The basement membrane of the persisting maternal blood vessels in the placenta of Callithrix jacchus.

Formation and morphology of the thickened basement membrane-like layer around the persisting maternal vessels of the Callithrix jacchus placenta were investigated from day 45 until term (day 142) using light, electron and immunofluorescence microscopy. Thickening occurs with the establishment of contacts between the vessels and the syncytiotrophoblast (day 48). Final thickness is reached at about day 100. The course of the vessels shows wide gaps where the maternal blood flows freely into the intertrabecular spaces. As revealed by electron microscopy, the extracellular sheath around the maternal vessels consists of an inner subendothelial basement membrane (3-6 microns) and an outer fibril-containing layer (2-4 microns). Cell debris is seen between the two layers and in the basement membrane. Plaques of granular and fine-filamentous material are incorporated into the fibril-containing layer. The synthesis of the basement membrane material is localized in the endothelial cells. Immunofluorescence microscopy reveals collagen types IV and V, laminin and heparan sulfate proteoglycan (BM-1) in the sheath around the persisting vessels. Fibronectin occurs only in certain areas or in the form of dots. Collagen types I and III are not seen in the region of the vascular wall. It can, therefore, be assumed that the subendothelial layer represents a genuine basement membrane; the fibrils consist of collagen type V and the plaques contain fibronectin. The existence of the thick perivascular sheath is attributed to the persistence and stability of the maternal vessels.