Enhanced bone marrow apoptosis in Kostmann syndrome. (A) Bone marrow biopsies from patient 2 were obtained prior to or after the initiation of G-CSF treatment, and sections were stained with hematoxylin-eosin and Giemsa for the morphologic evaluation of indices of apoptosis. Arrows indicate typical examples of apoptotic bodies in the pretreatment specimen. Apoptosis was significantly diminished in sections obtained during treatment with G-CSF. (B) Cell proliferation, as evidenced by immunohistochemical detection of the nuclear antigen Ki-67, is shown in bone marrow sections from patient 2 obtained prior to or after the initiation of G-CSF treatment. Similar data were obtained in all Kostmann patients included in the present study. Original magnification × 400.