The amniotic fluid compartment: the fetal habitat

Abstract

This study begins with a phylogenetic overview of the fetal membranes in terrestrial vertebrates. It is, of course, well known that the human amnion differs very little from that of other recent members of the phylum, although during evolution the development of the placenta has brought about extensive changes in the chorion. Much of the structure and many of the functions of both the amnion and chorion laeve can be interpreted from a comparative point of view (e.g., production of lipids). The next section deals with the ontogeny of the human amniotic sac, and the histogenesis and functional structure of the fetal membranes. It concludes with a structural analysis of the single layers of the fetal membranes at the end of pregnancy. Light and electron microscopy and histochemical examination are combined with experimental investigations; the striking differences and departure from any possible "norm", however, made the interpretation of morphological findings particularly difficult. Explanations for these differences are put forward: the texture of the connective tissue elements is only approximately determined and therefore gives rise to many irregularities; differences in the structure of cells depend upon their functional state; and the cells exist in groups which reciprocally exchange functional activity with each other. It is emphasized that the amniotic epithelium produces lipids which then appear in the same composition in the amniotic fluid. The fibroblasts in the amniotic connective tissue may, when necessary, be transformed into phagocytes (the so-called Hofbauer cells). Within the fetal membranes one can also observe glucose metabolism which, apart from the supply of local energy and the storage of glycogen, is concerned with production of phospholipids, triglycerides, glycoproteins, proteoglycans, and collagen (types I, III, and IV). A similar process of glucose metabolism is found in the trophoblast, especially the building-up of an extracellular matrix (the so-called fibrinoid), which seems to play a part in the formation of an immune barrier. An attempt is made to correlate the various differentiated cells of the trophoblast with particular functions. The vacuolated cells obviously store glucose. Others, as lectin-binding investigations have shown, are concerned with production of the extracellular matrix. They produce a three-dimensional latticework between the cells, and reach out as far as the borders of the chorionic connective tissue. In this regard, the microtexture of the amniotic and chorionic connective tissue provides a basis for a more precise understanding of their mechanical stability.(ABSTRACT TRUNCATED AT 400 WORDS)