Permeability of Intestinal Capillaries

Abstract

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, Os04 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 .

In vivo delivery of fluoresceinated dextrans to the murine growth plate: Imaging of three vascular routes by multiphoton microscopy

“…Dextrans are a standard tracer molecule to study nonselective transport that is neither active nor mediated by specific receptors across the vascular barrier (Simionescu et al, 1972(Simionescu et al, , 2002Heinzmann, 1980;Thorball, 1981). Their positive attributes are that they are stable, highly soluble, nontoxic, noncharged at the pH of normal blood, of biological origin, and available in a wide range of molecular weights (Thorball, 1981).…”

Morphometric Data on the Endothelium of Blood Capillaries

“…Data concerning the dimensions and frequency of fenestrae and the fractional area for which they account should be immediately useful in the current discussions on structural aspects of capillary permeability, since the diameter of these structures approaches closely that postulated for the large pores, (cf . 33, 34) and since there is evidence that the fenestrae are permeable to molecules large enough to act as large pore probes (7,8) . As already discussed in a previous paper (8), their fractional area is, however, much larger than that postulated for Table V for capillary dimensions and Table II for vesicle stomata frequency per µm 2 .…”

“…Among the latter, the plasmalemmal vesicles in the endothelium of muscular capillaries and the fenestrae in that of visceral capillaries are of particular importance, since they have been implicated in the movement of water and water-soluble molecules across the endothelial layer (2,(3)(4)(5)(6)(7)(8) . A number of incidental observations have already mentioned an uneven distribution of fenestrae and vesicles in a variety of capillaries (6,(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) .…”

Exocytosis of pinocytic contents by Chinese hamster ovary cells.

“…Exocytosis of pinocytic contents may occur as part of endocytic membrane cycling. In certain specialized cell types such as capillary endothelium (2,19), fetal yolk sac (11), and neonatal intestinal epithelial cells (14), exocytosis of pinocytic contents has previously been described. In these cell types pinocytic content exocytosis constitutes a physiological transport process for the movement of macromolecules from one side of a tissue layer to another.…”

“…The fate of pinocytic vesicle contents is generally thought to be delivery to and ultimate degradation within a lysosomal compartment except in certain specialized cell types (capillary endothelium [2,19], fetal yolk sac [1l], and neonatal intestinal epithelial cells [14]). Based on time-lapse photomicrography studies, Gey et al (4) speculated that pinocytized fluid whose fate could not be accounted for by transfer to other organelles was discharged from the cell .…”