during development, rod photoreceptor sensitivity determines dark-adapted, rod-mediated visual sensitivity.