ABSTRACT
The yolk sac is the first definitive fetal membrane of vertebrates. During embryonic development, hematopoiesis begins first in the yolk sac. This study was carried out to investigate the ultrastructural characteristics of the early rat extraembryonic visceral yolk sac membrane and hematopoietic cells localized in the yolk sac vitelline vessels. Pregnant albino rats were killed at gestational days 10-12. Yolk sac specimens were processed for light, transmission, and scanning electron microscopic examination. The study demonstrated the microscopic structure of the yolk sac membrane and the different hematopoietic cells in yolk sac vitelline vessels at this stage of gestation. The endoderm cells revealed a cytoplasm rich in cell organelles characteristic of endocytosis, phagocytosis, and active protein synthesis, as well as cell junctions between adjacent cells. The apices of the endoderm cells revealed numerous microvilli of two types and a whitish coat on top of the cells. In yolk sac vitelline vessels, a large number of blood cells were seen. Many cells were at different stages of mitosis. Ultrastructurally, blast cells including immature differentiating erythroblasts were recognized, together with a few mature erythrocytes. Furthermore, mature lymphocytes were also seen in vitelline vessels on gestational day 12. Yolk sac lymphocytes revealed a morphology typical of peripheral blood lymphocytes. This study demonstrated the importance of the yolk sac for adequate embryonic nourishment, as well as different blood elements necessary for potent immune response. The location of mature lymphocytes inside vitelline vessels suggests a possible emergence from the yolk sac membrane. However, future studies are needed to explore the exact origin of these cells