This is a cytospin from the CSF of a patient with L1 acute lymphoblastic leukemia (ALL) obtained at the time of diagnosis.
Notice the monotonous look to the cells present. The majority of the cells are blasts of moderate size with soft fine chromatin and a scant amount of basophilic cytoplasm. There is some irregularity and slight cleavage to the nuclear shape. Some of these blasts have cytoplasmic vacuoles. Though nucleoli are present in some of these cells, it not necessary for nucleoli to be present for these cells to be considered blasts. A high nuclear/cytoplasm ratio (N/C ratio) with large nuclei and sometimes scant cytoplasm define these cells as L1 lymphoblasts.
Notice the three small normal lymphocytes indicated by the arrows. The size and chromatin texture of these normal cells provide a contrast to the size and chromatin testure of the blasts.