SPERMIOGENESIS IN THE PULMONATE SNAIL, EUHADRA HICKONIS 1. ACROSOME FORMATION

Electron microscopical observations of the course of acrosomal differentiation in Euhadra hickonis show that the vesicular component of the mature acrosome is produced by early Golgi activity, whereas an equivalent amount of material that forms a basal component is added later to the outside of the vesicle. It is also suggested that similar material which concurrently accumulates against part of the outer surface of the nuclear envelope is finally incorporated into the basal part of the acrosome. In the early spermatid, which has a highly polymorphic nucleus, material derived from the well-developed Golgi complex accumulates within a network of tubules in its central maturing zone to form a single acrosomal vesicle ca. 150 nm in diameter. The next stage is characterized by the strikingly spherical shape of the nucleus, as well as by the addition of electron-dense material to the outside of the nuclear envelope over the future anterior surface, and to its inside in the posterior region where the centriolar fossa will form. At mid-spermiogenesis the Golgi complex moves posteriorly away from the acrosomal vesicle, which remains in the anterior cytoplasm. A growing mass of densely filamentous material forms a hollowed hemisphere around one side of the vesicle. This complex approaches the coated anterior part of the nuclear envelope, turning if necessary so that the filamentous material is in the lead, and the latter merges with the electron-dense material at the center of the coated area. As the late spermatid nucleus elongates, this material passes through a series of changes in arrangement and electron density, finally forming a homogeneously particulate element of medium density that surrounds the proximal half of the acrosomal vesicle and caps the slender tip of the nucleus in the mature spermatozoon.