Source : Webster's Revised Unabridged Dictionary (1913)

Fibrilla \Fi*bril"la\, n.; pl. {Fibrill[AE]}. [NL. See
   {Fibril}.]
   A minute thread of fiber, as one of the fibrous elements of a
   muscular fiber; a fibril.

Source : WordNet®

fibrillae
     See {fibrilla}