Pollen Tube Entrance in the Embryo Sac and Fertilization
In angiosperm plants the interaction between male and female reproductive organs results in double fertilization: the fusion of one sperm cell with the egg cell, and the fusion of the second sperm cell with the central cell. Most of our knowledge of the final stages of the sexual reproduction process, the entrance of the pollen tube in the embryo sac and the actual fertilization events, is based on microscopical observations of fixed and sectioned materials (Jensen 1972; Van Went and Willemse 1984). The embryo sac is positioned deeply inside the ovule, surrounded by a large quantity of untransparant sporophytic tissue, which prevents its direct observation, and the manipulation and experimental analysis of the events that take place. Since recent years it has become possible to separate the embryo sac and its composing cells from the surrounding sporophytic cells by the use of enzymatic maceration techniques (Hu et al. 1985; Zhou and Yang 1986; Mol 1986; Huang and Russell 1989; Wagner et al. 1989; Van Went and Kwee 1990; Kranz et al. 1991). These techniques allow the isolation of embryo sacs and composing cells in large quantities and in living and intact condition. Presently, in a number of laboratories molecular and cell biological research is in progress focused on the developmental and functional aspects of the female gametophyte and gametes. Also sperm cells can be isolated in living condition and large quantities (Theunis et al. 1991).
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