Spermatogenesis in Polytrichum juniperinum : I. The origin of the apical body and the elongation of the nucleus.

Spermatogenesis in Polytrichum juniperrinum includes a series of precise and coordinated morphogenetic movements among the organelles of the androcyte. The basal bodies, the underlying microtubules, and the multilayered structure (MLS) are positioned as an integrated unit at the periphery of the cell, and the nucleus migrates into contact with them. The shape of the nucleus begins to change, with the formation of an anterior projection, or beak. The mitochondrial sheath that has coalesced on the plastid divides to form the apical body, and elongation of the nucleus begins. The posterior basal body and one microtubule undergo lateral displacement from the rear forward, as elongation of the nucleus and the microtubules continues. The mitochondrial mass that is now the apical body grows rearward along the side of the elongated nucleus, and the two groups of microtubules in the MLS rearrange themselves. The lower elements of the MLS also participate in the morphogenetic rearrangement. By the time the nucleus has elongated once around the cell and the apical body has begun it s rearward growth, the lower elements of the MLS are found only beneath the anterior basal body. Subsequently these layers disappear from this location also.