Ex vivo magnetic resonance imaging of crystalline lens dimensions in chicken

[摘要] Purpose: A reduction in the power of thecrystalline lens during childhood is thought to be important in theemmetropization of the maturing eye. However, in humans and modelorganisms, little is known about the factors that determine thedimensions of the crystalline lens and in particular whether thesedifferent parameters (axial thickness, surface curvatures, equatorialdiameter, and volume) are under a common source of control or regulatedindependently of other aspects of eye size and shape. Methods: Using chickens from abroiler-layer experimental cross as a model system, three-dimensionalmagnetic resonance imaging (MRI) scans were obtained at 115-µmisotropic resolution for one eye of 501 individuals aged 3-weeks old.After fixation with paraformaldehyde, the excised eyes were scannedovernight (16 h) in groups of 16 arranged in a 2×2×4 array. Lensdimensions were calculated from each image by fitting athree-dimensional mesh model to the lens, using the semi-automatedanalysis program mri3dX. The lens dimensions were compared to measuresof eye and body size obtained in vivo using techniques that includedkeratometry and A-scan ultrasonography. Results: A striking finding was thataxial lens thickness measured using ex vivo MRI was only weaklycorrelated with lens thickness measured in vivo by ultrasonography(r=0.19, p<0.001). In addition, the MRI lens thickness estimates hada lower mean value and much higher variance. Indeed, about one-third ofcrystalline lenses showed a kidney-shaped appearance instead of thetypical biconvex shape. Since repeat MRI scans of the same eye showed ahigh degree of reproducibility for the scanning and mri3dX analysissteps (the correlation in repeat lens thickness measurements wasr=0.95, p<0.001) and a recent report has shown that paraformaldehydefixation induces a loss of water from the human crystalline lens, it islikely that the tissue fixation step caused a variable degree ofshrinkage and a change in shape to the lenses examined here. Despitethis serious source of imprecision, we found significant correlationsbetween lens volume and eye/body size (p<0.001) and between lensequatorial diameter and eye/body size (p<0.001) in these chickens. Conclusions: Our results suggest thatcertain aspects of lens size (specifically, lens volume and equatorialdiameter) are controlled by factors that also regulate the size of theeye and body (presumably, predominantly genetic factors). However,since it has been shown previously that axial lens thickness isregulated almost independently of eye and body size, these resultssuggest that different systems might operate to control lensvolume/diameter and lens thickness in normal chickens.

[学科分类] 生物化学/生物物理