Syncytia are multinuclear cells that can form either through normal biological processes, such as the mammalian placenta, or under the influence of certain pathogens, such as HIV, via fusion of the plasma membrane. Other examples include the skeletal muscle cells of mammals, the tapetal cells of plants, and the storage cells of Douglas-fir seeds. The polymorphonuclear leukocytes of mammals are not polynuclear cells, although the lobes of their nuclei are so deeply bifurcated that they can appear so under non-optimal microscopy.