Medically Assisted Conception An Agenda for Research (1989) / Chapter Skim
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Membrane Models: Evolution From the Fluid-Mosaic Standby
Membrane Models: Evolution From the Fluid-Mosaic Standby
Pages 155-175
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From page 155... ...
Since the presentation of the fluid-mosaic model, continuous application of pre-existing plus new techniques has resulted in additional quantitative details that must be integrated into the model. Careful rereading of this classic paper, as opposed to simple inspection of the much imitated schematic Figures (see Figure 1, left panel)
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From page 156... ...
proteins found in the original fluid-mosaic model illustrates an awareness that the external face of the membrane was complex, and the differential staining of the outer surface of cells (Revel, et al, 1960) provided strong evidence for the specialized distribution of carbohydrate on the cell surface.
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From page 157... ...
PUS'M ~EMB~ PHO~HOUPlD PLAYER it, 1 IS :~ O-YCOCALYX "SORSED .
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From page 158... ...
ASYMMETRY OF CELL SURFACES This concept also was considered in the original fluid-mosaic model, where the pictorial displays clearly indicate that components in the membrane were unequally distributed relative to the exterior and interior faces (Figure 11. Further, based on staining properties mentioned previously, carbohydrate components (eg., gangliosides)
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From page 159... ...
Since the phospholipid classes differ in the general types of acyl substituents, with PC and SPM having the more saturated members and PE, PS and PI having the more unsaturated components, this asymmetry in lipid by head group also introduces asymmetry in the hydrocarbon portion of the bilayer. To a first approximation, this asymmetry between outer and inner leaflets could result in differences in flexibility gradient (freedom of motion proceeding from the bilayer face into the interior)
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Preparation of lipid dispersions to yield putative bilayer, hexanonal and isotropic phases, followed by analyses by NMR (center panel) and electron microscopy (right panel)
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From page 161... ...
Much attention has been focused on these transitions in phase state, most often characterized using techniques such as differential scanning calorimetry (Figure 53. Abrupt changes in molecular order of the acyl groups with increasing temperature are observed for pure components, but addition of other lipids leg., cholesterol)
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From page 162... ...
Lipids can be grouped according to molecular parameters of molecular volume, surface area and depth of penetration into the bilayer. These considerations yield a limiting number of geometric shapes (cylinders, cones and inverted cones ~ which, in turn, can be related to the type of polymorphic phase form the system will assume when placed into water.
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From page 163... ...
OUTER LEAFLET INNER LEAFLET MODEL MEMBRANE (WITHOUT PROTEIN ~ Results of recent studies on differentiation of stem cells to erythrocytes (Rayler et al., 1985) reveal changes in the "inner vs outer" display of the polar headgroups of phospholipids (Figure 8~.
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From page 164... ...
and phospholipid exchange proteins allows selective alteration of the outer surface of the bilayer (provided the time of the experiment is limited to less that that required for transhilayer movement)
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Their tendencies to form unique molecular aggregates can result in atlered cell functions and shape. These distributions phospholipids provides an excellent framework for the design and interpretation of experiments dealing with a variety of membrane related events.
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From page 166... ...
are regulated in part by these membrane features assures that the topic will be intensively studied in the near future.RELATIVE SPEED OF MEMBRANE EVENTS Progress in the study membrane events related to reproductive processes undoubtably will proceed as a form of derivative science, where data from other areas (eg., erythrocyte and lymphocyte membrane - 166
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From page 167... ...
, is embarassingly informative. It alerts us to the extreme rapidity of membrane events described in other portions of this paper and to the fact that all too often the true ';cale of measurements is inappropriate for the events to be studied.
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From page 168... ...
Precursor-product assumptions would suggest the relationship of A before B before C It is possible that a detailed examination of the stoichiometry of the individual steps relative to overall pathway needs (see text for examples)
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In the first, very unique experimental designs are essential for any direct test of a postulated relationship of regulation of a cellular event involved in reproduction via a mechanism such as alteration of Nat - 169
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From page 170... ...
This may be untrue for events such as the epididymal maturation of sperm, in that the 7-14 days required for the process are sufficient to allow for extensive movement of phospholipids between the inner and outer leaflets of the bilayer. It is evident that study of membrane events is very immature, especially for complex membrane processes related to reproduction.
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From page 171... ...
As to be expected, subsequent contributions have refined the model by reemphasizing and clearly depicting the modes of molecular heterogeneity possible on any given surface. These unique orientations and motional properties of the molecules ultimately will allow discrimination among superficially similar, but uniquely regulated, membrane events.
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Dr. James Graham provided essential suggestions for Figures 12 and 13.
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From page 173... ...
H Hammerstedt 1988 Induction of motility, the acrosome reaction and egg penetration in epididymal ram sperm by liposomes.
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M van Deenen 19 8 4 Shape changes in human erythrocytes induced by replacement of the native phosphatidylcholine with species containing various fatty acids J
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H Hammerstedt 1987 In vitro induction of the acrosome reaction and hamster egg penetration in epididymal and ejaculated ram sperm.
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