Researchers at the University of Rochester Medical Center have discovered that the precursors of blood cells develop in the embryos of mammals sooner than previously thought. The discovery may ultimately help researchers to better understand under what conditions blood cells form, an essential piece of the puzzle for those looking to find ways to expand blood stem cells for human patients needing bone marrow transplants.
The abstracted findings of the study are being presented on May 4 at the 1998 Pediatric Societies' annual meeting in New Orleans. Using the mouse embryo as a model, the researchers sought to pinpoint the place and time that blood cells precursors first arise. "We wanted to know where and when embryonic cells become committed to blood cell fates, if blood cell precursors originate in the embryo or in the yolk sac, and at what point they show up in the body of the embryo," said Jim Palis, M.D., associate professor of pediatrics and the team's lead researcher.
Palis and his colleagues found that blood cell precursors originate soon after the first appearance of mesoderm cells in the embryo. These findings suggest that mesoderm cells become committed to blood cell fates during earliest stages of gastrulation.
Red blood cells form in two distinct waves. The first wave consists of primitive red blood cells that form in the yolk sac. The yolk sac is an organ outside of the forming embryo body that transfers nutrients from the mother to the embryo before the placenta is formed. The second wave consists of definitive red blood cells that form later in the body of the fetus and in adults. Using an assay system developed by co-authors Gordon Keller, Ph.D., and Marion Kennedy, Ph.D., of the National Jewish Medical Center in Denver that can distinguish between primitive and definitive red blood cell precursors, the research team found that definitive red blood cell precursors were found in the yolk sac before they were present within the body of the embryo. "We were able to find definitive red blood cell precursors inside the yolk sac even before the circulation begins or the liver develops," Palis said. This finding was unexpected since recent scientific theories have proposed that definitive red blood cell precursors arise inside the embryo.
Hematopoietic stem cells, which give rise to all circulating blood cells, are present in the bone marrow of the adult, the cord blood of the newborn, and the liver and central body region of the fetus. However, the developmental origin of these stem cells has not been clearly elucidated. Although Palis cautions that his team's basic research may not translate into an immediate benefit for humans, pinpointing the exact time and place that blood cell precursors naturally arise in the developmental process may provide clues to the origin of blood stem cells and the environmental conditions that support their expansion for use in bone marrow transplantation.