ELASTIC FIBERS ARE AN ESSENTIAL COMPONENT OF HUMAN PLACENTAL STEM VILLOUS STROMA AND AN INTEGRATED PART OF THE PERIVASCULAR CONTRACTILE SHEATH

The stroma of human placental stem villi is believed to consist only o f reticular and collagen fibres. In the present study we were able to show for the first time by light (orcein staining) and electron micros copy large amounts of elastic fibres in the stem villous stroma. Elect ron microscopically, homogeneous elastin was found alone or in associa tion with microfibrils. In addition, microfibrils were observed formin g long bands. These three structures, generally known to form elastic connective tissue, were seen in close connection with placental extrav ascular smooth muscle cells, which belong to the perivascular contract ile sheath (PVCS) of stem villi. Elastin was associated with these smo oth muscle cells and connected to collagen fibres via microfibrils. Co llagen fibres were additionally interconnected by spike-like structure s. Extravascular smooth muscle cells revealed numerous adhesion plaque s which occupied conspicuously long cytoplasmic faces of the plasma me mbrane. In cryostat sections, immunoreactivity of talin, an attachment protein of adhesion plaques linking intracellular a-actin filaments w ith extracellular fibronectin, was detected in extravascular and vascu lar (media) smooth muscle cells. The arrangement of placental extravas cular smooth muscle cells, elastic and collagen fibres suggests a func tional myofibroelastic unit within the PVCS, which surrounds the large foetal blood vessels possibly contributing to elasticity and supporti ng tensile and/or contracting forces within the stem villi.