Scanning electron microscopy of exposed surfaces of the porcine placenta.

The three-dimensional development of the separated parts of the porcine placenta from 9 Danish Landrace sows at gestational stages from 20 to 100 days was studied. After cautious separation of the allantochorion and the endometrium in a 1 mM buffered ethylenediaminetetraacetic acid solution, the separated parts were processed for scanning electron microscopy by routine methods. The macroscopic enlargement resulted from primary and secondary circular folds or plicae, which were permanent on the maternal side, whereas they were mainly non-permanent on the fetal side. The areolar placenta and interareolar placenta with formation of permanent microscopic folding on both sides were described. The observations of the separated parts yielded new information on the development of surface enlargement during gestation and revealed a hitherto unknown regular architecture of the endometrium by the formation of parallel primary ridges or rugae with secondary ridges or rugae at their sides subdividing the maternal troughs or fossae. This configuration on the maternal side explains the transformation of the regular chorionic ridges from the 63-day stage into bulbous protrusions at the 100-day stage. Based on these observations the precise terminology used above was proposed.