Ultrastructure of mucous areolae in the pig placenta

The mucous areolae in the pig placenta are characterized by the presence of acid mucopolysaccharides. They have different forms: areas of vascularized trophoblast in early pregnancy, small irregular areolae, vesicles and cysts. The ultrastructure of the mucous areolae in placentae from 18th to 113th day post coitum was investigated. The trophoblast lining the areolae consisted of polarized columnar cells of varied height and width. Only in large cysts that epithelium was extended and flat, only a few micrometers thick. On the free surface of the cells long microvilli formed groups resembling stereocilia. The hyaloplasm of these cells was extremely electron-dense. The apical parts of the cells were filled with mucous granules and exocytosis of their content into the areolar cavity was observed. High synthetic activity of numerous dictyosomes was noted near the nucleus. The basal parts of the cells were divided into lobular processes containing a significant number of elongated mitochondria. In many trophoblastic cells large heterogeneous vacuoles were present. Their content was extruded into areola cavity. The substance filling the mucous areola cavity was heterogenous with some cell debris and whole desquamated cells. The uterine epithelium covering the mucous areolae consisted of high columnar absorptive cells, with the brush border present on their free surface. Apical terminal web contained numerous small coated endocytotic vesicles with a light flocculent content. The filopodia and lamellipodia separated by the intercellular space formed a labyrinth in the basal parts of the cells. The mucous areolae differ from the regular areolae in the ultrastructural and functional features. Possible role of the mucous areolae is discussed.