Plasma membrane of Phycomyces: sequential changes in the structure during spore formation.

The plasma membrane of sporangiospores (spores) of Phycomyces blakeleeanus has distinct structural features that distinguish it from the plasma membrane of the sporangiophores. The plasma membrane of the spores is, however, derived from that of the sporangiophore. A study of sequential development of spores shows that the mass of cytoplasm in a sporangial initial is cleavaged by proliferation and subsequent anastomosis and membrane fusion of the clevage vesicles. The plasma membrane of the cleavage vesicles has a reversed curvature (inside out) to that of the sporangial plasma membrane. Otherwise, the distribution of intercalated membranous particles (Imp) on the fractured faces is similar to that of the plasma membrane of the sporangium. Lather, as a result of fusion of cleavage vesicles, curvature of the plasma membrane in the spore initials returns to their original state. In the processes of spore maturation, a new type of structural feature appears in the plasma membrane, which is not present either in the plasma membrane of spore initials or in the plasma membrane of sporangiophores or mycelia. This feature often is seen as large particles (30--35 nm) which are randomly distributed on the exoplasmic face (EF) with corresponding depressions on the plasmic half (PF) in freeze-fractured replicas. They increase in number with increasing spore maturity. These particles are, in fact, the inward protrusions of the plasma membrane of the spore. A change in the number of 5--8 nm intramembranous particles on the fractured faces is also noted during spor maturation, with an increase on the EF face and a corresponding decrease on the PF face. This alteration may result from vertical movement of Imp or from a change in the fracture plane as a consequence of alteration in the lipid content during maturation.