The microfibrils of connective tissue: I. Ultrastructure.

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.