Purpose: The purpose of this study was to develop a method for quantifying guinea pig ciliary muscle volume (CMV) and to determine its relationship to age and ocular biometric measurements.
Methods: Six albino guinea pigs’ eyes were collected at each of five ages (n = 30 eyes). Retinoscopy and photography were used to document refractive error, eye size, and eye shape. Serial sections through the excised eyes were made and then labeled with an α-smooth muscle actin antibody. The ciliary muscle was then visualized with an Olympus BX51 microscope, reconstructed with Stereo Investigator (MBF Bioscience), and analyzed using Neurolucida Explorer (MBF Bioscience). Full (using all sections) and partial (using a subset of sections) reconstruction methods were used to determine CMV.
Results: There was no significant difference between the full and partial volume determination methods (p = 0.86). The mean (±SD) CMV of the 1-, 10-, 20-, 30-, and 90-day-old eyes was 0.40 (±0.16) mm3, 0.48 (±0.13) mm3, 0.67 (±0.15) mm3, 0.86 (±0.35) mm3, and 1.09 (±0.63) mm3, respectively. Ciliary muscle volume was significantly correlated with log age (p = 0.001), ocular length (p = 0.003), limbal circumference (p = 0.01), and equatorial diameter (p = 0.003). It was not correlated with refractive error (p = 0.73) or eye shape (p = 0.60). Multivariate regression determined that biometric variables were not significantly associated with CMV after adjustment for age.
Conclusions: Three-dimensional reconstruction was an effective means of determining CMV. These data provide evidence that ciliary muscle growth occurs with age in tandem with eye size in normal albino guinea pigs. Additional work is needed to determine the relationship between CMV and abnormal ocular growth.