Acrosomal process

Tilney, L.G. Inoue, S. (1982). Acrosomal reaction or Thyone sperm. 11. The kinetics and possible mechanism of acrosomal process elongation. J. Cell Biol. 93, 820-827. 

Microfilaments of F actin traverse the microvilli in ordered bundles. The microfila-ments are attached to each other by actin-as-sociated proteins, particularly fimbrin and vil-lin. Calmodulin and a myosin-like ATPase connect the microfilaments laterally to the plasma membrane. Fodrin, another microfila-ment-associated protein, anchors the actin fibers to each other at the base, as well as attaching them to the cytoplasmic membrane and to a network of intermediate filaments. In this example, the microfilaments have a mainly static function. In other cases, actin is also involved in dynamic processes. These include muscle contraction (see p. 332), cell movement, phagocytosis by immune cells, the formation of microspikes and lamellipo-dia (cellular extensions), and the acrosomal process during the fusion of sperm with the egg cell. 

The acrosomal process of some invertebrate sperm cells is an actin cable that sometimes forms almost instantaneously by polymerization of the actin monomers and shoots out to penetrate the outer layers of the egg during fertilization (Chapter 32). The stereocilia, the "hairs" of the hair cells in the inner ear, contain bundles of actin filaments.302 Motion of the stereocilia caused by sound produces changes in the membrane potential of the cells initiating a nerve impulse. In certain lizards each hair cell contains about 75 stereocilia of lengths up to 30 pm and diameter 0.8 pm and containing more than 3000 actin filaments in a semicrystalline array. Microvilli (Fig. 1-6) contain longitudinal arrays of actin filaments. 

Another remarkable example is extension of the acrosomal process from a sperm cell of the horseshoe crab Limulus polyphemus at fertilization. A bundle of actin filaments in a crystalline state lies coiled around the base of the nucleus. At fertilization the bundle uncoils and slides through a tunnel in the nucleus to form a 60 pm-long acrosomal process within a few seconds. The uncoiled bundle is also crystalline. The coiled bundle is apparently overtwisted and an actin crosslinking protein scrum mediates the conformational alteration that takes place.339 A somewhat related process maybe involved in contraction of bacteriophage tails (pp. 363,364)... 

Way M, Sanders M, Garcia C et al. Sequence and domain organization of scruin, an actin-cross-linking protein in the acrosomal process of Limulus sperm. J Cell Biol 1995 128(l-2) 51-60. 

Owen C, DeRosier D. A 13-A map of the actin-scruin filament from the limulus acrosomal process. J CeU Biol 1993 123(2) 337-344. 

Filopodia. lamellipodia. stress libers, microvilli, acrosomal process... 

Figure 4. Scheme showing sperm-egg interaction in the abalone. 1. The sperm binds to the egg VE by the plasma membrane at the tip of the AV (AG), (F, flagellum M, mitochondrion N, nucleus). 2. The sperm acrosome reacts releasing lysin and the 18K protein from its anterior tip. 3. Lysin disrupts the fibers of the VE and the 18K coats the extending acrosome process as it extends. 4. The sperm passes through the hole in the VE and the membrane covering the tip of the acrosomal process fuses with the egg (from Vacquier and Lee, 1993). 

Swanson, W.J. and Vacquier, V.D. (1995b). Extraordinary divergence and positive Darwinian selection in a fusagenic protein coating the acrosomal process of abalone spermatozoa. Proc. Natl. Acad. Sci. USA. 92 4957-4962,... 

The acrosome reaction of L. anatina spermatozoa occurs through membrane fusion between the acrosomal outer membrane and the overlying plasma membrane at the anterior region of the acrosome. Acrosomal substance is released through the opening caused by membrane fusion followed by the formation of single acrosomal process. The acrosome reaction in L. anatina spermatozoa is typical of most marine invertebrates so far studied. 

In the acrosomal process, fine filaments which are derived from the subacrosomal substance extend along its longitudinal axis. The surface of the plasma membrane enclosing the acrosomal process is externally decorated with fuzzy materials. 

In some marine invertebrates, the acrosome reaction occurs via exocytosis of acrosome substance followed by the formation of one or more acrosomal processes which are responsible for the fusion of the sperm with the egg plasma membrane (Colwin and Colwin, 1961 Dan and Hagiwara, 1967 Tilney et ah, 1978). The acrosome reaction of Lingula anatina spermatozoa occurs through membrane fusion... 

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