Immunohistochemical study of alpha-fetoprotein in rat embryos during ontogenesis

In order to clarify the dynamics of alpha-fetoprotein (AFP) during ontogenesis, the appearance and localization of AFP and albumin in rat embryos were immunohistochemically investigated. AFP began to appear in the embryos on day 9 in some of the cells constituting the embryonic ectoderm. From day 10, both AFP and albumin became strongly stained in the yolk sac. The formation of liver anlage started on day 11; here AFP alone was clearly apparent in the primitive hepatocytes and also in some other cells, such as somites, or those of the central nervous system. After further development of the embryos, AFP, albumin, and transferrin were detected by means of serial section examination in the same hepatocytes. Besides being found in the liver, AFP began to be stained in the dorsal pancreas and mesonephric vesicles on day 12, in the stomach on day 13, and in the lung anlage and metanephros on day 14. Albumin was also localized in the same sites in these tissues, but the intensity of albumin staining was not as strong as that of AFP. Therefore, we assume that AFP plays an important role in the rapid growth of rats during the early gestational stages.