Immunohistochemical Comparison of Ontogenic Development of Bone Marrow Hematopoiesis in Two Different Ossification Systems

It is well known that the body skeleton is formed by two different types of ossification systems, endochondoral and intramembranous ossification. Bone marrow is the main site of active hematopoiesis after the formation of the bone marrow cavities. However, it is unclear whether the hematopoiesis in the bone marrow of two types of ossification is regulated by the same system or not. In this study, we focused on the ontogenic development of bone marrow hematopoiesis in two different ossification systems using mouse humeral bones and palatal process of maxillary bones. Immunohistochemical and RT-PCR analyses were performed to examine the development of hematopoiesis and the expression of cytokines related to hematopoiesis in the forming bone marrow (16-days gestation stage to 1-day postnatal stage). Immunohistochemical studies showed the sequential difference of hematopoiesis between two different ossification systems. In humeral bone marrow, granulopoiesis appeared first at E16, followed by erythropoiesis from E17. On the contrary, erythropoiesis preceded one day in the maxillary bone marrow at E18, one day before the detection of granulopoiesis. G-SCF and GM-CSF were expressed at every examined stage in both types of bones while M-CSF was not expressed in the humeral bone marrow at E16. Erythropoietin was detetcted in the endothelial cells and its expression was coincident with the onset of erythopoiesis. These results suggest the time kinetic and sequential differences of hematopoiesis in two different ossification systems, which might relate to the differences of hematopoietic microenvironment