Section 7
Chapter 6,715

The maternal-embryonic barrier in the early post-implantation mouse embryo: a morphological and functional study

Poelmann, R.E.; Mentink, M.M.

Scanning Electron Microscopy 1982(Pt 3): 1237-1247


ISSN/ISBN: 0586-5581
PMID: 7170605
Accession: 006714051

The yolk sac is the most important structure accomplishing nutrition of the early embryo before the establishment of the chorio-allantoic placenta. Four layers are involved, i.e., trophoblast, acellular Reichert's membrane, and parietal and visceral endoderm. Scanning electron microscope and transmission electron microscope studies show that the trophoblast cells, which line the uterine blood sinusoids, are fenestrated, and that the parietal endoderm consists of isolated cells with large intercellular spaces. Reichert's membrane, interposed between trophoblast and parietal endoderm, is the only continuous layer capable of blocking the passage of macromolecules. Horseradish peroxidase, ferritin and Thorotrast were found inside the yolk sac after injection of these substances into a tail vein of the pregnant mother. The parietal part of the yolk sac is not involved in a selection mechanism of molecules with a MW up to 480,000, i.e., ferritin. One of the possible functions of the trophoblast, the supply of native iron to the embryo in a stage of gestation when iron is needed for the formation of the 1st embryonic erythrocytes, is investigated with X-ray microanalysis. Trophoblast cells phagocytose thousands of maternal erythrocytes. These cells are degradated but the Hb-derived iron could not be detected by X-ray analysis. Iron is probably not stored in trophoblast cells in the form of ferritin, because ferritin can be shown by the X-ray technique, as is demonstrated by analysis of embryos which received an injection of exogenous ferirtin. The process of erythrophagocytosis is facilitated because the trophoblast cells are a part of the lining of the uterine sinusoids. In vascular microcorrosion casts of the maternal circulation, the imprint of these cells is demonstrated. After passing the parietal yolk sac, proteins are ingested by the visceral endoderm, as is demonstrated after injection of horseradish peroxidase or ferritin into the maternal circulation.

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