The microfibrils of connective tissue: I. Ultrastructure

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Abstract
The ultrastructure of connective tissue microfibrils was examined in two sites: the ciliary zonule of the eye and the foot pad, in 20‐day‐old mice perfused with glutaraldehyde. The microfibrils were classified into two categories, referred to as typical and atypical. Typical microfibrils predominate in both sites; they are unbranched, straight or gently curving, tubular structures of indefinite length with an overall diameter of 12.8 ± 1.7 nm in the zonule and 13.8 ± 2.8 nm in the foot pad. They are composed of two parts: tubule proper and surface band. The tubule is 7‐ to 10‐nm wide and characterized in cross section by an approximately pentagonal wall and an electron‐lucent lumen containing a 1‐ to 2‐nm bead referred to as a spherule. When longitudinal sections of microfibrils are examined at high magnification, the wall of the tubule does not appear as a continuous line but as a series of successive dots. The interpretation of these findings is that the tubule is composed of successive annular segments with an approximately pentagonal outline. The surface band is a 3‐nm‐wide, ribbon‐like structure wrapped around the tubule. The band has dense borders called tracks. Along the tracks, densely stained, 4.6‐nm‐long “spikes” are attached at 4.0‐nm intervals. The wrapping of the bands is somewhat irregular. They may be in a transverse position across single or several microfibrils, in which case each band might constitute a distinct belt; more frequently, the bands are oblique and appear to form a continuous helix. It is proposed that surface bands play a role in holding together the juxtaposed segments making up a tubule. A model has been constructed to represent the association of tubule and band into a typical microfibril. Atypical microfibrils, which are more common in foot pad than in ciliary zonule, appear wavy, lack a definite tubule, and are characterized by distorted, irregular surface bands. They are attributed to proteolysis of typical microfibrils.