Local differentiations within the endothelium of both muscular (diaphragm, myocardium) and visceral (pancreas, jejunal villi) capillaries have been studied in rats on sectioned and freeze-cleaved preparations. Four distinct parts have been recognized in the endothelial cells of all these vessels on the basis of subcellular components present in each part and on the basis of variations in the local frequency of plasmalemmal vesicles: (a) the parajunctional zone, (b) the peripheral zone, (c) the organelle region, and (d) the nuclear region. Our data indicate that ∼16, ∼7.0, and 8.5% of the endothelial cytoplasmic volume (in the peripheral zone) is accounted for by vesicles, their content, and their membranes, respectively. The average density of vesicular openings per µm2 is 78 in diaphragm, 89 in myocardium, 25 in pancreas, and 10 in jejunal mucosa capillaries. The frequency of fenestrae is 1.7 times as high in jejunal (26/µm2) as in pancreatic capillaries (15/µm2), the corresponding fractional areas being ∼9.5 and ∼6%, respectively, of the endothelial surface. Intercellular spaces occupy a relatively small area (∼0.08 to 0.2%) of the inner endothelial surface.
Whale skeletal muscle myoglobin (mol wt 17,800; molecular dimensions 25 x 34 x 42 Å) was used as a probe molecule for the pore systems of muscle capillaries. Diaphragms of Wistar-Furth rats were fixed in situ at intervals up to 4 h after the intravenous injection of the tracer, and myoglobin was localized in the tissue by a peroxidase reaction. Gel filtration of plasma samples proved that myoglobin molecules remained in circulation in native monomeric form. At 30–35 s postinjection, the tracer marked ∼75% of the plasmalemmal vesicles on the blood front of the endothelium, 15% of those located inside and none of those on the tissue front. At 45 s, the labeling of vesicles in the inner group reached 60% but remained nil for those on the tissue front. Marked vesicles appeared on the latter past 45 s and their frequency increased to ∼80% by 60–75 s, concomitantly with the appearance of myoglobin in the pericapillary spaces. Significant regional heterogeneity in initial labeling was found in the different segments of the endothelium (i.e., perinuclear cytoplasm, organelle region, cell periphery, and parajunctional zone). Up to 60 s, the intercellular junctions and spaces of the endothelium were free of myoglobin reaction product; thereafter, the latter was detected in the distal part of the intercellular spaces in concentration generally equal to or lower than that prevailing in the adjacent pericapillary space. The findings indicate that myoglobin molecules cross the endothelium of muscle capillaries primarily via plasmalemmal vesicles. Since a molecule of this size is supposed to exit through both pore systems, our results confirm the earlier conclusion that the plasmalemmal vesicles represent the large pore system; in addition, they suggest that the same structures are, at least in part, the structural equivalent of the small pore system of this type of capillaries.
The pathway followed by macromolecules across the wall of visceral capillaries has been studied by using a set of tracers of graded sizes, ranging in diameter from 100 A (ferritin) to 300 A (glycogen). Polysaccharide particles, i.e. dextran 75 (mol wt ∼75,000; diam ∼125 A), dextran 250 (mol wt 250,000; diam ∼225 A), shellfish glycogen (diam ∼200 A) and rabbit liver glycogen (diam ∼300 A), are well tolerated by Wistar-Furth rats and give no vascular reactions ascribable to histamine release. Good definition and high contrast of the tracer particles were obtained in a one-step fixation—in block staining of the tissues by a mixture containing aldehydes, OsO4 and lead citrate in phosphate or arsenate buffer, pH 7.4, followed by lead staining of sections. The glycogens and dextrans used move out of the plasma through the fenestrae and channels of the endothelium relatively fast (3–7 min) and create in the pericapillary spaces transient (2–5 min) concentration gradients centered on the fenestrated sectors of the capillary walls. The tracers also gained access to the plasmalemmal vesicles, first on the blood front and subsequently on the tissue front of the endothelium. The particles are temporarily retained by the basement membrane. No probe moved through the intercellular junctions. It is concluded that, in visceral capillaries, the fenestrae, channels, and plasmalemmal vesicles, viewed as related parts in a system of dynamic structures, are the structural equivalent of the large pore system.