Localization of intracellular calcium during the acrosome reaction in ram spermatozoa

Abstract

Calcium was identified by a pyroantimonate-osmium fixation technique in ram spermatozoa undergoing a spontaneous acrosome reaction induced by incubation of diluted semen at 39 degrees C. Intracellular calcium was only detected in diluted spermatozoa and increased in amount and distribution over 4 hr At 4 hr, the majority of the spermatozoa displayed ultrastructural evidence of an acrosome reaction. Calcium was initially evident on the outer acrosomal membrane in multiparticulate clusters, which were seen to be located on scalloped crests of acrosomal membrane as fusion developed; it was also located in the region of the acrosomal ridge beneath the outer acrosomal membrane. Vesiculation commenced just anterior to the equatorial segment and proceeded anteriorly. As vesiculation advanced, calcium particles became associated with the periphery of the vesicles attached in the region of the fusion between the two membranes, but were never seen inside the vesicles. The equatorial segment was not labelled until much later in the reaction, at which time calcium particles were also evident on the nuclear membrane; vesiculation of the equatorial segment was also noted at this time. Dense labelling of the postacrosomal dense lamina was seen in all incubated spermatozoa. At the anterior margin of this structure the labelling was seen to be in a "sawtooth" arrangement. The disposition of the calcium both temporally and spatially is discussed in relation to its possible mechanisms in bringing about membrane fusion.

Localizations of intracellular calcium and Ca2+-ATPase in hamster spermatogenic cells and spermatozoa

“…The results of our study suggest that more calcium deposits are present in the PM, acrosome membrane, acrosome reaction vesicles and matrices of the acrosome, and mitochondria in capacitated sperm compared to noncapacitated sperm only with very few or no reactive calcium precipitates. This is supported by previous observations in ram (Watson et al, 1995) and bull (Breitbart et al, 1984) spermatozoa. Current evidence indicates that the internalization of Ca 2þ leading to a rise in [Ca 2þ ] i is required during both capacitation and acrosomal exocytosis in mammalian spermatozoa (Fraser and McDermott, 1992).…”

A Differential Mechanism Is Involved During Heparin- and Cryopreservation-Induced Capacitation of Bovine Spermatozoa

“…However, it has been recently recognized that cryopreservation procedures (dilution, cooling, freezing/thawing) induce capacitation-like changes in spermatozoa [28,29]. Indeed, recent studies have reported similarities between the changes associated with capacitation and cryoinjury, such as plasma membrane reorganization and fluidization, and calcium influx to the spermatozoa [30].…”

Intracellular events and signaling pathways involved in sperm acquisition of fertilizing capacity and acrosome reaction

“…At later stages, Ca 2+ is localized in both post acrosomal dense lamina and on outer acrosomal membrane under the equatorial segment. These findings suggest that Ca 2+ may be implicated in the fusion process (92).…”