The infected myeloblasts from the circulating blood of chickens with this form of leukemia resemble closely, figure 1, the blast cells found in the bone marrow of normal birds of the same age. There is seen a large nucleus with a prominent nucleolus and often a nuclear notch which is best demonstrated by phase contrast as described in another section. A Golgi zone is located near the notch and consists of small vesicles and a few flattened sacs. A section usually includes several mitochondria and, also, several smaller round or ovoid bodies, designated earlier (9, 81) as gray bodies which probably are analogous to the microbodies, globoid structures, thought (82) to represent mitochondrial precursors. The endoplasmic reticulum is rather sparse and appears chiefly as small vesicles with a few longer profiles. Many ergastoplasmic granules are free in the cytoplasm. Recognizable virus particles occur in these sections only rarely, usually singly in small vesicles or vacuoles.

It is well to call attention in figure 1 to the presence of several vacuoles. As can be seen in phase contrast or after supravital staining with neutral red, such structures are arranged in a rosette pattern in or about the Golgi region. For the most part they are rather large and, in contrast to the appearance of monocytes under the same conditions, are relatively uniform in size. No difference is evident between the vacuoles of the normal myeloblast and those of cells from tissue cultures or circulating blood in size, staining reaction with neutral red, number, or distribution. Not all vacuoles are in this region, since a few may be scattered in other parts of the cells. In the vacuoles of myeloblasts associated with the virus, there may occur, as evident in figure 1, numbers of amorphous, darkly staining particles and, rarely, in the cells from the circulating blood, one or more virus particles of typical appearance (2, 88). It is notable, also, that there exists no definitive criterion for distinguishing every micro- or gray body from other structures of various sizes and similar staining reaction to osmium tetroxide.

Myeloblasts From Cultures Inadequate To Support Growth

As described earlier (84), myeloblasts may be maintained for long periods in culture media consisting of 20 percent chicken serum in a balanced saline solution in which the rate of cell multiplication is approximate^ the same as the rate of cell death. Under these conditions, despite the lack of increase in the cell population, there occurs a continuous release of myeloblastosis virus into the culture medium at rates varying from about 5 to 40 virus particles per cell per hour (28). Examination of thin sections of such cells reveals no feature, with the possible exception of larger numbers of small gray bodies, distinguishing them from cells studied immediately after removal from the circulating blood. Another possible difference is a tendency toward increased distinctness of cellular structures of cells from culture as compared with those taken directly from the bird. This impression would be difficult to quantitate, but the interpretation seems justified on the basis of examination of many sections of these cells. The state of the cell taken directly from the chicken may possibly be related to a relatively poor nutrient state of the blood of the leukemic bird (35).