The derivation of three-dimensional (3-D) stratified neural retina from pluripotent stem cells has permitted investigations of human photoreceptors. We have generated a H9 human embryonic stem cell subclone that carries a GFP reporter under the control of the promoter of cone-rod homeobox (CRX), an established marker of post-mitotic photoreceptor precursors. The CRXp-GFP reporter replicates endogenous CRX expression in vitro when the H9 subclone is induced to form self-organizing 3-D retina-like tissue. At day 37, CRX+ photoreceptors appear in the basal or middle part of neural retina and migrate to apical side by day 67. Temporal and spatial patterns of retinal cell type markers recapitulate the predicted sequence of development. Cone gene expression is concomitant with CRX, whereas rod differentiation factor NRL is first observed at day 67. At day 90, robust expression of NRL and its target nuclear receptor NR2E3 is evident in many CRX+ cells, while minimal S-opsin and no rhodopsin or L/M-opsin is present. The transcriptome profile, by RNA-seq, of developing human photoreceptors is remarkably concordant with mRNA and immunohistochemistry data available for human fetal retina although many targets of CRX, including phototransduction genes, exhibit a significant delay in expression. We report on temporal changes in gene signatures, including expression of cell surface markers and transcription factors; these expression changes should assist in isolation of photoreceptors at distinct stages of differentiation and in delineating co-expression networks. Our studies establish the first global expression database of developing human photoreceptors, providing a reference map for functional studies in retinal cultures.

We describe a simple, rapid and highly selective protocol for the primary culture of Schwann cells in vitro from freshly dissociated adult rat nerve. The protocol is based on a selective culture medium comprising both mitogens (forskolin and optionally N2 supplement plus bovine pituitary extract), to stimulate growth of Schwann cells, plus an inhibitory substrate to simultaneously restrict fibroblast overgrowth (D-valine), contained in DMEM. This protocol differs from other available methods in that it uses the preferential capacity of Schwann cells to metabolize D-valine because of the difference in expression of a D-amino acid oxidase (DAAO) enzyme between Schwann cells and fibroblasts plus the presence of a selective mitogen to stimulate growth of Schwann cells. This permits derivation of highly pure Schwann cells directly from fresh adult nerve. Average Schwann cell purities of 97% can be achieved after 19 d without pre-degeneration, purification or antimitotic steps.