Cytoplasmic matrix: old and new questions.

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Abstract
Conferenceson thenatureofprotoplasmhavenotbeen in vogueforperhapssome 30-oddyears,inpartforreasonsthat Iwishtodiscuss, but,withtheturningoftime'skaleidoscope, new patternsoforganization havenow emergedand a new synthesis isperhapspossible .Cellbiologyispassingfroman analytical phaseinwhich eachcomponent ofthecellwas isolated and characterized apartfromothercomponentstoa new phaseinwhichtheintegration ofthesecomponentsto giverecognizablecellbehavioronce againiscommanding considerable attention .Inhisintroduction tothissupplement, Porterpointsoutthatthesequestionson thecytoplasmic matrixand theintegration ofcellular functionarea partof ourheritagethatextendsbackatleast100yearsandwerethe subjectof intensiveexperimentalinquiryin the firstfew decadesofthiscentury.At least,we can confronttheold questionswithnew perspectives and perhapsfindimportant new questionstoaskofthecell. Our heritagehastwoaspects, thefirstandolderonebeing a viewofthecytomatrixbasedon physicalchemistry.Porter remindsusofthecolloidal and coacervatechemistryofthe cytoplasm.Thisistheviewofprotoplasmasathixotropic gel and areturntoexplanations ofcellbehaviorbasedon sol-gel transformations, forexample.The otherviewofthecytoma- trixisstructural, and althoughitechoesancientideas,itis basedprimarilyon thehigh-voltage electronmicroscopyof cellwholemountsaspracticed inthelastdecadeorso.Porter referstothisbasicstructural organization ofthecytomatrix asthemicrotrabecular lattice (MTL).'The MTL connectsor entrapsallformedstructures inthecelland influences, aswe shallsee,themost intimateproperties ofmoleculesinthe cytoplasm.The cytoplasmhastwo phases:cytosoland cyto- matrix.Intherecentpast,onlythecytosolic fractionofthe cytoplasmhasbeenconsidered, butthepaperspresentedin thissupplementshow us thata new biochemsitryof the cytomatrixisrapidlybeingdeveloped. The major filamentsystemsof the cytomatrixarethe subjectofthearticles by Stossel, Steinertetal.,Olmstedet al.,and Pollardetal.The manner inwhichtheseelements areinterconnected and theirspatialspecification withinthe cellconcernValleeetal.,Tucker,Tiwarietal.,Tilneyand Tilney,Mangeat and Burridge,Millerand Solomon, Moo- sekeretal.,Condeelisetal.,Geigeretal.,and Karsentietal. Rebhun and Fisher,Steams,Johnsonetal.,Pickett-Heaps et al.,Sandovaletal.,Rodewald,and McNiven and Porter discusscytomatrixdynamics.The articles byClegg,Horowitz andMiller,MastroandKeith,Paine,andParsegianandRau concernthedistinction betweenthecytomatrixand cytosol . They indicate thatthemedium isnotthewholemessage.An overviewofcellularcompartmentationisprovidedin the articles byFeyetal.,Fulton,Masters,andLaseketal.Means etal.provideuswitha discussion ofmechanismsthatplace thecytomatrixunderenvironmentalcontrol.The oldand new questionsthatIhavementionedwillperhapsbeofsome useinlinkingtogetherthearticles presentedhereandindicat- ingthelooseendsfrom whichfutureconceptsofthecyto- matrixcanbewoven.