The Brownian Movements of Pinocytic Vesicles

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SUMMARY Measurements were made of the diameters, numbers and other parameters of small, smooth‐surfaced vesicles in the endothelium of blood capillaries and lymphatics, and in the mesothelial cells of the diaphragms of mice. Some measurements were also made on the aortic endothelium. With a few exceptions, there were no morphological differences between the various sites. It was found that between 25% and 35% of the non‐nuclear cell volume was composed of vesicles, whose membranes accounted for about 55% of their volumes. Their internal volumes were ∼ 70,000 nm3, totalling ∼ 0·04 μm3/μm2 of luminal surface area. For each 1 μm2 there were ∼ 135 vesicles attached to each surface membrane of the cell, and between ∼ 200 and ∼ 350 vesicles lying free in the cytoplasm. There was probably a slight amount of shrinkage during the preparation of the material, and the true linear dimensions were probably ∼ 105% of those actually observed. Thus the values for the internal volumes were probably ∼ 85,000 nm3 and ∼005 μm3 respectively; the vesicular numbers were probably ∼ 125 attached to each surface and between ∼ 175 and ∼ 300 free. The vesicles attached to the plasma membranes often had quite long stalks; these were estimated to be ∼ 30 nm at the moment of rupture. Thus the vesicles must be released an appreciable distance away from the membrane. This modifies the conclusions of Shea & Karnovsky (1966), since it can now be shown that Brownian movement alone is capable of accounting for the release of the vesicles, their movements within the cells and their transportation of material. By combining these results with others estimating the endothelial permeability coefficients, it can be calculated that the average free lifetime of a vesicle is ∼1½ sec, from union with one plasma membrane to the next. It can also be shown that the average time of such an attachment is ∼ 2½ sec. There are many possible sources of error relating to these measurements; they must only be regarded as tentative. It appears likely, however, that they are of about the correct order of magnitude as they accord well with other data.

Section: Small Vesicles and Endothelial Permeabilitysupporting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The dimensions and numbers of small vesicles in cells, endothelial and mesothelial and the significance of these for endothelial permeability*

Casley‐Smith¹

1969

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“…What is shown is that cellular energy is not required for particles to enter these bodies, for them to separate from the plasma membrane, nor for them subsequently to fuse with one another. (The energy needed for vesicular movements could well be supplied by thermal energy-' Brownian movements '- Casley-Smith, 1963a, by 1969cShea & Karnovsky, 1966.) It is likely that the reason for the difference in the energy required for uptake into the two classes of vesicles is that many small vesicles already exist in the cells, frequently opening at the plasma membranes; particles have merely to enter them.…”

Section: Discussionmentioning

confidence: 99%

Endocytosis: The different energy requirements for the uptake of particles by small and large vesicles into peritoneal macrophages

Casley‐Smith

1969

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SUMMARY The uptake of various, different‐sized particles by macrophages was studied using the electron microscope. In addition to observing normal cells, cells were examined which had been inhibited by exposing them to low temperature (4°C), and to a number of metabolic poisons. It was found that large particles (> 01 μm) enter the cells and are contained in large vesicles (0·1–5 μm). Small particles (< 50 nm) may also enter the cells by this process. They enter most frequently, however, by passing into small (∼ 70 nm) vesicles. These may later coalesce and their contents adhere to give a second kind of large vesicle. The various inhibitors prevented the ingestion of the large particles (and of the small particles en masse) into large vesicles, but did not prevent their initial adsorption onto the plasma membranes. They did not prevent nearly normal numbers of small particles from entering the pre‐existing small vesicles, nor their subsequent fusion into the second class of large vesicles.

“…This has received experimental verification, but with vesicles slightly larger than the small ones (Casley-Smith, 1963b). Shea & Karnovsky (1966) assumed that if a vesicle contacts a plasma membrane there is a probability of unity that they will fuse.…”

Section: Introductionmentioning

confidence: 99%

The passage of cytoplasmic vesicles across endothelial and mesothelial cells

Smith

Chin

1971

The proportions of labelled cytoplasmic vesicles, at increasing distances across mouse heart endothelium and diaphragmatic mesothelium, were studied using the following electron-microscopical tracers : ferritin, horseradish peroxidase and sodium ferrocyanide. The cells were incubated in Hank's solution containing the tracer for periods of 2 sec up to 30 min.Peroxidase labelled the vesicles well, but ferrocyanide may have escaped from them into the cytoplasm. Both passed down the intercellular junctions. Ferritin showed evidence of molecular sieving when entering the vesicles, and probably also suffered from this on leaving them. It was possible to allow for these effects.The vesicles containing tracers were found to have traversed the cells after only a few seconds; by approximately 10 sec a steady-state was observed, after which there were constant proportions of labelled vesicles. These proportions decreased slightly, but not sharply, across the cells. It was found that the observed distributions across the cells were very similar to those predicted by diffusion theory, assuming the following postulates: (1) the vesicles are moved solely by Brownian motion; (2) a low (i.e. 01 = 0.05) probability that a collision with a plasma membrane results in fusion. If however it was assumed that the fusion probability ( a ) were unity, the observed distributions of labelled vesicles differed very significantly from those predicted. A possible explanation of the observed low value of 01 is suggested, based on mutually-repelling charges on the vesicles and membranes.If 01 is given a value of around 0.05, the observations agree with calculated predictions that the median transit times for vesicles through cells 0.3-03 pm wide are approximately 3-5 sec and that the cytoplasmic viscosity is approximately 0.2-0-3 poise. The predictions of vesicular median free lives of 3-5 sec and median attachment times of 2-3 sec also received confirmation, as did the prediction that some 409,, of the released vesicles regain the membrane on the opposite side of the cell.