Trichuris muris: structure and formation of the egg polar plugs

The structure and development of the polar plugs of Trichuris muris eggs were observed by light and electron microscopy. Initial stages in plug formation commenced in the spermatheca of the adult female where two polar papillae became delimited from the remainder of the oocyte cytoplasm. These papillae exhibited a discrete PAS positive reaction and were regarded as prospective plug regions from which cytoplasmic granules were absent. Glycogen rosettes, initially concentrated in these prospective areas, were later transformed into an irregular fine network of chitin-protein microfibrils. This arrangement of microfibrils was in contrast to that of the surrounding collar region which displayed a distinct lamellate organization. The fully developed polar plug was of lower electron density than the shell. Each plug was covered externally by the vitelline layer and lined internally by the lipid layer of the eggshell.