Final stages of erythrophagocytosis in the sheep placenta.

In trophoblastic epithelial cells of the sheep placenta the final stages of erythrocyte breakdown within the lysosomal apparatus were studied at the ultrastructural level. As a result of hemoglobin digestion lysosomes containing hemoglobin-derived pigments (HDP) were formed. The HDP-lysosomes were acid phosphatase-positive, highly electron-dense bodies of round to irregular shape containing whorled membranous formations. The accumulation of these lysosomes in epithelial cells led to fusion resulting in the formation of conglomerates. At the end of the gestation period the amount of HDP-lysosomes and their conglomerates markedly increased. In addition to erythrocytes the trophoblastic epithelial cells in the erythrophagocytic regions phagocytosed maternal leukocytes and neighbouring epithelial cells and giant cells. By gradual accumulation of HDP-lysosomes and remnants of phagocytosed cells, highly electron-dense acid phosphatase-positive residual bodies of variable appearance were formed within the epithelial cells. At the end of pregnancy the spaces between juxtaposed villi of the trophoblastic epithelium in the erythrophagocytic zones were occluded by apposition of the epithelial cells. In these occluded regions an increase in highly electron-dense large-sized residual bodies (15-22 micrometer of dimension) occurred as a result of multiple cell phagocytosis in combination with fusion. In these residual bodies the numerous incorporated HDP-lysosomes and the remnants of phagocytosed cells could still be recognized.