Immunoelectron microscopy of endothelial cells in rat incisor suggests that most basement membrane components are produced by young cells, whereas heparan sulfate proteoglycan is produced by both young and old cells.

When periodontal capillaries of rat incisor tooth were immunostained for four basement membrane components (laminin, collagen IV, fibronectin, heparan sulfate proteoglycan), all four were detected in the secretory organelles of endothelial cells located within 3 mm of the tooth's proximal end, but only the proteoglycan was observed in cells located 4 mm away and beyond (Experiment I). [3H]-Thymidine autoradiography revealed that the endothelial cells located at the tooth's proximal end were young and actively dividing, whereas those located 4 mm or more away were older and generally quiescent (Experiment II). Since immunostaining of a cell's secretory organelles for a given substance indicates production of this substance, the first experiment shows that endothelial cells at the proximal end produce the four basement membrane components. The second experiment discloses that these cells are young. As for the endothelial cells located 4 mm or more beyond the proximal end, the first experiment reveals that they produce only heparan sulfate proteoglycan, while the second shows that they are relatively old. Production of laminin, collagen IV, and fibronectin only by young cells implies that these substances are long-lived and stable components of basement membrane, whereas production of the proteoglycan by both young and old cells implies that it is labile and continually replaced.