Electron microscopic findings for diagnosis of breast lesions.

The normal mammary gland can be roughly divided into the large duct close to the nipple and the terminal duct located within the lobulus. Both the large duct and the terminal duct are composed of epithelial cells and myoepithelial cells. The epithelial cells can be divided into light and dark cells using electron density. Heterochromatin is the predominant type of chromatin found in normal mammary glands. The cytoplasm of myoepithelial cells contains a number of fine filaments that possess dense patches. The myoepithelial cells of the large duct have a large process with a crablike appearance that protrudes from the cytoplasm. The myoepithelial cells of the terminal duct, by contrast, assume a relatively flat form and are approximately parallel to the epithelial-stromal junction. If the nuclei of the epithelial cells of normal mammary glands and benign breast lesions are compared with those of malignant breast lesions, the latter are primarily oval or circular in shape whereas the former often show marked notches. The predominant chromatin is heterochromatin in noncancer cells and euchromatin in cancer cells. The intracytoplasmic lumen (ICL) can be roughly divided into two types. The ICL is frequently seen in breast cancers, especially scirrhous carcinoma and lobular carcinoma. Invasive ductal carcinoma can be divided into three types: papillotubular carcinoma, solid-tubular carcinoma, and scirrhous carcinoma. Scirrhous carcinoma can be divided into two subtypes: scirrhous carcinoma in the broader sense of the term (characterized by scirrhous invasion of the stroma by papillotubular carcinoma or solid-tubular carcinoma), and scirrhous carcinoma in the narrower sense of the term (characterized by linear or cluster-like invasion of the stroma without forming ducts). Ultrastructural characteristics of scirrhous carcinoma in the narrow sense are bright cytoplasm (seen in most cells) and euchromatin (seen in all cells of this type of carcinoma). In cases of papillotubular carcinoma, solid-tubular carcinoma, and scirrhous carcinoma in the broad sense, euchromatin is predominant but sporadic cells with heterochromatin are also seen. Adenoid cystic carcinoma and carcinoid tumor of the breast are histological types of breast carcinoma that show characteristic features under an electron microscope. Ultrastructurally, the former shows a pseudocyst and true lumen whereas the latter presents numerous neuroendocrine granules within the cytoplasm. Breast carcinoma shows several ultrastructural characteristics that are useful in differential diagnosis. Therefore, it is advisable to take electron microscopic findings into account when evaluating or diagnosing breast lesions.