The macrogametes of Toxoplasma gondii develop within the epithelial cells of the cat ileum. As they develop the nucleus enlarges and dense patches of chromatin which were present in the nucleoplasm, disappear. Polysaccharide granules and lipid globules appear in the cytoplasm and increase in number during development. The wall-forming bodies of Type I (WFB I) appear before the wall-forming bodies of Type II (WFB II); WFB I are smaller, more osmiophilic and more numerous than the WFB II. The WFB I appear to form from vesicles produced by the smooth endoplasmic reticulum, and the WFB II form within the lacunae of the rough endoplasmic reticulum. Double membraned vacuoles appear to form from the nuclear membranes but the function of these is unknown. Throughout development the macrogamete retains a normal pellicle which possesses numerous micropores. The first evidence of oocyst wall formation is the appearance of participate matter in the parasitophorous vacuole which precipitates to form Layer 1 of the oocyst wall. Layers 2 and 3 are unit membranes which form between Layer 1 and the pellicle. During this development the organism has an organelle complement similar to that of the macrogamete. Layers 4 and 5 form between Layer 3 and the pellicle. Layer 4 is osmiophilic and its formation is accompanied by the disappearance of WFB I. Layer 5 is less osmiophilic than Layer 4 and its formation is accompanied by the disappearance of WFB II. The two innermost layers (Layers 4 & 5) in the oocyst wall of Toxoplasma are similar to those found in Isospora spp. and Eimeria spp. Toxoplasma seems to be unusual in that, firstly, it possesses an additional 3 layers and, secondly, all 5 layers are formed outside the pellicle of the original macrogamete.

The ultrastructural development of the macrogamete and formation of the oocyst wall of Toxoplasma gondii.

Ultrastructural development of macrogamete and formation of oocyst wall of toxoplasma-gondii

The development of three chicken coccidia (Eimeria tenella, E. acervulina andE. maxima) was studied by means of light and electron microscopy. One group of chickens infected with 6000–20000 oocysts received a single dose of 5 mg/kg body weight (comparable to approx. 25 ppm in the feed) Bay Vg 7183 or Bay Vi 9142 orally (on day 3 or 4 p.i.), whereas others received two doses (on days 3 and 4 or on days 4 and 5 p.i.). The animals were killed on days 3, 4, 5, 6 and 7 p.i. and parts of the mucosa were dissected from the caecum (E. tenella), the ileum (E. maxima) and the duodenum (E. acervulina). Significant damage was observed in comparison to the controls, affecting nearly all of the parasites in those animals that had been treated twice, whereas some of the parasites remained microscopically unchanged after only one treatment. In general, the perinuclear space, mitochondria and the endoplasmatic reticulum were found to be considerably enlarged. Nuclear divisions were disturbed in schizonts and microgamonts, thus resulting in a greatly reduced production of parasites. The most important damage occurring in the macrogamonts concerned the wall-forming bodies II. As they burst, the formation of intact oocyst-walls was hindered, even if fertilization had taken place.

The effects of sym. Triazinones on developmental stages of Eimeria tenella, E. maxima and E. acervulina: a light and electron microscopical study.

SYNOPSIS
Fine structure of gametocytes and oocyst formation of Sarcocystis sp. from Quiscalus quiscula Linnaeus grown in cultured embryonic bovine kidney cells was studied. Microgametocytes measured up to ∼5 μm diameter. During nuclear division of the microgametocyte, dense plaques were found adjacent to the nucleus just beneath the pellicle; occasionally microtubules were present within these plaques. These microtubules subsequently formed 2 basal bodies with a bundle of 4 microtubules between them. Microgametocytes also contained numerous mitochondria, micropores, granules, vacuoles, and free ribosomes. Each microgamete was covered by a single membrane and consisted of 2 basal bodies, 2 flagella, a bundle of 4 microtubules, a perforatorium, a mitochondrion, and a long dense nucleus which extended anteriorly and posteriorly beyond the mitochondrion. The bundle of 4 microtubules is thought to be the rudiment of a 3rd flagellum. Macrogametes were covered by a double membrane pellicle, and contained a large nucleus (∼2.5 μm), vacuoles, and a dilated nuclear envelope connected with the rough endoplasmic reticulum (ER). In young macrogametes (∼4 μm), the ER was arranged in concentric rows in the cortical region, and several sizes of dense granules were found in the cytoplasm. However, in later stages (∼8 μm) the ER was irregularly arranged and was dilated with numerous cisternae; only large dark granules remained and a few scattered polysaccharide granules were found. No Golgi apparatus or micropores were observed. After the disappearance of dark granules 5 concentric membranes appeared. Four of these fused to form an oocyst wall composed of a dense outer layer (∼66 nm thick) and a thin inner layer (∼7 nm). The 5th or innermost membrane surrounded the cytoplasmic mass which was covered by a 2-layered pellicle and contained a nucleus, small amounts of ER, large vacuoles, and mitochondria. The sexual stages described greatly resemble those of Eimeria and Toxoplasma.

Fine structure of gametogony and oocyst formation in Sarcocystis sp. in cell culture.

SYNOPSIS Oocysts of Isospora serini and Isospora canaria from the canary Serinus canarius were broken added to a cell suspension, fixed in Karnovsky's fluid, and studied in the electron microscope The oocyst wall of each species had an electron_ lucent inner layer, a more osmiophilic middle layer and an outer layer of electron-lucent (I serini) or electron-dense material interspersed with some electron-lucent material (1 canaria) A few, relatively large lipid-like bodies were present in the outer or middle layer of the oocyst wall of I canaria As many as 9 membranes were present in the oocyst wall of I canaria and 3 in that of I serini When exposed to a trypsin-sodium taurocholate fluid, sporozoites of I serini excysted from 5-month-old sporocysts in vitro, but not from sporocysts stored for more than 6 months No excystation occurred in l5-month-old I canaria sporocysts Similarities and differences in excystation between I serini and other Isospora, Eimeria, and Sarcocystis species are discussed R ECENTL Y, Box (1) dt'scrihed the exogenous stages of Iso­ s para serini (Aragao, 1933), which may be synonymous with Atoxoplasma, and Isospora canaria Box, 1975, from canaries, Serinus canarius Linnaeus When examined in the light micro­ scope the sporulated oocysts of these species are very similar structurally and differ only slightly in size Our electron-micro­ scopic findings with regard to the similarities and differences of the oocyst walls of I serini and I canaria, as well as light­ microscopic observations on excystation of I serini, are described herein

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1681&context=parasitologyfacpubs

Fine Structure of the Oocyst Walls of Isospora serini and Isospora canaria and Excystation of Isospora serini From the Canary, Serinus canarius L.

The fine structure of the macrogametocyte, macrogamete and the early stages in the formation of the oocyst wall of Eimeria acervulina have been described and compared with other species of Eimeria. It has been shown that type II wall-forming bodies are formed in cisternae of the granular endoplasmic reticulum in association with Golgi complexes. The developing oocyst has been shown to be enclosed by three membranes: the outermost is the original membrane of the merozoite and macrogametocyte; the second membrane appears when the wall-forming bodies are appearing in the cytoplasm of the macrogametocyte and the innermost membrane appears after fertilization. The wall-forming bodies discharge their contents between the innermost membrane and the middle membrane.We wish to thank Mr P. L. Long for much valuable discussion, Mrs B. Fisher for technical assistance and Mr B. Carter for assistance with the photography.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0031182000071262

B. J. Millard

Papers

The structure and development of the macrogamete and oocyst of Eimeria acervulina.