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In the bone marrow, an elaborate stroma forms the structural basis of the hemopoietic microenvironment. In this study, two different types of stromal cells were identified with certainty on tissue sections of intact bone marrow of rats and mice using light and electron microscopic histochemistry: (a) a fibroblast-type of reticulum cell which is characterized by having alkaline phosphatase associated with its plasma membrane. We refer to this cell as the alkaline-phosphatase-positive reticulum cell (Al-RC). It is closely associated with granulocytic precursors, particularly myeloblasts and neutrophilic promyelocytes. These reticulum cells may be found throughout the marrow but are concentrated near the endosteum. (b) a macrophage-type of reticulum cell which is characterized by its abundance of lysosomal acid phosphatase and is mainly associated with erythroid precursors (as observed by others). In contrast to the above-mentioned cell type, this latter cell was found to be distributed uniformly throughout the marrow. We speculate that the Al-RC are mesenchymal stromal cells necessary for granulocytic differentiation in bone marrow.

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