Urea-modified lignin was prepared according to the Mannich reaction and well characterized by Fourier transform infrared spectrometer, elemental ananlyses, and scanning electron microscopy (SEM). Ammonium polyphosphate (APP) and urea-modified lignin were added into poly(lactic acid) (PLA) as a novel intumescent flame-retardant (IFR) system to improve flame retardancy of PLA. The flammability of IFR-PLA composites was studied using limiting oxygen index, UL-94 vertical burning method and cone calorimeter test, and their thermal stability was evaluated by thermogravimetric analysis. The results showed that the urea-modified lignin combined with APP exhibited much better flame retardancy and thermal stability than that of the combination of virgin lignin and APP. The improvement may be attributed to the better char morphology with more phosphoric char evidenced by SEM and X-ray photoelectron spectroscopy.

The yolk sac is the first site of hematopoiesis in the mammalian embryo. However, little is known about the initial stem cells in the yolk sac. We have isolated hematopoietic stem cells from early mouse embryonic yolk sac by using a sequential protocol of nonadherence to plastic, density gradient centrifugation, immunocytoadherence, and cell sorting. Isolated, nonadherent, density < 1.077-g/cm3, surface antigen AA4.1+, wheat germ agglutinin bright (WGAbright) cells give rise to multiple lineages, including T cells, B cells, and myeloid cells, as detected by using fetal thymus organ culture, S17 stromal feeder layers, or methylcellulose culture colony-forming cells, respectively. AA4.1+, WGAbright cells expressed high levels of heat-stable antigen (HSA) and CD45 (Ly-5) but did not significantly express major histocompatibility complex antigens, CD44, or Sca-1. Peak stem cell concentration is reached by day 11, before stem cells can be found in the liver, omentum, or thymus. In vivo long-term reconstitution of lethally irradiated mice was effected by as few as 720 AA4.1+, WGAbright yolk sac cells, but it required addition of a subset of bone marrow cells capable of providing immediate (short-term) radiation protection. Yolk sac donor-derived T cells, B cells, and macrophages were readily identified 6 months after transfer of yolk sac-derived stem cells. We suggest that, because of their cell surface phenotype as well as their capacity to differentiate in vitro and in vivo, the cells isolated from the mouse embryonic yolk sac may include the most primitive hematopoietic pluripotential stem cells yet identified.

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Abstract. The yolk sac is the first site of hematopoiesis during mammalian development. The yolk sac is also the first site of blood vessel development. Development of the blood islands in the yolk sac is an integrated process in which these two developmental events, hematopoiesis and vasculogenesis, proceed in concert. This review focuses on mouse yolk sac hematopoietic stem cells (YS-HSC), describing their differentiation in vitro and in vivo. YS-HSC go through a progressive series of changes prior to the initiation of lineage-specific differentiation. Experiments tracing their origins from postulated hemangioblasts, and the subsequent interaction between these stem cells and yolk sac endothelial cells are described. Differences between the extraembryonic YS-HSC and HSC found later within the embryo, perinatally or in adults, are described. YS-HSC have greater reproductive capability than HSC obtained from fetal liver, umbilical cord blood or adult bone marrow; they do not yet express major histocompatibility complex-associated antigens and they are able to reconstitute adult immunocompromised animals even when introduced in small numbers (<100 cells/mouse). With recent results demonstrating the feasibility of expanding YS-HSC in vitro as well as of introducing new genes into these cells by transfection, the YS-HSC shows promise both as a means of achieving long-term restitution of hematopoiesis across histocompatibility barriers and as a self-renewing vehicle for gene transfer.

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