Grace S. Lee

Retinoid-Induced Limb Malformations

The developing limb has been studied extensively and is a useful model to study morphogenesis. During embryogenesis, limb formation is initiated as a budding off from the embryonic lateral body wall. Limb pattern is specified by a series of epithelial-mesenchymal interactions, directing proximodistal, dorsoventral and anteroposterior axes. Vitamin A metabolites, especially retinoic acid, are known to play an important role in limb development, and the effects of retinoic acid may be mediated through the retinoid receptor signaling pathways. Accumulated evidence has shown that inadequate levels (excess or deficiency) of retinoic acid cause a wide range of limb malformations. Some species have the capacity to regenerate amputated limbs, and retinoids certainly affect this process, but there is debate regarding the extent that regeneration recapitulates development. In this review, phenotypic features, pathogenesis and the molecular basis of retinoid-induced limb malformations are discussed with a description of normal limb development and endogenous retinoid pathways.

Genetic and pathologic aspects of retinoic acid-induced limb malformations in the mouse†

Because all-trans retinoic acid (atRA) is teratogenic in all species tested and many of the specific defects induced are common across the phylogenetic spectrum, it would be logical to predict that murine strain differences in teratology to this agent are minimal. However, for specific defects, strain susceptibilities are vastly different. Studies with atRA have shown stark differences between C57BL/6 and SWV mouse strains in susceptibility to postaxial forelimb ectrodactyly and ectopic hindlimb formation, with the C57 strain being more susceptible for both defects. Various approaches were used to determine why these strains differ in susceptibility, but the mechanisms remain unknown. Hindlimb duplications were hypothesized to be caused by the formation of ectopic posterior body axes. For forelimb ectrodactyly, a locus on chromosome 11, Rafar, has linkage to the strain difference, and mRNA localization has shown that specific genes (Fgf8, Dlx3, Bmp4, and Sp8) in the postaxial preAER (prior to formation of the apical ectodermal ridge) of the developing limb bud (the site of the defect) were downregulated hours after atRA administration more in the susceptible C57 than in the SWV strain. Because both atRA and divalent cadmium induce postaxial forelimb ectrodactyly (right-sided predominance) at a high rate in C57BL/6 and low in the SWV strain, there is debate as to whether they share a common mechanism. These teratogens cause a greater-than-additive level of forelimb ectrodactyly when coadministered at low doses, but cadmium does not induce ectopic hindlimb formation. The hypothesis is that these agents have separate molecular pathologic pathways that converge to perturb a common anatomic structure.

Lid wiper epitheliopathy: The influence of multiple lid eversions and exposure time

PURPOSEnTo investigate the effect of multiple lid eversions on lid wiper epitheliopathy (LWE), along with the effect of cumulative lid exposure time and the patterns of associated staining.nnnMETHODSnThe increase in area of lid wiper staining with lissamine green was compared by everting both the upper eyelids of each subject (i.e. contralateral design), with one eye being everted once for 45u2009s and the fellow eyelid everted three times, each time for 15u2009s. This pattern of contralateral eversion was repeated with a total of three eversions in one eye and nine eversions in the fellow eye, with each eye totalling 135u2009s cumulative exposure to eversion over about 9u2009min. The LWE area of staining was objectively quantified from slit lamp photography images captured at every lid eversion by 2 masked observers. Two-way repeated measures ANOVAs were used to determine the effect of number of lid eversions and cumulative exposure time on the amount of staining caused. Each image was also categorized into its primary LWE staining pattern, by a masked observer.nnnRESULTSnThe multiple eversions condition caused significantly greater LWE than the single eversion condition (pu2009<u20090.001), while cumulative exposure time did not have a significant effect on LWE (pu2009=u20090.137). Classification of the primary staining patterns revealed that with more eyelid eversions there was a shift from mostly no staining to minor patterns (short horizontal bands and vertical streaks) and then to more extensive patterns (broad horizontal bands and comb-shaped).nnnCONCLUSIONSnThe number of eyelid eversions is a confounding factor that should be controlled when investigating LWE, in particular when considering the link with dry eye or contact lens discomfort. However the cumulative exposure time did not appear to influence the LWE magnitude.