Normal Corneal Thickness and Endothelial Cell Density in Rhesus Macaques (Macaca mulatta)

Abstract

Purpose: To define the normal range of central corneal thickness (CCT) and corneal endothelial cell density (ECD) in rhesus macaques (Macaca mulatta) and the effects of age, body weight, sex, and intraocular pressure (IOP) on these parameters.

Methods: Ophthalmic examinations were performed on 144 rhesus macaques without anterior segment pathology. The CCT was measured via ultrasound pachymetry (USP) and specular microscopy, and the ECD was semiautomatically and manually counted using specular microscopy. Rebound tonometry was used to measure IOP. Linear regression and mixed-effects linear regression models were used to evaluate the effects of age, body weight, sex, and IOP on CCT and ECD.

Results: We included 98 females and 46 males with an age range of 0.2 to 29.4 years. The mean CCT by USP and specular microscopy were 483 ± 39 and 463 ± 33 µm, respectively, and were statistically different (P < 0.001). The ECDs were 2717 ± 423 and 2747 ± 438 cells/mm2 by semiautomated and manual analysis, respectively. Corneal endothelial degeneration was identified in one aged rhesus macaque.

Conclusions: The mean USP and specular microscopy CCT values differed significantly, whereas the semiautomatic and manual ECD did not. The CCT was associated with the IOP and sex, whereas the ECD was associated with body weight and age (P < 0.05). As in humans, corneal disease in rhesus macaques is uncommon.

Translational relevance: Establishing reference values is fundamental to use rhesus macaques as a model for corneal disease or to identify toxicity in studies of ocular drugs or devices.

Figure 1.

Age and sex of the 144 rhesus macaques examined in this study. The NHPs were divided into 6 age groups: 0 to 4 years (8 females, 10 males), 5 to 9 years (19 females, 13 males), 10 to 14 years (17 females, 9 males), 15 to 20 years, (23 females, 8 males), 20 to 24 years (30 females, 4 males), and 25 to 30 years (1 female, 2 males). Females were overrepresented (n = 98 [68%]).

Figure 2.

Scatterplots and Bland–Altman plots showing difference between USP and specular measurements for CCT (A1, A2), and difference between ECD semiautomatic and manual measurements (B1, B2) in 143 primates without corneal disease. For the Bland–Altman plots (A2, B2), the vertical axis shows the difference among the two types of measurements and the horizontal axis is the mean value of the two types of measurements. The dashed lines represent the 95% limits of agreement and the black line the mean of the differences between the two types of measurements. The CCC was 0.47 (0.36–0.57) for CCT measurements, and 0.88 (0.83–0.91) for ECD, indicating moderate and strong agreement between techniques, respectively.

Figure 3.

Scatterplot demonstrates a direct relationship between CCT measured with USP and IOP measured by rebound tonometry 286 eyes of 143 primates with healthy corneas. The area in gray corresponds with the 95% confidence interval. For every 1.26-mm Hg increase in IOP the CCT increases by 100 µm (P = 0.015; R² = 0.07).

Figure 4.

Scatterplot showing an indirect relationship between (A) ECD and body weight, and (B) ECD and age in 143 eyes of 143 primates with healthy corneas. For each 1-kg increase in body weight, the ECD decreases by 29 cells/mm² (P = 0.006; R² = 0.17). For each additional year of age, ECD decreases by 23 cells/mm² (P < 0.0001; R² = 0.22). The area in gray corresponds with the 95% confidence interval.

Figure 5.

Corneal endothelial cell appearance using specular microscope in healthy rhesus macaques at different ages. Although the cell morphology remains regular and mostly hexagonal, a lower ECD and increased cell area were observed in older individuals. Rhesus macaques of 0.4 years (A) (3061 cells/mm²; mean ± standard deviation cell area, 366 ± 69 µm²), 10.8 years (B) (3088 cells/mm²; mean cell area, 323 ± 51), 22.6 years (C) (2497 cells/mm²; mean cell area, 400 ± 24 µm²), and 29.4 years (D) (1853 cells/mm²; mean cell area, 539 ± 17) are shown. White dots in the center of some cells were placed manually for analytical processing. Scale for reference in (D) applies to all images.

Figure 6.

A geriatric male rhesus macaque with low corneal ECD, mild pleomorphism, and polymegathism in both eyes. In a scatterplot for analyzing outliers (A), the value for the right eye (OD) and the left eye (OS) of the rhesus macaque is outside of the cluster. The area in gray corresponds with the 95% confidence interval. Anterior segment appearance was normal OD (B) and OS (C). Specular microscopy revealed low ECD at 19.3 years old (D) (1507 cells/mm², OD) and at 22.6 years old (E) (1086 cells/mm², OS). Both eyes also had larger cells when compared with other primates (663 ± 121 µm² and 920 ± 244 µm² for OD and OS, respectively). Subjective loss of hexagonality of the endothelial cells was also apparent. The inset in (D) includes detail of the specular microscopy at same magnifications of a 21.4-year-old female with normal corneal endothelial morphology (ECD 2457 cells/mm²).