The Megakaryocyte Lineage Arises in the Yolk Sac and Generates an Initial Wave of Large Embryonic Platelets in the Early Mammalian Embryo.
Authors: Palis James , Tober Joanna , Vemishetti Radhika , Koniski Anne , Waugh Richard 
Published: October 2019

Abstract
Two distinct waves of hematopoietic progenitors originate in the yolk sac of the mammalian embryo. The first “primitive” wave contains primitive erythroid and macrophage progenitors that generate the embryo’s first red cells (Palis, et al. Development126:5073, 1999). The second “definitive” wave consists of definitive erythroid (BFU-E) and multiple myeloid progenitors that arise later in the yolk sac and are subsequently found in the fetal liver and postnatal marrow. While megakaryocyte progenitors have been detected in the yolk sac, the ontogeny of the megakaryocyte lineage is poorly understood. Furthermore, it is not been determined when platelets first enter the bloodstream of the mouse embryo. The presence and size of platelets in the embryonic bloodstream were examined by microscopy after staining with anti-GPV antibodies. Rare platelets were identified in 2 of 4 litters of mice at E10.5. These platelets were very large with a diameter of 4.2 ± 0.4 (mean ± SEM) microns. Platelet size remained large (4.0-3.8 microns) at E11.5–E12.5, but decreased to 3.3-3.2 microns in diameter between E13.5 and E15.5 of gestation. At birth, the mean platelet diameter (2.8 ± 0.04 microns) was similar to that of adult mice (2.7 microns). These results indicate that large, embryonic platelets begin to circulate in the mouse embryo beginning at E10.5 and raise the possibility that embryonic, fetal, and adult waves of platelets are produced during mammalian embryogenesis. Using a collagen-based culture system, megakaryocyte progenitors (Meg-CFC) were first identified in late primitive streak embryos (E7.25), concomitant with primitive erythroid progenitors. Meg-CFC numbers subsequently expand in the yolk sac along with BFU-E before the development of the fetal liver. To examine the relationship of the megakaryocyte and primitive erythroid lineages in the yolk sac, we stained hematopoietic colonies grown in collagen for 5–10 days with anti-GP1bβ (megakaryocyte) and anti-βH1-globin (primitive erythroid) antibodies. As expected, the majority of the stained colonies were primitive erythroid (containing only βH1-globin-positive cells) and approximately 15% of the colonies were megakaryocyte (containing only GP1bβ-positive cells). However, 15% of the colonies contained both GP1bβ- and βH1-globin-positive cells consistent with an origin from bipotential primitive erythroid/megakaryocyte progenitors. Furthermore, proplatelet formation was evident in both unipotential and bipotential megakaryocyte colonies cultured for 10 days. Our studies support the concept that the megakaryocyte and primitive erythroid lineages originate in the yolk sac from a bipotential precursor. This parallels lineage relationships in the bone marrow which contains bipotential definitive erythroid/megakaryocyte progenitors. Finally, we hypothesize that yolk sac-derived “primitive” Meg-CFC give rise to the first embryonic platelets that enter the bloodstream soon after the onset of circulation as the fetal liver becomes a hematopoietic organ.