An axon of a hippocampal granule cell, including dentate gyrus granule cell and CA3 granule cell, characterized by expansions (mossy fiber expansions) giving the fibers a mossy appearance. These unmyelinated axons were first described by Ramon y Cajal. [ http://www.ncbi.nlm.nih.gov/pubmed/17765709 http://www.neurolex.org/wiki/nlx_subcell_100312 http://www.ncbi.nlm.nih.gov/pubmed/24336151 http://www.ncbi.nlm.nih.gov/pubmed/20554881 ]