Early Yolk Sac Research Articles

Migration of erythropoietic and prebursal stem cells from the early chicken embryo to the yolk sac.

Lymphocyte development and ontogenetic changes in erythroid cells have been studied in chick-chick yolk sac-embryo chimeras constructed of histoincompatible partners. The results obtained indicate that the early chick yolk sac produces transiently erythroid stem cells whereas definitive erythrocytes are derived from the intraembryonic stem cells. Such a change from the yolk sac-derived cells into embryo-derived cells is not observed in the lymphocytes which are exclusively derived from the embryo-borne stem cells. Experiments with cell transfers from the chimeric yolk sacs demonstrate that erythropoietic and prebursal stem cells migrate from the early embryo to the yolk sac during the second to the seventh day of incubation. The results obtained also exclude the de novo generation of prebursal stem cells in the yolk sac.

Studies on mouse autoreactive cells: III. Fetal development of autorosette-forming cells

Self-recognition assessed by rosette formation by lymphocytes with erythrocytes of syngeneic or autologous origin is a very primitive function that is present before lymphoid system development proper in the thymus. Autologous rosette-forming cells (A-RFC) have been found in the very early yolk sac of pregnant mice of 10–11 days gestation. Moreover, when these 10- to 11-days' gestation pregnant mice were subcutaneously injected with facteur thymique sérique (FTS) 1 day before A-RFC examination, it appears that FTS reduces the number of A-RFC in the yolk sac by 63%. Thus it has not been possible to determine whether FTS acted by changing the migration capacity or the expression of receptors on the cell surface.

Ultrastructural changes of the early visceral yolk sac layer of mouse embryos after maternal injection of trypan blue

The ultrastructural features of the early visceral yolk sac epithelium of normal mouse embryos on day 9 were compared to those whose mothers had received a single subcutaneous injection of 100mg/kg trypan blue on day 8. The following results were obtained: In normal embryos the visceral yolk sac cells are predominantly characterized by numerous membrane bounded inclusions localized in the supranuclear cytoplasm. In embryos of animals treated with trypan, blue, at about 12h after injection large single and only partly membrane bounded vacuoles are observed occupying most of the apical cytoplasm. Up to 24h after injection large cytoplasmic areas are seen which are in a stage of autodigestion possibly due to leakage of the vacuolar content. These alterations are exclusively limited to the visceral yolk sac epithelium whereas in the cells of the embryonic part, e.g. head process, no changes could be found. The observations are discussed in relation to the mechanism of the teratogenic activity of trypan blue.

Embryonic Origin of the Mouse Macrophage

Abstract Progenitor cells capable of giving rise to functional macrophages in in vitro culture were first detectable in the fetal yolk sac of the mouse between day 7 and 8 of gestation. Macrophage progenitors were not detectable outside the yolk sac until day 11. Although the early yolk sac contained macrophage precursors, no cells with the morphologic or functional characteristics of mouse promonocytes or more mature macrophages were observed. Promonocytes and macrophages were first identified in the 10-day yolk sac and 11-day fetal liver. These cells were characterized by surface receptors for IgG immunoglobulin, peroxidase activity (promonocytes), glass adherence, and phagocytosis of a large yeast particle (macrophages). From these observations, we conclude that the early fetal yolk sac is the embryonic site of origin of the macrophage precursor and that this precursor is "proximal" to the promonocyte on the pathway of sequential macrophage maturation.

Differentiation of the Yolk Sac of the Chick Studied by Electron Microscopy

ABSTRACT The marginal cells of the area opaca and early yolk sac are extended distally into long processes which may measure up to 500μ in length by as little as μ in depth. In whole mounts of blastoderms, examined by fight microscopy, these processes can be seen forming a clear rim to the blastoderm. They possess cytoplasmic granules, but no mitochondria or endoplasmic reticulum. It is suggested that these processes enable the blastoderm to migrate under the vitelline membrane. The endoderm of the region lying immediately proximal to the edge has been found to be composed of cells and not of a syncytium as is generally supposed. These cells are frequently separated by patches of modified yolk, consisting of a meshwork of membranes. These membranes possess a fine structure similar to that of myelin figures and it is suggested that they are formed by hydration of yolk lipids by water drawn through the vitelline membrane from the egg white. This meshwork corresponds with the region known at this stage as the periblast by some light microscopists. No evidence about the nature of the periblast in early cleavage is presented. The incidence of desmosomes in the area opaca appears to increase between 1 and 2 days of incubation. It is shown that each cell membrane retains its anatomical identity throughout the desmosome. Structural changes begin to take place in the intracellular yolk drops of the area vasculosa just before the establishment of the yolk sac circulation. These changes are considered to be the result of enzyme activity and it is suggested that partly digested yolk is carried in the blood stream to the area pellucida. These events exemplify the nicety of timing involved in many embryonic processes, for just as the intracellular yolk drops of the area pellucida become used up, so, it seems, new supplies of yolk start to arrive from the area vasculosa. Other changes which take place as the area opaca becomes colonized by mesoderm are described. They include: the re-arrangement of the ectoderm cells and changes in their shape; the loss of dorsal villi from the ectoderm cells and the appearance of more desmosomes and to nofilaments in them; changes in shape and arrangement of the endoderm cells, which become epithelial, acquiring desmosomes and tonofilaments. Pinocytic-like indentations appear at the lower border of the endoderm cells. The mesenchyme cells are described and it is noted that collagen-like fibrils appear at the same time.