Identification of Promonocytes and Monocytoid Precursors in Acute Leukaemia of Adults: Ultrastructural and Cytochemical Observations

Abstract

Summary. Examination of 25 cases of adult non‐lymphoid acute leukaemia by electron microscopy and esterase cytochemistry revealed that most cases contained a majority of classifiable cells, however undifferentiated they may have appeared by routine light microscopy. The major proliferating cell in most cases (72%) was found to be a promonocyte or monocytoid precursor cell, characterized by extreme nuclear convolution, large cytoplasmic bundles of microfilaments, and numerous small electron‐dense lysosomal granules, which strongly stained for alpha‐naphthyl acetate esterase. Pure granulocytic cases, representing a minority (24%), were composed of blasts and typical promyelocytes and myelocytes, staining for Naphthol‐AS‐D chloroacetate esterase. Only one case was totally composed of undifferentiated, unclassifiable blasts. These results show that electron microscopy and cytochemistry increase the accuracy of classification of leukaemia and that the true incidence of monocytic leukaemia may be much higher than generally recognized.