heparin - binding regions of several proteins , such as neural cell adhesion molecule , fibronectin , laminin , midkine , and anti - thrombin iii have been shown to promote neurite extension on two - dimensional surfaces . the effect of heparin - binding peptides on neurite extension through three - dimensional matrices was investigated by culturing embryonic chick dorsal root ganglia within fibrin gels containing chemically attached heparin - binding peptide . the length of neurites within fibrin gels containing cross - linked hbp was increased by more than 70 % over extension through fibrin gels containing no peptide . the hbp sequence of antithrombin iii was incorporated into the fibrin gel as the c - terminal domain of a bidomain , chimeric peptide ; the n - terminal second domain of this peptide contained the α2 - plasmin inhibitor substrate for factor xiiia . factor xiiia , a transglutaminase , was used to chemically attach the hbp - containing chimeric peptide to the fibrin gels during polymerization . the amount of hbp cross - linked into the fibrin gels was determined , after degradation by plasmin using gel permeation chromatography , to be approximately 8 moles of peptide per mole fibrinogen . a peptide , where the cross - linking glutamine was replaced with glycine , showed no increase in extension in comparison with fibrin gels . the addition of heparin to the gel precursors resulted in no increase in neurite extension in comparison with fibrin gels . hbps promote neurite extension by binding to cell surface proteoglycans on the drg .