Hepar uterinum: a history of ideas on fetal nutrition.

The means of fetal nutrition has been debated for over two millennia, with the controversy of oral versus parenteral nutrition already in the Corpus Hippocraticum. In 1587 Aranzio rejected connections between maternal and fetal blood vessels, and coined the term "hepar uterinum" for the placenta. From the 16th to the 18th century, a fervent debate focused on the type and extent of connection between maternal and fetal vessels, which was finally settled by Hunter's injection experiment in 1774. But up to the middle of the 19th century, an important nutritive function was attributed to amniotic fluid. When with the discovery of oxygen the placenta's respiratory function became understood, its nutritional function fell from grace. Most scientists realized reluctantly that the organ had numerous functions. As late as in the 19th century, the advent of microscopy allowed cell theory to develop, and analytical chemistry furthered the understanding of the transport of nutrients across the placenta. The identification of the syncytiotrophoblast made passive diffusion unlikely. Radioisotopes, molecular biology and the fluid mosaic model of the cell membrane revealed active transport mechanisms for nearly all macronutrients.