Ultrastructure of ventral membranes of rat hepatocytes spread on type IV collagen.

Rat hepatocytes obtained by means of liver perfusion with collagenase were allowed to spread on type IV collagen coated coverslips for 20 h. Interference reflection microscopy revealed a peripheral ring of dark spots. Carbon replicas of ventral membranes left attached to coverslips after lysis and squirting provided high resolution information on the ultrastructure of the protoplasmic surface. Correlative light and electron microscopy of the same ventral membrane showed that the area of the peripheral 'adhesion annulus' was rich in clathrin-coated structures (sheets, pits and vesicles). In vertical thin sections of hepatocyte monolayers numerous small smooth vesicles were observed piled up below the peripheral portion of the cell. These findings suggest high cytotic activity at the cell periphery during spreading. No bundles of microfilaments were observed in cells after squirting or in sections, but a ring of filaments at the cell periphery could be seen in many cells in whole mount preparations after treatment with Triton X-100. The absence of microfilaments associated with the points of adhesion indicates a cytoskeleton independent adhesion mechanism in hepatocytes during the first 20 h of spreading.