A subset of human choroid plexus epithelial cells exhibit mitochondrial eccentricity and distinct expression of the pigmentation-associated enzyme TYRP1
For decades, ultrastructural evaluation of epithelial cells in diverse organ systems has demonstrated the existence of two subtypes identified by stark differences in cytoplasmic electron density -- so-called light and dark epithelial cells. Choroid plexus (CP) epithelial cells are key regulators of CSF homeostasis and are one of many specialized epithelial linings that exhibit this bimodal phenotype. Despite longstanding acknowledgement, it has been difficult to assess the potential significance of adult human light and dark CP epithelial cells due to a lack of characterization beyond electron microscopy (EM). We present the first transcriptomic analysis of adult human CP epithelial cells and denote the existence of four epithelial subpopulations, one of which is defined by elevated expression of TYRP1 -- a melanocyte-associated tyrosine-related protein involved in cellular pigmentation and proliferation. TYRP1-high cells also downregulate genes related to cilia function (which is consistent with observations of dark cell identity in organoids) and upregulate genes associated with pathways related to cell cycling, stress, and iron regulation. Our data provide an explanation of the molecular underpinning of adult human light and dark cell identity and serve as a resource for investigations of epithelial heterogeneity in the CP and other organs where dark cells are found.