Ultrastructural observation of capillary sprouts in the dental organs of rat molars.

The morphological events associated with the formation of capillary sprouts were examined in the dental organs to elucidate the morphological processes of angiogenesis in epithelial tissues. Mandibular first molars of 2-day-old male rats were processed for routine observation by transmission electron microscopy. In angiogenic regions, capillaries were located in the caves delineated by outer enamel epithelium, and extravasated erythrocytes were often observed in the stellate reticulum. At the capillary sprout, endothelial cells were amoeboidal in shape, and extended their filopodia to the fluffy extracellular matrix. On the other hand, each endothelial cell interdigitated with each process to form a tubule, and bulbous cell processes filled the luminal side of capillaries. Coated vesicles or pits 50-90 nm in diameter lined the luminal plasma membrane and that comprising the base of filopodial processes. Some endothelial sprouting tips (30 of 47 sprouts) were associated with pericyte-like cells. However, pericyte-like cells were observed in most of the caves where capillaries were located or destined to be located. We investigated the dynamic status of endothelial cells via parameters such as cell elongation and spreading which were supported by the additional observation of membranous components such as coated vesicles at the capillary sprouting region. It was also suggested that pericyte-like cells participated in angiogenesis of epithelial tissues by guiding the sprouts and sealing labile junctions between endothelial cells.