The permeability of capillaries among the small granule-containing cells in rat superior cervical ganglia: an ultrastructural lanthanum tracer study.

The permeability of blood capillaries associated with small granule-containing (SGC) cells in rat superior cervical ganglia was investigated at ultrastructural level by employing ionic lanthanum as an electron dense tracer. In rat superior cervical ganglia, the majority of blood capillaries were nonfenestrated. Both fenestrated and nonfenestrated capillaries were observed in the area associated with SGC cells. Lanthanum tracer was observed in the luminal surface, the interendothelial cleft and the subendothelial perivascular spaces of both fenestrated and nonfenestrated capillaries associated with SGC cells. The external lamina of the Schwann cell which surrounded the neurons, nerve fibres and SGC cells were clearly delineated by the lanthanum tracer. Furthermore, the perineuronal space, the periaxonal space, and the pericellular space of the SGC cells were readily accessible to the lanthanum ion. The results demonstrated an absence of blood-nerve barrier, blood-ganglionic and blood-SGC cell barrier to the lanthanum ion in the parenchymal area of the SGC cells in rat superior cervical ganglia. It is proposed that lanthanum may pass through the endothelial cells via 1) the fenestrae of fenestrated capillaries, 2) the intercellular junctions of both fenestrated and nonfenestrated capillaries, i.e., a paracellular pathway; and 3) the process of endocytosis/exocytosis, i.e., a transcellular pathway, to reach the subendothelial space and be distributed in the parenchyma of SGC cells in rat superior cervical ganglia.