Figure 8.

Structure and cholesterol distribution of protrusions in mature cortical fibers of the embryonic chicken lens fibers. A: SEM showing the distribution of numerous protrusions (arrows) from the corners of fiber cells in the inner cortex. Several small sockets (open arrows) of ball-and-sockets are also seen in the inner cortex. Note that two different sizes of protrusions from adjacent cells are often paired together for interlocking. B: Freeze-fracture TEM showing the absence of any gap junction on protrusions (p), although two gap junctions (gj) are found in close proximity to the protrusions (p). C: Freeze-fracture immunogold labeling confirms the absence of labeling for the Cx46 antibody in the protrusions (p). Instead, Cx46 antibody specifically labels the nearby large gap junction (gj). D: Thin-section TEM reveals the complex configuration of protrusions (p) without the association of gap junction (open arrow) with the protrusions. Surprisingly, several adherens junctions (paired arrows) are found associated with the neck portion of these protrusions. E: Filipin cytochemistry in conjunction with freeze-fracture TEM shows that a cluster of the protrusions (p) contain consistently high amounts of cholesterol (filipin-cholesterol-complexes, FCCs). F: At higher magnification, the protrusions display a high density of membrane cholesterol (i.e., 402 FCCs/μm² membrane). Note that the adjacent gap junction (gj), classified as the cholesterol-rich subtype, contains only one half of FCCs (i.e., 188 FCCs/μm² membrane) distributed in the protrusions. G: A top-viewed protrusion (p) in the cytoplasm showing a high density distribution of filipin-cholesterol-complexes. The scale bar indicates 1 μm in A, 500 nm in B, C, D, E, and 200 nm in F, and G.