Morphology of abalone spermatozoa before and after the acrosome reaction

Abstract

Spermatozoa of the red abalone, Haliotis rufescens, consist of a bullet-shaped acrosome granule (2.5 × 1.1 μm), a nucleus (4.2 × 1.0 μm), a midpiece (0.75 × 1.3 μm) consisting of five mitochondria, a pair of centrioles and a series of vesicles, and a flagellum (45 × 0.25 μm). The large acrosome granule contains two distinct components: (1) an ovoid electron-dense body in the anterior apex of the granule (approximately 0.9 × 1.0 μm), and (2) a less dense, homogeneous material at the granule posterior. A bundle of preformed actin filaments (about 2.4 μm in length) protrude from the anteriorly located nuclear fossa into the acrosomal fossa located at the posterior of the acrosome granule. The acrosome reaction of Haliotis sperm can be triggered by 30 mM Ca2+ in the sea water, the ionophore A23187, or the aging of sperm. Upon reaction, the acrosome granule releases water-soluble components (thought to be the less electron-dense, posterior material of the granule, leaving a ghost-like empty sac with a wall approximately 0.03 μm in thickness. The extension of the actin-cored acrosome process occurs immediately after the exocytosis of the acrosome granule. The process (7 μm in length × 0.2 μm in width) extends through the sheath-like ghost of the acrosome granule. The inner surface of the ghost is studded with globules (about 0.04 μm diameter). The outer surface of the acrosome process is coated with dense material which may function in sperm-egg adhesion. Abalone sperm are of the “primitive” type of sperm morphology.