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The motility of growth cones of embryonic peripheral neurons is not inhibited by contact with the surfaces of neurites or of non-neuronal cells. Rather, growth cones and microspikes adhere to other cell surfaces and often respond with forward movement and elongation in contact with other cells, as they do on adhesive surfaces in vitro. Furthermore, non-neuronal cells do not display contact inhibition when they contact growth cones or neurites. If anything, surface motility and ruffling is stimulated by contact with a neuronal cell surface and some non-neuronal cells prefer to migrate along neurites rather than on the surface of the culture dish. These observations on the contact behaviour of cells from peripheral nerve ganglia imply that the surfaces of embryonic neurons differ from those of non-neuronal cells in that the neuronal surfaces do no elicit the typical contact inhibition response.

Cationic ferritin binds in a time and concentration dependent manner to all surfaces of ciliary ganglion neurons in culture except "mounds" and "veils". In chase experiments, bound ferritin clears from the cell surfaces and forms larger and larger patches, even at low temperatures. Binding of cationic ferritin is inhibited by poly-I-lysine, potentiated by poly-L-glutamate, and not affected by neuraminidase (acylneuraminyl in HBSS, and then exposed to cationic ferritin. The cells were then washed and placed in 2.5% glutaraldehyde for 30 min. Alternatively, a 6 min chase intervened before the second glutaraldehyde fixation. For "cold" (0-4°) treatments (explained in text), all solutions (except glutaraldehyde) were kept on ice, pipettes were chilled, and dishes treated or chased in the cold were put directly on cracked ice with several pieces of ice on top of the dish cover. Neuraminidase (acylneuraminyl hydrolase, EC 3.2.1.18) (Boeringer Mannheim, Clostridial, 250,ug/ml in HBSS with 75 ,tg/ml of bovine serum albumin; pH 7.0-7.1; tests for activity by M. R. Bernfield and S. Banarjee to be reported) was applied to washed, living cells for 10 or 20 min followed by washing of cells in HBSS, exposure to cationic ferritin (500 ,g/ml, 30 or 60 sec), HBSS washing, and fixation. Alternatively, cells were first labeled with ferritin (500 gtg/ml, 30 sec), washed and then exposed to the enzyme for 10 or 20 min. Control dishes run concurrently received the same series of washes and HBSS or HBSS plus albumin for equivalent times. In one experiment, cells were prefixed with 2.5% glutaraldehyde, washed in HBSS, placed in 0.1 M NH4Cl in HBSS for 4.5 hr, washed again, and then treated with neuraminidase in HBSS at pH 5.0 for 10 min. After repeated washing in HBSS, cationic ferritin (500 ,ug/ml, 30 sec) was applied. The cells were then washed and fixed again in glutaraldehyde.Hyaluronidase (hyaluronoglucosidase, hyaluronate 4-glycanhydrolase, EC 3.2.1.35; bovine testicular VI, Sigma; 3 ,tg/ml in Ca++-and Mg++-free HBSS at pH 7.1) or chondroitin ABC Iyase (EC 4.2.2.4) (Miles Laboratory; used at 1.0 or 0.2 units/ml in Ca++-and Mg++-free HBSS at pH 7.1) was applied to cells for 20 min, followed by washing, ferritin treatment, washing, and fixation. These enzymes normally remove extracellular materials from living cells or tissues (10, 11).Some cells were washed, exposed to poly-L-lysine.HBr (Miles; 500 ,ug/ml in HBSS for 5 min), washed, treated with ferritin (500 ,tg/ml, 30 sec), washed, and fixed. Others were pretreated with poly-L-glutamate-HBr (Sigma; 500,Ag/ml in HBSS for 60 sec) and treated similarly. In these and all other special proce-4100

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