We examined long bones from 42 human embryos and fetuses whose gestational ages ranged from 6 to 28 weeks. Bone rudiment sections were stained using a panel of monoclonal antibodies directed at antigens expressed by hematopoietic cells, endothelial cells, smooth muscle cells, fibro-blasts, and stromal cells, to describe the events preceding and accompanying the onset of hematopoiesis in the diaphyseal region. Five distinct stages were identified. Stage I (6.6 to 8.5 gestational weeks [gw]) was that of entirely cartilaginous rudiments: chondrocytes were dilated and capillaries with CD34+ endothelial cells were observed in the perichondral limb mesenchyme. At stage II (8.5-9 gw) chondrolysis was actively proceeding; numerous CD68+ cells were observed, interspersed within the marrow cavity. Stage III (9 to 10.5 gw) was characterized by the development of the vascular bed in the absence of detectable hematopoiesis. At mid-diaphysis, specific structures that we named primary logettes were discernible; they consisted of small chambers of connective tissue, framed by a loose network of CD45-negative cells organized around an arteriole and limited from the surrounding sinus by a clearcut lining of CD34+ endothelial cells flanked on their abluminal side by alpha SM actin+ myoid cells. Stage IV (10.5-15 gw) was characterized by the onset of hematopoiesis. Hematopoietic cells were found exclusively in the primary logettes that had considerably increased in size. Logettes filled with hematopoietic cells were immersed within large and almost empty vascular sinuses. Logettes were attached by a short pedicle to connective tissue adjacent to bone/cartilage remaining formations; this tissue contained very rare hematopoietic cells. Logettes were few, usually less than 10 per long bone, and found solely in the diaphyseal area. Most hematopoietic cells found inside logettes were CD15+ myelocytes; rarely seen were glycophorin A+ immature erythroblasts and CD34+ nonendothelial cells. Hematopoietic cells within the logettes were in contact with alpha SM actin+ myoid cells and flattened endothelial-like (although consistently CD34-negative), aligned cells limiting small capillary lumina. Stage V (16 gw onward) was that of final organization of the long bones with areas of fully calcified bone and areas of dense hematopoiesis where logettes were no longer visible. This study shows three major features of incipient long bone hematopoiesis: 1) absence of CD34+ hematopoietic precursors before the onset of hematopoiesis and extreme rarity of those in the emerging blood-forming marrow, 2) predominance of granulopoiesis, and 3) exclusive development in specific structures organized by vascular cells. This study also suggests that CD68+ cells are instrumental in the chondrolysis process while vascular cells (endothelial and myoid cells) may be the critical microenvironment at the onset of hematopoiesis.