Many mature retinal cell types, including first-born RGCs,
37 are derived from multipotent RPCs. Identification of RGCs requires confirmed expression of a combination of markers because while all are expressed in RGCs, some are also expressed in other cell types outside of the eye. Published RGC markers include βIII TUBULIN (TUBB3), POU class 4 homeobox 1 (POU4F1, also known as BRN3A), POU class 4 homeobox 2 (POU4F2, also known as BRN3B), ISLET1, atonal homolog 7 (ATOH7, also known as Math5 in mouse), HUC, HUD, thy-1 cell surface antigen (THY1), medium polypeptide neurofilament (NEFM), ribosome production factor 1 homolog (RPF1), and opsin 4 (OPN4, also known as melanopsin). βIII TUBULIN, ELAV 3/4, and NEFM are expressed in other neuronal cells, and therefore not exclusive to RGCs. OPN4 is expressed by a small subset of intrinsically photosensitive RGCs.
38–41 Atoh7 is expressed at the onset of RGC differentiation.
42 Downstream of Atoh7
, and mediating RGC differentiation, are the transcription factors Brn3b
43 and ISL LIM homeobox 1 (Isl1, also known as ISLET1).
44 Brn3b is expressed in most RGC subtypes, and thus serves as a common RGC marker.
45 Functionally, Brn3b is required for specification of RGC fate from RPCs.
46 The functions of Brn3a, Brn3b, and Brn3c transcription factors are conserved in mouse and human neural development
47 ; however, Brn3a and Brn3c are only expressed after the onset of RGC differentiation.
45 Isl1 is another transcription factor expressed in several tissues during development, including the retina.
48 In mice, Isl1 is expressed after the initial wave of RGC generation to modulate differentiation and survival.
44 Thy1 is a plasma membrane glycoprotein that is expressed on most RGCs within the retina
49 and also on a range of other cells throughout the body
50 including amacrine and Müller cells in the retina. Nevertheless, Thy1 is frequently used as a surface marker to isolate primary RGCs from eye samples.
51,52 While the exact function of Thy1 within the retina and in RGCs is unclear, it has been implicated in retinal development and neurite outgrowth.
53