In the first place, if docking areas arise in biological membranes via a process of selective crystallization, this would provide an explanation for the numerous observations that different types of membrane microdomains can exist in eukaryotic cells. It is well known that fast and slow freezing of solutions results in very different molecular arrangements because during progressive freezing crystals form preferentially between molecules that fit together well. In the same way, under physiological conditions, formation of membrane docks should occur very progressively and between similar molecules, or between specific combinations of gangliosides, other sphingolipids and sterol derivatives as well as particular transmembrane or GPI-anchored proteins.
One of the earliest observations suggesting the existence of membrane microdomains was that viral surface proteins, such as the influenza virus hemagglutinin, were incorporated into detergent-insoluble fractions as they transited through the Golgi complex . What better example can there be of proteins that tend to homoassociate to form an organized structure than those viral surface molecules, which will ultimately assemble around what is effectively a microcrystal, the viral particle? Viruses may in fact be taking advantage of the tendency of biological membranes to promote homomeric crystalline arrangements for the assembling of their membranes and/or capsids, whilst excluding other cellular proteins. Results obtained by electron paramagnetic resonance (EPR) using material from influenza virus particles have in fact revealed the existence of SLOT (SLow Oxygen Transport) domains, where the 14-EASL probe was much less mobile, and exchanged with the rest of the membrane at remarkably slow rates . Binding to a solid-state dock would explain the probe's decreased mobility, whilst it's imperfect incorporation to the crystalline structures could result in the persistence of exchanges with the surrounding liquid domains.
What may be the consequences of rigid membrane structures for intracellular signalling? One of the most obvious and immediate consequences of the appearance of rigid structures within the plasma membrane would be an alteration of its impermeability [12, 13], resulting in ion fluxes. In mammals, this would presumably be dominated by calcium (and sodium) penetrating inside the cell. Such fluxes may in fact represent very early steps of a signalling cascade, and would appear particularly appropriate for driving phenomena such as chemotaxis. Early in evolution, such ion fluxes may have constituted a mean for receptor-mediated signalling in unicellular organisms, before the appearance of bona-fide ion channels.
Regarding signalling pathways calling upon the recruitment of downstream signalling molecules to the receptor, one can easily envisage that the stable multi-molecular assembly of particular GPI-anchored molecules, or of certain gangliosides, could represent a seed for specific recruitment of downstream signalling molecules such as Src family kinases, heterotrimeric G proteins or Ras. This could offer a rational explanation for the observation that crosslinking GPI-linked molecules can result in specific intracellular signalling [14, 15], and explain how gangliosides can act as co-receptors for FGF .
One further elaboration on the system comes from the recent observations that initial signaling via Fas or CD40 can trigger the release of acid sphingomyelinase from intracellular vesicles . This enzyme turns sphingomyelin into ceramide, which will then favour the formation of the compacted lipid domains necessary for recruitment of additional Fas or CD40 molecules . These results would in fact tie in very well with an earlier study by Massey on the effect of ceramides on the biophysical properties of bilayers reconstituted in vitro. This author concluded that under certain conditions, particularly at low cholesterol concentrations, the activity of sphingomyelinase could result in the appearance of gel phase areas at physiological temperatures .
To conclude, I would like to dwell briefly upon the concept of maintenance of the liquid state of the membrane by the cell itself. The hypothesis presented here supposes that organized, stable domain formation in biological membranes represents a sufficiently marginal thermodynamic gain to require priming for it to occur efficiently. But this also means that in this metastable situation, the formation of such structures will have a tendency to develop spontaneously, albeit much more slowly and progressively than after priming. If all this holds true, one could predict that mechanisms would exist allowing cells to actively disrupt such solid areas arising spontaneously. Proteins associating with the cytoskeleton and having an intrinsic affinity for microdomains arising spontaneously could have just such a role, much like a stirrer can prevent water from freezing below 0°C. Conversely, when the development of those docks is not spontaneous, but driven by physiological stimuli such as receptor multimerisation, the same stirring mechanisms could have a role in driving the recruitment of additional components and in bringing surrounding domains in Lo state to the signalling platform, as has been documented for cells of the immune system in several reports (see  for review). Once docks have truly formed, they will represent thermodynamically stable structures that would probably be very difficult to disrupt. After membrane docks have formed and duly performed their function, their persistence at the cell surface would presumably become undesirable, and particular mechanisms probably exist allowing cells to eliminate such structures actively and rapidly. The very localised and transient existence of such membrane structures having adopted a gel phase could in fact be the main reason for their having escaped detection despite intense interest in this field of investigation.
For a cell to get rid of crystallised membrane docks, I see three obvious solutions:
Shedding: For certain cell types, the direct disposal of such undesirable membrane areas into the extra cellular milieu may well be the preferred route.
Endocytosis: For other cell types, such an answer may be unsuitable, either because it would degrade their direct environment, or because it would be too metabolically costly. Such cells would therefore internalize the areas harbouring a solid state, and, in the cell types expressing caveolin, this may well be one of the main roles of caveolae, the flask shaped membrane structures that are enriched in membrane components of high hydrophobicity . After all the components that can be recovered and recycled have been extracted by the cell's own machinery in the endosomal pathway, it can be expected that certain components of the docks' structure may remain in particularly stable arrangements, thus precluding their disassembly. Such structures, possibly in the form of minute vesicles, would then presumably have either to remain inside the cell, or be secreted by exocytosis, to be cleaned up by scavenger cells such as macrophages, that would be better equipped enzymatically for their disposal.
It seems that exosomes  would be very good potential candidates for the products of the two types of excretory phenomena described above.
Trogocytosis (From trogo, to nibble in ancient Greek.): Corresponding to the active capture of membrane fragments by another cell. This phenomenon seems to occur very broadly among cells of the immune system. Indeed, after the formation of an immune synapse, lymphocytes will extract a significant portion of the components of the plasma membrane of the other cell that was involved in the formation of that synapse [21, 22]. As was seen above, pathogens such as viruses would rely on membrane docks for their assembly, and the immune system could have adapted to recognise the assembly of such structures as danger signals. By nibbling rigid areas from the surface of other cells, lymphocytes, and possibly other leukocyte types, may not only be surveying their neighbouring cells for the development of dangerous pathogens, but may also have an important role in the refuse disposal of membrane docks that may be unwanted at the surface of resting healthy cells.