Unusual features of HCs. (A) During embryonic development,
retinal progenitor cells (RPCs) undergo mitosis near the outer limiting
membrane and subsequently migrate to their appropriate retinal layer. HC
precursors do not migrate directly to the prospective HC layer, but rather
bypass this layer and migrate basally to the ganglion cell layer (GCL) before
changing direction to migrate apically towards the HC layer. This second phase
of HC migration has been shown to depend on Lim1. (B) Upon the
completion of HC migration, the HCs are organized in a non-random spatial
arrangement (mosaic) within the outer retina. HC spacing has been shown to be
regulated by homotypic repulsive interactions mediated by transient, apically
directed neurites (arrowheads). (C) Zebrafish and chick retinae contain
committed progenitor cells that divide to produce only HCs, and in the
zebrafish do so within the HC stratum. (D,E) Subsequent to cell
cycle exit, homotypic interactions restrain dendritic overlap, as these
processes stratify and form synaptic contacts with their afferents in the
outer plexiform layer (OPL). (F) In a mouse model of retinoblastoma,
fully differentiated HCs re-enter the cell cycle and give rise to aggressively
metastatic tumors. ONL, outer nuclear layer; INL, inner nuclear layer; IPL,
inner plexiform layer; onbl, outer neuroblastic layer; inbl, inner
neuroblastic layer; RPE, retinal pigment epithelium.