Endothelial vesiculo-vacuolar organelles, pockets and multi-layered fenestrated lamellae in the capillaries of the mouse carotid body

Fenestrated capillaries represent the basic structural unit in the carotid body. They mediate a characteristic hyperpermeability state in this organ. Endothelial fenestrae and plasmalemmal vesicles are of particular importance in this respect. The present electron microscopic study of the capillaries of the mouse carotid body demonstrates prominent endothelial cell structures that are suggested to be closely related to endothelial fenestrae and plasmalemmal vesicles. These structures include: (1) Vesiculo-vacuolar organelles formed by fusion and intercommunication of vesicles and vacuoles of variable dimensions. (2) Pockets in the form of fenestrated membrane-bound vacuoles that communicate either with the capillary lumen, pericapillary space or both via multiple apertures or fenestrae. (3) Multi-layered fenestrated lamellae where the endothelial cytoplasm is divided into multiple attenuated sheets provided with several fenestrae. The latter two structures are preferentially located in the thick perinuclear region of the endothelial cell. Their fenestrae are always distributed in linear series and show close similarity to the usual chains of fenestrae in the attenuated periphery of the endothelial cells. The individual apertures of the fenestrated vacuoles and multi-layered fenestrated lamellae are closely similar to the stomata of fully opened plasmalemmal vesicles suggesting a relationship between them. Morphological and morphometrical analysis of a series of fenestrae belonging to these structures revealed that they are identical to the usual chains of fenestrae in the attenuated periphery of the endothelial cells.