Rat bone marrow was fixed in glutaraldehyde, postfixed in osmium tetroxide, and processed for electron microscopy. The myeloid cells were arranged in order of maturation according to their successive compartments.
On the basis of their differences in form, substructure, volume, and density five morphologically distinct types of developing granules are to be observed in neutrophil, two in eosinophil, and four in basophil, cells. Primordial granules appear in the interphase of the myeloblast, respectively in the early promyelocytes. The first granules in the neutrophils are pale, of homogeneous structure. These granules grow gradually denser with increasing condensation. In the myelocyte stage polymorphism is more pronounced. In the granulocytes, vacuoles and dense-cored vacuoles indicate the sites of granules. In the eosinophil line, the “basophilic bodies” decrease in number during differentiation. The eosinophil granules show fewer variations in the course of maturation than the neutrophils. The immature forms of the basophil granules are relatively large, pale, and of globular structure; they undergo condensation and show gradually higher density.
Sites of granulogenesis in the rat are first of all the Golgi apparatus and, possibly, the cisternae in the endoplasmic reticulum. On occasion, bodies in a transitional stage between a mitochondrium and a granule can be observed, but whether they may have a bearing on the problem of granulogenesis is an open question.