Structure of the Plasma Membrane
An
Electron-Microscope Study
By WALTER STOECKENIUS, M.D.
Electron micrographs of the cell membrane is compatible with the assumption that it
consists of a bimolecular leaflet of lipid coated on both sides with protein.
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plasma meinbranie are polar lipids and protein and that the lipids are probably present
in the form of a bimolecular leaflet. Such
leaflets form spontaneously when lipid extracts from tissues are brought into contact
with water. They can be prepared for eleetron microscopy by the same techniques as
those used for whole tissues, e.g., fixation with
OS04, dehydration in acetone, embedding in
methacrylate, and sectioning. X-ray diffraction diagrams, taken after every step of this
procedure, show that the basic lamellar structure of the lipid is preserved by this technique.2 In such preparations one bimolecular
leaflet appears in the electron micrographs
as a triple-layered structure similar to but
not identical with the image of the plasma
membrane. It consists of two outer dense
bands approximately 8 A wide and a central
light layer of approximately 25 A (fig. 2).
This appearance can be shown3, 4 to be due to
an accumulation of osmniumn around the hydro-
THIN SECTIONS of fixed and embedded
tissue from plants and animals show, in
the electron microscope, a thin dense layer
on the surface of the protoplasmn which in all
probability corresponds to the plasma membrane postulated by the physiologists. In
many instances it can be seen to be a triplelayered structure. eonsisting of two outer
dense layers and a central lighter layer, each
approximately 25 A wide (fig. 1). It has been
termed the unit membrane.' The resolution
obtainable in sueh mierographs is of the same
magnitude as the size of smaller organic moleeules, but for an interpretation of the observed contrast distribution in terms of the
molecular arrangement in the membrane.
additional information is needed.
The preceding papers of this symposiun
have shown that the main constituents of the
From the Rockefeller Institute, Newx York, N. Y.
Supported by Grant RG-6977 fronm the National
Institutes of Health, IT. S. Public Health Service.
Figure 1
Electron micrograph of a section through a rabbit erythrocyte. Fixed with 0804.
Contrast was increased by "staining" the section with lead hydroxide. The triplelayered structure of the membrane is clearly visible. In this case the two outer dense
lines are narrower than those found in the unit membranes of most other cell types.
(1620/60; X 280,000.)
Figure 2
Electron micrograph of a highly swollen phospholipid preparation after Os04-fixation
and sectioning. The bimolecular leaflets are separated by large spaces originally containing water. (9301/59; X 280,000.)
1066
Circulation, Volume XXVI, November 1962
SYMPOSIIJM ON T'HE PLASMA MEMBRANE
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Citrcultton, 1'olrtmr XXVI, Novt mWbf r 19f6.2
-
.
-
'}::':hig h:*e:;K
*--X
0-
>25 A
*.: t:.A:
Figure 3
Score of the molecublr structure f/nfl 0//smium
dlistr ibuttion of the bimolecular leaiflets showvn in
lfiyc/r . The emnpty circles represent tfl hylrophidie groups of the phospholipid molecules, the
b1w/h (lots thle 0t//i///m1 dle])osited in ti/Pe str/tct//r'e'.
~2:
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Figure 4
E1lectron flifcroyraph of' biimolecular leal-gets of phospholipid coatedl on both surfaces with
protein. lF'ixed wiith OsO8 . The contrast has been enhanced by staining the section
(/4/hic/fh lh,ayftoi.f/.
l
Ict7 iple-l/) oi4? str-fture is //hi ;cisiblie w,here the plane of the
lipidl leafileIs is oricn)ltcfl applpoximtltel,/j /101///l to the ]pla/e of the sccti'on. (X 280+,000.)
Figure 5
Schcme o t f he //oleful ar fa/n
'Unge//nt i7n/ the str?ucturi-e sh/o/rn i/)/. fif10/c 4. :/the protein molecuie s ar e
represe//ted bY thbe zigf ocJ lines.
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SYMPOSIUM ON THE PLASMA MEMBRANE
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philic groups of the lipid molecules on the
surface of the leaflet (fig. 3).
The main difference in appearance between
the bimolecular leaflet of lipid and the cell
mnembrane is the lesser density and width of
the outer layers in the case of the lipid leaflet.
If an appropriate protein, e.g., globin, is
added to the solution in which the lipid leaflets form, this protein is adsorbed onto the
surface of the lipid layers and increases the
density and the width of the outer dark bands
seen in the electron micrographs to 25 A or
more depending on the amount of protein
present (figs. 4 and 5). Therefore, in such
preparations, structures are found which are
known to consist of one bimolecular layer of
lipid with a layer of protein on both surfaces
and which are identical in appearance to the
image of the plasma membrane obtained by
the same techniques. It follows that the structure of the plasma membrane as observed in
high resolution electron micrographs is consistent with the molecular arrangement rendered in the Davson-Danielli model.
The observations and considerations that
led Davson and Danielli to propose this model
are largely independent of the morphological
findings presented here. Moreover, Finean,5
using still another approach, arrives at the
same conclusion. The evidence that the Davson-Danielli model correctly represents the
molecular architecture of the plasma mem-
Circulation, Volume XXVI, November 1962
1069
brane is therefore very strong. But this model
onlv describes the general features of the
membrane. Considerable refinement will be
necessary before active tranisport and other
phenomena can be linked to the molecular
organization. There is some indication from
recent electron-microscopical work that the
material constituting the inner surface of the
membrane is different from that on the outer
surface and that specific enzymes may be
bound to one surface. Although much more
information is needed, it seems evident that
the electron microscope is a promising tool
in membrane research.
References
1. ROBERTSON, J. D.: The molecular structure and
contact relationships of cell membranes. Progr.
Biophys. 10: 343, 1960.
2. STOECKENIUS, W., SCHULMAN, J. H., AND PRINCE,
L. M.: The structure of microemulsions as
observed with the electron microscope. KolloidZschr. 169: 170, 1960.
3. STOECKENIUS, W.: Osmium tetroxide fixation
of lipids. In Proc. Eur. Reg. Conf. on Electron
Microscopy. Delft, 1960, vol. 2, p. 716.
4. STOECKENIUS, W.: The molecular structure of
lipid-wvater systems and cell membrane models,
studied with the electron microscope. In
The Interpretation of Ultrastructure. Symp.
Internat. Soc. Cell Biol., Bern, 1961. Exp.
Cell Research, in press.
5. FINEAN, J. B.: The nature and stability of the
plasma membrane. In Symposium on the
Plasma MIembrane. Circulation, suppl., 26:
1151, 1962.
Structure of the Plasma Membrane: An Electron-Microscope Study
WALTER STOECKENIUS
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Circulation. 1962;26:1066-1069
doi: 10.1161/01.CIR.26.5.1066
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