ECM proteins, including VN, FN, collagen IV, tenascin-C, and laminin, have been identified along the BM or within the stroma directly underlying putative LSCs,
56 and key roles for these proteins in SC maintenance have been identified. For example, cells that rapidly adhere to collagen IV have higher DNA label retention,
24 a universal SC feature indicative of slow-cycling cells.
3 In addition, FN has been shown to delay the onset of terminal differentiation by triggering involucrin expression,
57 and VN maintains SCs in culture.
36,50 Similarly, cell surface receptors for ECM molecules (integrins) are involved in corneal wound healing,
27,58 are often expressed in a polarized manner along BMs,
59 potentiate limbal epithelial outgrowth from tissue explants placed on HAM,
60 and support an undifferentiated phenotype on the same bioscaffold.
61 Interestingly, a VN/VN-receptor (αvβ5) axis has been implicated in promoting cell migration and spreading in explant outgrowth,
62 an approach analogous to ours.
14 Therefore, it is tempting to speculate that VN released from CLs and engagement with its receptor may be a mode by which cells transfer between CLs and the ocular surface. Consolidating this view was the observation of VN deposits directly beneath the donor-transplanted cells, along the recipient corneal BM (
Fig. 5), a region normally void of this factor (
Fig. 3). Although VN appears to be one of the major proteins involved in supporting the expansion of limbal epithelial cells on silicone hydrogel lenses and in the transfer of these cells in organotypic models, it is likely that the highly related molecule FN might also play a role.
63 We have previously identified FN on cell-laden CLs before transplantation
14 and detected its presence in the CL proteome; however, this factor was not stably bound to the lens polymer (data not shown) and was, therefore, not pursued in our investigations. It is also possible that other ECM proteins were bound but were below the sensitivity of our proteomic approach. Notably, a host of ECM and BM proteins have been identified within the limbal zone
22 that may be of equal importance to or greater importance than VN in supporting LSC.