An ultrastructural comparison of the uptake and transport of horseradish peroxidase by the rat visceral yolk‐sac placenta during mid‐ and late gestation

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Abstract
Horseradish peroxidase was used to explore electron microscopically the uptake and transport of protein by the rat visceral yolk sac at 2 minutes, 6 hours, and 12 hours following a single intravenous injection into pregnant rats on day 12 and day 21.5 of gestation.On both days, the visceral endoderm absorbed peroxidase via micropinocytosis at 2 min postinjection, but not at the 6 and 12 hr intervals. At the latter two postinjection intervals on day 12, peroxidase was localized mainly at two sites, i.e., within intraepithelial supranuclear storage vacuoles, and within the vitelline endothelium deep to the visceral endoderm. On day 21.5 at 6 and 12 hr postinjection, peroxidase was localized in supranuclear storage vacuoles, though most of it was within tubular structures and small vacuoles in the paranuclear and infranuclear cytoplasm of the endoderm. Many dense vacuoles were in close anatomical relationship with the basal cell membrane. Deep to the endoderm a few macrophages were actively engulfing peroxidase, but reaction product was rarely found in the fenestrated vitelline endothelium.The results presented differ from the previous physiological and anatomical transport studies of the visceral yolk sac in that protein (peroxidase) transport was observed deep to the endoderm during both mid‐ and late gestation.

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