Ultrastructural Identification of the Mouse

A LTHOUGH MAMMALIAN BASOPHILS and mast cells may share a common hematopoietic precursor,’ these cells express different functions and biochemical properties.2’3 In many species, basophils participate prominently in delayed hypersensitivity to biologically important antigens such as contact allergens, soluble proteins, parasites, allografts, viruses, and tumors.2’3 The mouse had long been considered devoid of such reactions; however, recent evidence suggests that certain strains can develop significant basophilia in response to antigenic stimulation.4 With recent reports of mouse cell lines containing histamine and metachromatic granules, ’2 the distinction between basophils and mast cells has become increasingly obscure. Some cell lines have been identifled as mast cells based on ultrastructural6” and functional6’8 evidence. Others have been tentatively called basophils5 or basophil/mast cells.7 The mouse basophil is known only through preliminary light microscopic reports,4 and neither its ultrastructural nor functional properties have been described. Electron microscopy can be used to differentiate basophils from mast cells. These cell types have exhibited distinct ultrastructural features in all mammalian species where they have been carefully studied.2 We have used transmission electron microscopy to establish the identity of the mouse basophil. We describe here the ultrastructure of basophils in normal mouse bone marrow both in vivo and in primary cultures in vitro.