The formation of a cell surface protrusion, such as a lamellipodium or filopodium, at the leading edge of a migrating cell. [ http://www.ncbi.nlm.nih.gov/pubmed/11944043 http://www.ncbi.nlm.nih.gov/pubmed/14657486 https://www.worldcat.org/search?q=bn%3A0815316194 ]