Spreading offibroblasts inmediumcontaining cytochalasin B: Formation oflamellar cytoplasm asacombination ofseveral functionally different processes (pseudopodial reaction/microtubules/microfilaments/concanavalin A)

Normal cultured mousefibroblasts spreading onsolid substrate extend andattach numerous pseudopods; la- mellar cytoplasm iseventually formed fromtheattached pseudopods. Fibroblasts spreading inthepresence ofcytocha- lasin B(CB) formdenovoasystem ofarbor-like branched pro- cesses rather thanlamellar cytoplasm. Thegrowing andfully formed arbor-like processes, incontrast tonormal lamellar cytoplasm, havelowcontractility andareunable toclear patched concanavalin A receptors fromtheir surfaces; their attachment sites arenotassociated with microfilament bundles. Thecells spreading inmedium containing CBandColcemid donotformwell-organized branched structures butextend and attach numerous unstable pseudopods. Itissuggested that normal formation oflamellar cytoplasm canberegarded asa combination ofseveral functionally different processes: (a)of rudimentary pseudopodial reactions resistant toCBandCol- cemid; (b) ofCB-sensitive lamellization; and(c) ofColcemid- sensitive stabilization. Spreading ofcultured fibroblasts onvarious solid substrates is aconvenient process forinvestigation ofmechanisms ofcell shape alterations andofcell attachment tononcellular surfaces. Morphology ofspreading hadbeen studied indetail (see review inref. 1). Ithasbeen shown that spreading isaresult ofexten- sion andattachment ofnumerous pseudopods. These pseudo- podial reactions eventually lead toformation ofspecial sub- strate-attached structure, lamellar cytoplasm. Pseudopods and lamellar cytoplasm formed inthe course ofspreading arehighly contractile; they also have thedistinctive ability toclear their surfaces fromthepatches ofreceptors crosslinked byacorre- sponding ligand (2, 3). Numerous microfilament bundles are formed inthecourse ofspreading; theends ofthese bundles are associated withfocal cell-substrate contacts (4-6). Radial growth ofmicrotubules into thelamellar cytoplasm isanother characteristic feature ofspreading (7). Toanalyze further thecellular reactions involved in spreading wehave examined thealterations inthecourse ofthis process caused bycytochalasin B(CB). Theeffects ofCBonthe fully spread fibroblasts arewell known: itdestroys partially or completely thestructures consisting ofactin microfilaments, itinhibits pseudopodial activity andother cellular movements, andittransforms lamellar cytoplasm into anarbor-like structure consisting ofbranched cytoplasmic processes (8-11). Asfirst shown byGoldman andKnipe (12), CBdoes notcompletely stop thespreading offibroblasts: acell canformdenovoan arbor-like structure inthepresence ofthis inhibitor. Intheex- periments described below wecompared theproperties of pseudopods andofarbor-like structures formed inthepresence ofCBwith those ofthestructures formed inthecourse ofnor-