The ultrastructural pathology of congenital murine toxoplasmic retinochoroiditis. Part I: The localization and morphology of Toxoplasma cysts in the retina.

This study describes the ultrastructural characteristics of retinal parasitization by Toxoplasma gondii in a congenital mouse model. Forty-two eyes from infected mice, 18-22-weeks-old, and 24 control eyes were initially studied by light microscopy of semithin sections. Twenty-six eyes from infected animals and six from the controls were further investigated by transmission electron microscopy. A total of 13 Toxoplasma cysts was found in samples of the retinas of six eyes from five infected animals. These were located in the inner retina, particularly the ganglion-cell layer, but in no other ocular tissue. The cyst wall interdigitated with the host cell which in most cases was probably glial in origin (Müller cell). Two cysts showed evidence of parasitization of neural cells. The individual Toxoplasma cystozoites demonstrated characteristic ultrastructural features. There was no evidence of morphological changes indicative of toxicity to surrounding retinal tissues, and the associated inflammatory cell reaction (described in Dutton, McMenamin, Hay and Cameron, 1986b) was remote from the parasite. There was no morphological evidence of rupture or degeneration of cysts. No free parasites (endozoites) or pseudocysts were observed.