SPERM CELLS IN POLLEN TUBES OF NICOTIANA-TABACUM-L - 3-DIMENSIONAL RECONSTRUCTION, CYTOPLASMIC DIMINUTION, AND QUANTITATIVE CYTOLOGY

The organization of the sperm cells and vegetative nucleus (male germ unit) of Nicotiana tabacum was examined 18 h after semi-vivo pollination using transmission electron microscopy, computer-assisted serial section reconstruction and quantitative cytology. Based on a measurement of 11 cellular parameters in nine reconstructed sperm cell pairs, there are no statistically significant differences between the two cells. The S(vn) is characterized by a strap-shaped cytoplasmic extension that is physically associated with the surface of the vegetative nucleus. The nucleus is located adjacent to the sperm crosswall, with sperm organelles being distributed between the nucleus and the extension. The S(ua) is a tapered cell with cytoplasmic areas at both poles and deep axial invaginations near the crosswall. This cell has a centrally-located nucleus and a largely polar distribution of organelles. Three mechanisms for cytoplasmic diminution were observed that appear to contribute actively to the loss of cytoplasmic volume and organelles: (1) enucleated cytoplasmic body production in the S(ua); (2) vesiculation at the tip of the cytoplasmic projection of the S(vn); and (3) vesicle-containing body accumulation in the periplasm of both the S(vn) and S(ua).