Spermiogenesis and Cytochemistry of the Functional Spermatozoon of the Rotifer Asplanchna brightwelli

Electron microscopic observations indicate that circular-shaped spermatocytes transform in development into elongated forms, groups of which are confluent through a common area of cytoplasm. Terminally, they assume a pyriform shape with an anteriorly-projecting flagellum. Cytodifferentiation begins with fusion of the membranes of the nuclear envelope and the consequent elimination of the perinuclear space. Midway through the fusion process a biphasic layer of granules appears adjacent to the cytoplasmic side of the fused nuclear envelope. Immediately following, myelin-like whorls can be seen within tubular invaginations of the sperm plasma membrane, into which the differentiating flagellum extends during development. Concomitantly, rough endoplasmic reticulum can be seen blebbing vesicles toward golgi composed of 4–6 parallel cisternae. The vesicles become aligned parallel to the golgi stack and fuse with each other to become the most immature golgi cisternum. Immediately thereafter donut-shaped bodies appear at the mature face of the golgi apparatus. Consumately, the spensiatids assume a pyriform outline, the flagellum reenters the cytoplasm by reverse exocytosis, and the axonemal microtubules become partitioned from the main body of the spermatozoon by a narrow isthmus of apposed plasma membrane. The peripherally distributed donut-shaped bodies were shown with the aid of electron microscopy to contain a β-glycerophosphatase. The enzyme appears to be localized between the inner and outer concentric membranes of these organelles and not within the central lumen. A periodic acid-silver methenamine reaction reveals staining characteristics for glycoprotein in the donut-shaped bodies, also localized between the inner and outer concentric membranes and not within the central lumen. Although dehiscence in the presence of oocytes was not demonstrated, the silver methenamine and β-glycerophosphatase localization between the two concentric membranes of the donut-shaped bodies, and the apparent origin of these organelles from the golgi apparatus during spermiogenesis of the functional spermatozoon, indicate a possible acrosomal function.