Development of the Polar Filament‐Polaroplast Complex in a Microsporidian Parasite*

SYNOPSIS. The development of the polar filament in a microsporidian parasite was studied in the electron microscope. The polar filament is a peculiar and complex organelle with intricate anatomical relationships to other structures in the mature spore. The characteristic ultrastructure of the formative and mature stages of the polar filament made it possible to trace its development and study the interactions among various organelles during its formation. In sporoblasts the polar filament develops sequentially from 3 different regions. The base of the filament appears first and is derived from a dense body. The anterior part of the filament is formed from electron dense material located in the perinuclear cisterna and in agranular endoplasmic reticulum. The base and the anterior part of the filament move toward each other and fuse. Subsequently, the posterior part of the filament develops from the posterior part of the Golgi complex. The polar sac and the polaroplast surrounding the anterior segment of the filament are formed from the anterior region of the Golgi complex.