Characterization of trophoblast cell populations by lectin histochemistry in canine placenta during development.

The aim of this study was to identify and characterize populations of trophoblast cells in canine placenta during different stages of fetal development using lectin histochemistry. Dogs have endotheliochorial placentation and trophoblast cell invasion continues after chorioallantois villous penetration early in pregnancy, leading to formation of a labyrinth. Specialized subpopulations of cells differentiate, such as syncytial trophoblast that invades the maternal epithelium early in placentation and surrounds and forms intimate cuffs around maternal blood vessels. Marginal haematomata, which are lined by specialized phagocytic cytotrophoblast cells, form by mid-gestation. Invasive 'extravillous' cells advance into and remodel maternal endometrial tissues further. Placentas and attached uterine tissues were collected and sampled from six bitches at mid-gestation (days 31-33 of gestation) and 12 females in late gestation (day 42-term) for characterization of these tissues and identification of other populations of trophoblast cells. Uterine tissues from nonpregnant bitches were also collected at oestrus (n = 2) and during the luteal phase (n = 1). In histochemical studies, two of six biotinylated lectins that were tested stained cytotrophoblast and syncytial trophoblast cell populations differentially. Arachis hypogaea agglutinin (PNA) was specific for cytotrophoblasts in placental tissue lining villi and cytotrophoblastic cells with phagocytic or absorptive phenotypes in the necrotic zone at mid-gestation. In late gestation, cytotrophoblast cells with an absorptive phenotype at the interface between the labyrinth and lacunar glandular chambers were stained with PNA. Staining of other cells was minimal, with the exception of deep endometrial glands. Lectin binding using Maclura pomifera agglutinin (MPL) specifically stained the same cells as PNA and the population of invasive syncytial trophoblast cells remodelling maternal blood vessels and small maternal vessels at the materno-fetal interface, as well as trophoblast cells within necrotic zones at mid-gestation. Both lectins were positive for phagocytic cytotrophoblast cells lining the haematophagus organs. The results of this study demonstrate that lectin histochemistry is a useful tool for staining subpopulations of cytotrophoblast and syncytial trophoblast cells.