A comparative study of primary and secondary granules in monocytopoiesis and myelopoiesis of mouse bone marrow.

The differentiation and maturation of monocytes and neutrophil granulocytes were studied in bone marrow of normal mice by electron microscopy and cytochemical assessment of peroxidatic activity. The granule populations of the mature cells of bone marrow were identified and investigated to obtain a basis for the analysis of the earlier stages of maturation. Mature monocytes and neutrophils showed primary and secondary granules, and mature neutrophils had more of both kinds. The size, shape, and number of primary granules proved to offer the most reliable criteria for distinguishing promonocytes and promyelocytes. The primary granules of monocytes were smaller than those of mature neutrophils and were either spherical (smallest diameter 50-200 nm) or elongate (100 X 400 nm). Both granules had a homogeneous matrix. The granules of the granulocytes were either spherical (smallest diameter 200-300 nm) or elongate (150-200 X 300-500 nm), and some of them had a crystalline inclusion.