Erythropoiesis in the bone marrow of the fetal rabbit.

Abstract

AbstractAlthough the extravascular origin of erythrocytes from undifferentiated mesenchymal or reticular cells in mammalian bone marrow is generally accepted, the morphological evidence on which this concept is based has not conclusively ruled out the possibility of endothelial cell contributions to erythropoeisis or of the preferential localization of developing erythrocytes within endothelial cell‐lined spaces. Since conventional methods of tissue preparation have produced artifacts which obscure the fine details of marrow architecture, the re‐examination of this problem using newer histological and fixation methods permits a more critical study of the bone marrow in nearly artifact free sections.Ribs and long bones of 65 rabbits, ranging from 18 days of gestation to the second day after birth were removed, fixed intact with 10% aqueous acrolein, decalcified in 5% aqueous nitric acid and embedded in plexiglass‐methacrylate. Two micron sections stained with toluidine blue were examined with the light microscope. Serial paraffin sections of formal‐Zenker fixed material also were examined.Morphological studies conclusively indicate that erythrocytes develop extravascularly arising from mesenchymal or reticular cells in the fetal bone marrow. Mature erythrocytes enter the circulation through discontinuities in the sinusoidal walls. Neither endothelial cells or blood‐borne lymphocytes make an apparent contribution to erythropoiesis. The first hemopoietic cells to form in the fetal marrow are determined and develop along the erythrocytic line. These proerythroblasts initially arise randomly in the marrow parenchyma and are not in obvious association with the sinusoids. Subsequent maturation and proliferation of the primitive erythrocytic cells result in the formation of colonies of erythrocytic cells at all stages of development. As these colonies enlarge, the erythrocytic elements come in close association with the sinusoids. In later stages of marrow development, developing erythrocytic and granulocytic cells become intermixed and more randomly associated in the extravascular space of the marrow.