Electron microscopy of MM in effusions. The nuclear pleomorphism is normally more extensive than seen in cytomorphology (a, b). Typically, the apical surface of MM cells is covered by long, slender microvilli, completely devoid of any glycocalyx, and malignancy is revealed by the finding of neolumina, i.e. apical cell membranes with microvilli as either a cytoplasmic vacuole (c) or in-between cells with connecting desmosomes, replacing the basolateral membrane (d). Cytoplasmic filaments are often seen surrounding the nucleus as a ‘scarf' (e), sometimes forming coarser tonofibril structures (f; see also fig. 3g).