POSTPARTUM ERYTHROPHAGOCYTOSIS, IRON STORAGE AND IRON SECRETION IN THE ENDOMETRIUM OF THE TREE SHREW (TUPAIA) DURING PREGNANCY

Tree shrews (Tupaia belangeri) develop a bidiscoid endotheliochorial p lacenta. In addition, histiotrophe secreted by uterine glands is absor bed by the paraplacental trophoblast. Histiotrophe which is rich in ir on is necessary for erythropoiesis in the young embryo. This report is part of a study of the accumulation and metabolism of iron in the end ometrium of precisely dated pregnant Tupaia belangeri by application o f electron spectroscopy and histochemistry. In the endometrium of tree shrews which had been pregnant at least once, iron-laden granules wer e present in macrophages and secreting cells of uterine glands. Iron a ccumulated in the endometrium shortly after parturition, when macropha ges phagocytosed erythrocytes at small haematomas 0.2-0.5 mm in diamet er. These haematomas arose during parturition after bleeding into the uterine stroma when the placental discs were detached. At 24 h after p arturition the following structural consequences of the erythrolysosom al breakdown of phagocytosed erythrocytes could be observed: free cyto solic siderin granules, membrane-bound siderosomes, telolysosomes (som e of which contained myelin figures or lipid droplets) and mixed telol ysosomes (containing membranous stacks and siderin granules). During t he lysosomal degradation of phagocytosed erythrocytes, iron was transf erred from haemoglobin into a different macromolecular compound. Elect ron energy loss spectra detected from inside siderosomes indicated an iron-oxygen compound, and high-power bright field electron micrographs of siderosomes demonstrated the ultrastructural pattern characteristi c of ferritin. At about d 12 of a new pregnancy, macrophages containin g siderosomes closely approached the bases of secreting cells of endom etrial glands. This strongly suggests that iron is transferred from th e macrophages to the glandular cells. Within the glandular cells, iron -rich histiotrophe was synthesised and released into the glandular lum en. Within the uterine cavity this histiotrophe was absorbed by the om phalopleure. We suggest that among eutherians, postpartum erythrophago cytosis, the transfer of iron from macrophages to uterine glands, and the paraplacental uptake of iron, represent an ancestral mechanism of iron supply to the embryo.