Electron Microscopy of the Lymphocyte 



63 



Nucleoplasm: 



C.\ toplasm: 



Mitochondria: 



Endoplasmic 

 reticulum: 



Lymphocyte 



dark areas distributed in 

 essentially patternless 

 fashion 



clear appearance 



large, moderate number 



small, round or oval pro- 

 files, sparse 



Monocyte 



dark band inside of nu- 

 clear membrane, some 

 dark areas within nu- 

 cleus 



speckled appearance 



small, numerous 



small, round or oval pro- 

 files, very numerous 



holds true regardless of the dimensions of the cell. A large lymphocyte is 

 clearly distinct from a monocyte. But some agranulocytes in normal, human 

 blood possess morphologic characteristics that make uncertain their identi- 

 fication as either monocytes or lymphocytes. For example, the cell in Figure 

 4-12 has an indifferent pattern of nucleoplasmic differentiation and large 

 mitochondria resembling those of a lymphocyte. But its cytoplasm has 

 many profiles of endoplasmic reticulum and is darker than lymphocytic 

 cytoplasm, both characteristics suggesting a monocyte. Observation of many 

 agranulocytes of uncertain identity has failed to reveal any pattern govern- 

 ing the mixture of confusing characteristics. Mitochondria, endoplasmic 

 reticulum, general cytoplasmic characteristics, and nucleoplasmic patterns 

 may be combined in a single cell in the forms characteristic of either cell 

 type so as to produce all possible varieties of structural intergrade. 



Some agranulocytes in normal peripheral blood possess nucleoplasm that 

 is either uncertainly differentiated (Fig. 4-13) or of even density throughout 

 (Fig. 4-14). The evidence of progressive nucleoplasmic differentiation 

 afforded by the leukemias, notably lymphatic leukemia, (Figs. 4-8, 4-15) 

 seems to justify the classification of these cells as lymphoblasts. 



SUMMARY 



The evidence now available from the electron microscopy of normal and 

 leukemic human blood 11 may be used to compare the lymphocyte and its 

 developmental series to the other blood cells and their developmental series. 

 In brief, the evidence from the human leukemias indicates that the parent 

 cell of the bone marrow usually called a myeloblast (stem cell) and the 

 parent cell of the monocytic series, the monoblast, are morphologically in- 

 distinguishable from each other under the electron microscope. But the 

 parent (ell of the lymphocytic series, the lymphoblast, is always readily 

 identifiable as such. There is never any danger of confusing it with any 



