The first-appearance of neutrophils in the human fetal bone marrow cavity

We sought to define the time of first-appearance of neutrophils within the developing human bone marrow cavity, and to compare this with the time of a ppearance of G-CSF and its receptor (G-CSF-R) at that site. We hypothesized that the onset of G-CSF production is an initiation signal for neutrophil production within the marrow cavity, and that therefore G-CSF mRNA and G-CS F protein in the marrow cavity would immediately precede the first-appearan ce of neutrophils. To test this, we determined the time of first-appearance of neutrophils in the clavicular marrow space using a monoclonal antibody against myeloperoxidase (MPOAb), and then validated these findings by flow cytometric analyses, for neutrophil cell-surface markers, of cells flushed from the marrow cavity. After thus defining the time of first-appearance of neutrophils, specific mRNA transcripts for G-CSF and G-CSF-R were sought f rom clavicles of varying gestational ages, using RT-PCR, and the presence o f these proteins in the clavicles were sought using immunohistochemistry. W e observed that; (1) MPO + cells first appeared in the clavicular marrow ca vity between the 10 to 11th weeks post-conception, (2) Flow cytometric anal yses confirmed that these MPO + marrow cells included CD11b +, CD15 + neutr ophils, (3) Transcripts for G-CSF and G-CSF-R, and the specific G-CSF and G -CSF-R proteins, were present in the clavicles by 6 weeks postconception, 4 to 5 weeks before the first-appearance of neutrophils. Thus, neutrophils f irst appear in the human clavicular marrow at 10-11 weeks post-conception, and G-CSF and G-CSF-R are present in the developing bone rudiment preceding the appearance of neutrophils. It is unclear whether neutrophils arise in the marrow cavity in response to the onset of production of G-CSF or to oth er initiation signals. (C) 1998 Elsevier Science B.V. All rights reserved.