Melanocytes populate the mouse choroid earlier in development than previously described

Abstract

Purpose : Whilst the migration and differentiation of cutaneous melanoblasts is well documented, little is known about the timing of melanoblast migration to the choroid. Many studies maintain that melanoblasts migrate into the uveal tract during the last trimester of human development. As part of a wider investigation of choroidal melanocytes, with relevance to uveal melanoma, this study was designed to define the appearance of melanoblasts in the developing mouse eye and their relation to the choroid, particularly the vasculature and immune cells.

Methods : Naïve B6(Cg)-Tyr^c-2J/J albino mice were used, age is denoted as embryonic day (E), postnatal (P) or adult. Stages studied included E15.5 (n=1), E18.5 (n=4), P0 (n=7), P2 (n=4), P4 (n=4), P6 (n=4), P8 (n=4) and adult (n=4). Whole eyes, posterior segments, or dissected choroids were stained with antibodies against TRP2 [melanoblasts], Iba-1 [myeloid cells] or Ib4 [vascular endothelium/myeloid cells]. Confocal microscopy was used to analyse wholemounts and immunoreactive cells were quantified with Imaris (Bitplane). One-way ANOVA with Tukey’s post-hoc test was used to assess statistical significance.

Results : Melanoblasts were present in the embryonic choroid as early as E15.5 with density increasing until they formed multiple layers beneath the choriocapillaris. Melanoblasts were observed posteriorly adjacent to the optic nerve, at the equatorial choroid and anteriorly to the optic cup margin. A reduced number of melanoblasts and less dense vasculature was noted anteriorly compared to the posterior peri-optic region. Melanoblast density significantly increased at P0 compared to E18.5 (695.2±87.1 vs. 381.4±45.8 cells/mm², P=0.031). Melanoblasts were also found in the mesenchyme around the developing eye cup prior to differentiation of vascular elements. Lastly, myeloid cells were present at high densities from the earliest time points but attain equivalent densities to that of adults by P8 (556.4±73.6 cells/mm²).

Conclusions : Contrary to previous studies we have discovered that melanoblasts are present at the earliest stages of eye development. Further, melanoblasts did not move in uniform wave-like patterns into the choroid. Finally, we did not observe a consistent relationship with melanoblasts and vascular elements; whilst melanoblasts appear earlier in the choroid, they seem to have a close association with the blood vessels at later stages of development.

This is a 2020 ARVO Annual Meeting abstract.