Expression of Luminal Progenitor Marker CD117 in the Human Breast Gland.

CD117 is a putative marker of luminal progenitor cells in the human breast. However, so far mapping the expression pattern of CD117 within the normal gland has not been reported. Here, we examined the anatomical distribution of CD117-expressing cells in lobular and ductal structures by immunohistochemistry. The presence of CD117-positive luminal cells could be divided into three distinct patterns: (1) contiguous, with coherent positive cells and rare negative cells interspaced; (2) patched, with a roughly equal frequency of positive and negative cells distributed focally; or (3) scattered, with few or no positive cells in the structure. Generally, a patched or scattered expression pattern was more frequent in lobules compared with ducts. Furthermore, an age-correlated increase in heterogeneity was observed. When comparing women below and above 21 years of age this heterogeneity was evident for both lobules and ducts. Although CD117-expression was generally segregated from luminal-lineage transcription factor GATA3-positive cells, some did co-express both markers. Finally, co-staining with Ki-67 revealed that a prominent part of cycling cells belonged to the CD117-positive population. Together these data demonstrate the presence of a CD117-expressing progenitor compartment with the capacity to replenish the luminal lineage of the breast gland.