Placentation Fundamentals

Viviparity and the development of a placenta are two of the major reasons for the success of the vertebrates in colonising all habitats, both terrestrial and aquatic.

This book is mainly concerned with providing pictorial and descriptive data to facilitate broad comparisons of structures and secretions both during development and in their final form with a view of relating them to the functional activity of the placenta. In the viviparous vertebrate amniotes, both reptilian and mammalian, the remarkably wide variety of structures in the mature placenta develops from a similar repertoire of extraembryonic membranes as typified in the amniote egg. The diversity can be further simplified by use of straightforward structural criteria. The major problem is to relate those criteria to the wide range of functions served by the placenta. An appreciable number of viviparous vertebrate non amniotes, fish and amphibia, have developed analogous structures which serve similar functions to the extraembryonic membranes, These will be discussed briefly as they offer further insights into the requirements for successful fetal development. Even a few invertebrate species utilise placental analogues, a tribute to the advantages of viviparity (see Chap. 9.2) in particular environments (Hagan 1951; Woollacott and Zimmer 1975; Amoroso et al. 1979; Bone et al. 1985).