Membrane vesicles from trophoblast cells as models for placental exchange studies

One of the more important functions of the placenta is the transport of solutes between the maternal and the fetal compartments. Studies of transport in either the intact or perfused whole placenta have described the general mechanisms involved in the transfer of particular solutes (for recent reviews, see Young, 1981). These approaches have certain limitations with respect to defining specific transport phenomena. If separate transport properties exist at the maternal and fetal plasma membranes of the trophoblast, studies with whole tissue may fail to distinguish them. Furthermore, the studies of transport of nutrients are hampered by the metabolic activity present in the whole placenta. Isolated plasma membrane vesicles have recently been used in an effort to overcome some of the problems inherent in whole tissue approaches. At the onset I should point out that in this review I am using the term 'vesicle' in a restricted sense to denote a plasma membrane derived enclosed structure prepared for transport studies. The use of isolated membrane vesicles in renal and intestinal epithelia has been reviewed (Hopfer, 1978; Murer and Kinne, 1980). Membrane vesicles from the placental trophoblast provide an experimental system where the specific transport mechanisms located in the maternal facing plasma membrane and those present at the fetal face can be separately examined. An understanding of placental transport could then be reconstructed from knowledge of the properties of each of the two membranes. This article will discuss methods of preparation of trophoblast plasma membrane vesicles, measurements of transport in these vesicles and the types of transport mechanisms which have been defined to date.