Orderly particle arrays on the mitochondrial outer membrane in rapidly‐frozen sperm

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Abstract
By deep-etching and rotary replication of unfixed, non-cryoprotected tissue frozen on a helium-cooled copper block, previously undemonstrable organellar surfaces and intramembranous structures can be examined. Among the more remarkable features of mammalian spermatozoa thus prepared are the highly ordered particulate arrays on the surface of the mitochondrial outer membrane. In the midpiece of the sperm, mitochondria curl around dense fibers and the axoneme. The surface of the mitochondrion that faces the plasmalemma carries closely packed rods in haphazard dispersement, composed of two to four 70-to 80-Å particles, <20 Å apart, while the concave aspect of the organelle contains rods in stepladder pattern. These ladders are parallel, with their paticles in neighboring rungs apparently in register at a 40–45° angle relative to the mitochondrial axis. This organizational disparity between the convex and concave surfaces of the organelle not only affords evidence of a new mitochondrial substructure, but represents a type of topographical heterogeneity rarely found except within specialized areas of the plasma membrane. Other novel findings in the sperm cell include the observation of “lipid” tracts flanking intramembranous particle-strands in the plasmalemma of the cytoplasmic droplet, and a gridiron design on the cytoplasmic faces of the droplet's microcisternae, as well as both within and atop its plasma membrane—a motif consistent with the presence of exocytotic or endocytotic activity in this portion of the cell. Additional recent observations are the differing internal and external periodicities of axonemal microtubules and the subunit structure of rectangles on the tail surface overlying the intramembranous particles of the zipper.