Immunophenotyping of the human bulge region: the quest to define useful <i>in situ</i> markers for human epithelial hair follicle stem cells and their niche

TLDR

Epithelial hair follicle stem cells were discovered in the human bulge, sparking a quest to identify reliable in‑situ markers. This study evaluates published immunoreactivity patterns and seeks to identify markers for in‑situ detection of human eHFSCs and their niche. The authors examined cryosections from at least five donors using standard and high‑sensitivity immunohistology, then quantified marker intensity in defined reference areas. They found CK15, CK19 and CD200 as useful positive markers, CD34, connexin43 and nestin as negative markers, β1 integrin and Lhx2 not upregulated, and tenascin‑C upregulated in the bulge, indicating that a limited panel of positive and negative markers is needed for practical identification.

Abstract

Abstract: Since the discovery of epithelial hair follicle stem cells (eHFSCs) in the bulge of human hair follicles (HFs) an important quest has started: to define useful markers. In the current study, we contribute to this by critically evaluating corresponding published immunoreactivity (IR) patterns, and by attempting to identify markers for the in situ identification of human eHFSCs and their niche. For this, human scalp skin cryosections of at least five different individuals were examined, employing standard immunohistology as well as increased sensitivity methods. Defined reference areas were compared by quantitative immunohistochemistry for the relative intensity of their specific IR. According to our experience, the most useful positive markers for human bulge cells turned out to be cytokeratin 15, cytokeratin 19 and CD200, but were not exclusive, while β1 integrin and Lhx2 IR were not upregulated by human bulge keratinocytes. Absent IR for CD34, connexin43 and nestin on human bulge cells may be exploited as negative markers. α6 integrin, fibronectin, nidogen, fibrillin‐1 and latent transforming growth factor (TGF)‐beta‐binding protein‐1 were expressed throughout the connective tissue sheath of human HFs. On the other hand, tenascin‐C was upregulated in the bulge and may thus constitute a component of the bulge stem cell niche of human HFs. These immunophenotyping results shed further light on the in situ expression patterns of claimed follicular ‘stem cell markers’ and suggest that not a single marker alone but only the use of a limited corresponding panel of positive and negative markers may offer a reasonable and pragmatic compromise for identifying human bulge stem cells in situ .