Microfibril

Definition
noun, plural: microfibrils
(1) (cell biology) A microtubule or microfilament within the cell; an extremely small, submicroscopic cellular fiber.
(2) (histology) A tubular structure composed of collagen, which may occur in solitary or in groups in the extracellular matrix.
(3) (botany) The fundamental structural unit of the cell wall. It is made of an inner core of about fifty parallel chains of cellulose and other polymers in a paracrystalline array.
(4) (zootomy) The smallest unit that collectively forms the fibril.
Supplement
(Botany) In higher plants and many algae, the microfibrils are made of cellulose. They are extremely long relative to their width (about 10nm in diameter). They provide great tensile strength due to their spatial structure. The cellulose molecules are oriented parallel to the long axis of the microfibril in a paracrystalline array. The microfibrils are held in place by the wall matrix.
In some fungi, the microfibrils are composed of chitin whereas in few algae, mannan or xylan.
(Zootomy) In animals, the microfibrils are submicroscopic structures that are grouped to form the fibril. The fibrils in turn are grouped to form the fiber. The bundle of fibers forms the tendon.
Word origin: Greek mīkro-, from mīkros, small + New Latin fibrilla, diminutive of Latin fibra, fiber.

Related forms: microfibrillar (adjective)