my·o·fi·bril (m???-f??br?l, -f?b?r?l)n. Any of the threadlike fibrils that make up the contractile part of a striated muscle fiber and are made up chiefly of actin and myosin filaments.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.myofibril (?ma???fa?br?l) n (Biology) one of cylindrical organelles found in muscle fibreCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014my?o?fi?bril (?ma? ??fa? br?l, -?f?b r?l) n. a contractile fibril of skeletal muscle, composed mainly of actin and myosin. [1895?1900] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.