Melanoblasts Populate the Mouse Choroid Earlier in Development Than Previously Described

Abstract

Human choroidal melanocytes become evident in the last trimester of development, but very little is known about them. To better understand normal and diseased choroidal melanocyte biology we examined their precursors, melanoblasts (MB), in mouse eyes during development, particularly their relation to the developing vasculature and immune cells. METHODS. Naïve B6(Cg)-Tyr c-2J /J albino mice were used between embryonic (E) day 15.5 and postnatal (P) day 8, with adult controls. Whole eyes, posterior segments, or dissected choroidal wholemounts were stained with antibodies against tyrosinase-related protein 2, ionized calcium binding adaptor molecule-1 or isolectin B4, and examined by confocal microscopy. Immunoreactive cell numbers in the choroid were quantified with Imaris. One-way ANOVA with Tukey's post hoc test assessed statistical significance. RESULTS. Small numbers of MB were present in the presumptive choroid at E15.5 and E18.5. The density significantly increased between E18.5 (381.4 ± 45.8 cells/mm 2) and P0 (695.2 ± 87.1 cells/mm 2 ; P = 0.032). In postnatal eyes MB increased in density and formed multiple layers beneath the choriocapillaris. MB in the periocular mesenchyme preceded the appearance of vascular structures at E15.5. Myeloid cells (Ionized calcium binding adaptor molecule-1-positive) were also present at high densities from this time, and attained adult-equivalent densities by P8 (556.4 ± 73.6 cells/mm 2). CONCLUSIONS. We demonstrate that choroidal MB and myeloid cells are both present at very early stages of mouse eye development (E15.5). Although MB and vascularization seemed to be unlinked early in choroidal development, they were closely associated at later stages. MB did not migrate into the choroid in waves, nor did they have a consistent relationship with nerves.

Comprehensive single-cell transcriptomic analysis of embryonic melanoblasts uncovers lineage-specific mechanisms of melanoma metastasis and therapy resistance

“…This includes melanoblasts in the dermis, epidermis and hair follicles, along peripheral nerves and surrounding the dorsal root ganglion (DRG) – as reported by Adameyko and colleagues 5 . We also noted melanoblasts clustered in collagen-rich regions and surrounding vasculature, which has only previously been reported of embryonic melanoblasts populating the choroid (eye) 17 . Using scRNA-seq we queried melanoblast subpopulations at two developmentally relevant embryonic stages (Fig.…”

Melanocytes in regenerative medicine applications and disease modeling

“…Other melanoblasts migrate towards the follicle bulb, maturing into melanocytes by transiently expressing c-KIT [ 35 ]. In the eye, melanoblasts are observed in the choroid mesenchyme at E15.5 in mice, while pigmented melanocytes are observed in the human choroid at week 27 [ 25 , 36 ].…”

“…By E9, melanoblasts are primarily localized in the head and face, with fewer in cervical and trunk regions (Fig. 1B) [24][25][26]. At E10.5, they enter the dorso-lateral pathway and begin to differentiate into melanocytes, a process regulated by WNT signaling [27].…”

“…Melanocytes also reach brain meninges in mice, rats and humans, and are present in human buccal mucosa from 20 weeks of gestation (reviewed in [31]) [32][33][34]. At E15.5, melanoblasts reside in the hair follicle bulge, where some transition into melanocyte stem cells (McSCs) [25]. Other melanoblasts migrate towards the follicle bulb, maturing into melanocytes by transiently expressing c-KIT [35].…”

Molecular and Cellular Regulations in the Development of the Choroidal Circulation System

“…Although the choroid includes other neural crest derivatives, such as stromal cells and melanocytes, neural crest cells do not differentiate into melanocytes in the ocular region. Melanocytes are differentiated in other sites and migrate into the choroid in late gestation at 27 GW in humans and E 15.5 in mice [ 18 ].…”