Observations on the Fine Structure of the “Cyst Stage” of Besnoitia jellisoni

SYNOPSIS. Light and electron microscope studies of the “cyst” of Besnoitia jellisoni indicate that it consists of an extracellular wall, a large, sometimes multinucleate, host cell, and an intracellular vacuole containing the parasites. The “cyst” wall has fine fibrils and small dense granules embedded in an election‐lucid matrix. The wall may be formed from a secretion of the enclosed host cell. The plasma membrane of the host cell is very irregular, being modified into microvillar or pseudopodial extensions. Small vesicles and invaginations of the plasma membrane indicate mioropinocytosis. The one to several large lobular nuclei lie in a thick area of cytoplasm which is filled with rough endoplasmic reticulum and many mitochondria with lamellar cristae. The parasite‐containing vacuole is limited by a vacuolar membrane which has many blebs suggesting a transfer of materials into the vacuole. The “cyst” organisms are crescentic or piriform and are enclosed by a pellicle consisting of outer and inner membranes. Twenty‐two subpellicular fibrils extend longitudinally adjacent to the inner membrane from the anterior polar ring to a posterior ring. A micropyle is situated laterally in the pelliole near the level of the nucleus. A conold and several associated paired organelles are present at the anterior end. Microuemes, more abundant in older organisms, are also present in the anterior portion of the parasite. A Golgi apparatus lies adjacent and anterior to the nucleus. One or more mitochondria with saccular cristae, ovoid glycogen bodies, free ribosomes and occasional vacuoles are also present. Organisms within the “cyst” multiply by endodyogeny.