Dysregulated erythopoiesis in kit^D814Vflox deleter-Cre neonatal mice. The kit^D814Vflox deleter-Cre genotype, which results in early and general deletion of the stop element and expression of kitD814V in all cells expressing wt kit, causes perinatal lethality in approximately 75% of newborn mice and is associated with a hyperproliferative dysregulation of the erythroid lineage. (A) FACS analysis of peripheral blood from newborn mutant animals displays an abnormal population of small, nongranulated cells (region R1). Gating on this population (lower panel) reveals that these cells express the erythroid marker Ter119 and that approximately 50% of the cells take up the dye propidium iodide and are therefore dying or dead. (B) Positive staining with the cell-permeant dye Draq5 provides additional evidence that this population (R1) does not represent normal erythrocytes but indeed nucleated cells. These cells also express the transferrin receptor (CD71) but not CD45 (bottom panel). Blood smears (C, 40×/0.75 objective and 63×1.3 NA oil objective) show large numbers of small cells with dense nuclei and sparse basophilic cytoplasm resembling erythroblasts, but also numerous nuclear shadows (*) represent remnants of cells, indicating the presence of a cell population with increased mechanical fragility. Whole-mount sections of bitransgenic newborns demonstrate drastically increased numbers of nucleated Ki67 cells in blood vessels (Ki67 immunostaining: D, control; E, mutant, 20×/0.7 NA objective). The liver of the bitransgenic mice (F, mutant; G, control, 20×/0.7 NA objective) contained a massively increased number of hematopoietic cells (dense, dark blue nuclei, hematoxylin and eosin staining). Flow cytometry of fetal liver homogenates for the markers CD45 and Ter119 gated on DNA containing cells reveals an increase of nucleated Ter119⁺ cells within the liver of kit^D814Vflox deleter-Cre mutants (I) versus control (H). Dramatic increase in hematocrit (J) in mutants (☐) versus control animals (○).