Chromatin Arrangement and Axis Formation in the Spermiogenesis of a Pulmonate Snail

Nuclear change in relation to axis formation and condensation during spermiogenesis was investigated in the snail, Physa acuta. In the early spermatid, characteristic thick layers (termed apical and basal plates) are formed on two sides of a nuclear envelope. Soon after the formation of these plates, a developing acrosome and a flagellum attach externally to the center of the apical and basal plates, respectively. However, most (presumably all) of the chromatin filaments become attached all over the inner surface of the apical and basal plates. This means that the plates themselves are actually the specialized forms of the nuclear envelope to which chromatin filaments become connected; by means of these plates, the chromatin filaments become arranged in parallel to the antero-posterior axis as the nucleus elongates. This suggests that the formation of these two thick layers on opposing surfaces of the nucleus primarily determines the antero-posterior axis of the spermatid and the direction of the arrangement of chromatin. The flattening of the nucleus prior to elongation is caused mainly by the enlargement of the basal plate. Subsequent nuclear shaping and condensation are discussed in relation to the change in the surface structures of the nucleus and the organization of the microtubules.