Immunolocalization and endocytosis of the uterine secretory protein, uteroferrin, in pre-implantation pig trophectoderm on day 11 of pregnancy.

Uteroferrin is a progesterone-induced, iron-binding glycoprotein secreted by the glandular epithelium of the pig endometrium. Evidence is presented that maternal uteroferrin is present in trophectoderm of preimplantation pig blastocysts on day 11 of pregnancy. Although [35S]-methionine was not incorporated into uteroferrin during in vitro culture of blastocysts, solubilized tissue extracts from 10-20-mm-diameter blastocysts contained uteroferrin by western blotting with monospecific antiserum to uteroferrin. Uteroferrin was detected in the apical and basolateral cytoplasm of trophectoderm by immunocytochemistry of paraffin-embedded blastocysts. Immunostaining was excluded from cells of the endoderm and the inner cell mass. Furthermore, blastocysts internalized fluorescein-labeled uteroferrin from medium during in vitro culture in a temperature-dependent manner. Fluorescent label was located in apical and basolateral cytoplasm in a punctate distribution, and clustered in the supranuclear region of trophectoderm. Addition of a threefold excess of unlabeled uteroferrin to culture medium did not inhibit uptake. These results suggest that the pre-implantation pig blastocyst actively endocytoses uteroferrin from glandular secretions in utero. Uptake was restricted to trophectoderm.