Distribution and characterization of anionic sites in trophoblast and capillary basal laminas of human placental villi

The distribution of anionic sites was studied in the trophoblastic and fetal capillary basal laminas of developing human placental villi with the cationic stain ruthenium red. At 7-12 weeks of gestation the trophoblastic basal lamina (TBL) contained ruthenium red-positive granules in a quasi-regular array throughout the lamina densa or sometimes concentrated at the interstitial surface of the lamina densa. The capillary basal lamina (CBL) (and anionic sites) were not present at this age. Anionic sites were also associated with collagen or reticular fibrils. At term, the TBL was largely devoid of anionic sites except for some distributed along its interstitial surface. The CBL was present in later gestation and sometimes had arrays of anionic sites. In order to characterize the anionic sites, minced pieces of villi were incubated in the presence or absence of either chondroitinase ABC, heparitinase, neuraminidase, or Streptomyces hyaluronidase in appropriate buffer systems. Incubation of early villi with heparitinase resulted in the disappearance of the TBL-associated sites. Chondroitinase ABC appeared to reduce staining of collagen-associated sites. In term villi, heparitinase removed those few sites still associated with the TBL but did not affect sites associated with the CBL or collagen. Chondroitinase ABC resulted in the disappearance of all anionic sites. In later gestation, a number of developmentally important macromolecules are transported across the trophoblast and enter the fetal capillaries. We conclude that the absence of an array of polyanionic sites from the term placenta TBL and the reduction in the amount of extracellular matrix intervening between the trophoblast and capillaries are adaptations to enhance the exchange of macromolecules across the placenta.