The abluminal membrane and most attached caveolae were devoid of terbium labeling. selleck inhibitor Dual axis tilting
generated tomograms with better resolution than those acquired from single axis tilting. Reconstructed tomograms revealed discreet, unattached vesicles both labeled with terbium (Figure 3 and Video S1a) and unlabeled (Figure 4 and Video S2). Thresholding and surface rendering of a labeled free vesicle clarified its relationship with other vesicular structures and surface membranes (Video S1b). Translation of a single orthoslice through the model verified the accuracy of the model representing terbium deposition and the vesicle interior (Video S1c). A similar tomographic series through an unlabeled vesicle showed it appearing and disappearing without any connection with
other vesicular compartments (Figure 4, Video S2) In another tilt series, a large membranous compartment was revealed to be connected to selleck both luminal and abluminal membranes (Figure 5). Only the luminal membrane of the compartment exhibited bound terbium indicating the absence of glycocalyx on the abluminal portion of the compartment (Video S3). This structure represents a large thoroughfare through the capillary wall not commonly seen in continuous capillaries. In several regions, vesicles labeled with terbium were attached to the abluminal membrane by a stoma (mouth) (Figure 6, Video S4) and presented the possibility of a transendothelial Carteolol HCl channel. In one instance, a single tomographic
slice indicated a transendothelial channel open to both luminal and abluminal surfaces (Figure 6A). A tomographic series acquired at one of these locations showed a labeled abluminal vesicle that appeared connected to the lumen of the capillary (Figure 7). Creating a model of this vesicular compartment interior by thresholding the terbium revealed a channel-like structure through the capillary wall (Video S5a). An animated journey through the channel was generated with Amira beginning at the abluminal stoma (mouth) of a caveola, a rotation of the camera perspective in mid-channel and backing out through the luminal side (Video S5b) The anatomical correlates of transport pathways across continuous capillary walls have long been a subject of vigorous debate [4,11,18,20,21,23]. Pappenheimer et al.  postulated the existence of a single system of small pores (3–5 nm radius) to account for microvascular permeability. Grotte  introduced the concept of an additional smaller population of large pores (15–25 nm radius) to account for the transport of larger solutes. These estimates were based on the transendothelial transport dynamics of a range of different-sized solutes. Recent estimates of the ratio of large pores to small pores in skeletal muscle capillaries are about 1/5000 .