Intralobular stromal fibroblasts in the resting human mammary gland: ultrastructural properties and intercellular relationships

The intralobular stroma of the resting human mammary gland was studied by thin-sectioning transmission electron microscopy on tissue obtained from reduction mammoplasty or adjacent to fibroadenomas. The arrangement of delimiting fibroblasts around the epithelium and their possession of contacts as shown by earlier studies were confirmed. Observations on other ultrastructural aspects show these cells to have abundant rough endoplasmic reticulum, a well developed Golgi apparatus and vesicles considered to contain collagen-precursor, and conflict with earlier data. The fine structure of those fibroblasts without close epithelial proximity has been described for the first time and was similar to that of delimiting fibroblasts. Other features applicable to both categories of fibroblast included: variation in overall staining intensity, structures resembling hemi-desmosomes, simple junctions, and close juxtapositions of cell surfaces forming 'contacts' with other fibroblasts as well as with mononuclear cells (lymphocytes, plasma cells, mast cells, macrophages). These cell contacts have not been documented previously. Contacts were structurally undifferentiated, forming spaces 20-50 nm wide which were occluded by lanthanum nitrate. Intralobular stromal cells were therefore organised into a kind of reticulum. The possible functions of this network are discussed. The concept of the epithelial-stromal junction is re-assessed in the light of these observations.