Morphological analysis of oocyst wall formation in cryptosporidia: Is the sporocyst really missing?
|Fakulta / Pracoviště MU|
|Popis||The development of Cryptosporidium muris (strain TS03) oocysts was mapped in detail using a combined microscopic approach, supported by a freeze etching. Our data show that the oocyst wall starts to form shortly after fertilization of the macrogamonts by the microgametes. Two types of wall forming bodies (WFB) of different electron density occur exclusively in sexual stages: mature macrogamonts and zygotes. The WFB, located beneath the zygote pellicle, disintegrate into small particles and migrate into the space between pellicular membranes. Additional membranes seem to develop beneath the pellicle so that four or more membranes can be seen enveloping more advanced zygote stages. Developing oocysts are enveloped by a parasitophorous sac and their wall comprises three layers. The outermost one, considered to be a ‘true oocyst’, is very fragile and this can be the reason that it usually remains unnoticed. In endogenous stages, this layer is usually separated from inner two layers and often almost unnoticeable as it is adjacent to the inner membrane of the parasitophorous sac. The middle thin and the innermost thick layers, on which the characteristic suture can be seen, form the wall of a ‘sporocyst’. Fully sporulated ‘sporocysts’, found either in stomach or faeces, are released from parasitophorous sac and frequently lack the outermost layer (‘oocyst’). Under scanning electron microscope, they exhibit either smooth or wrinkly surface, presumably depending on the oocyst wall thickness. Generally, we can resume that the thickness of their wall is not uniform and varies significantly even in fully sporulated oocysts, thus it is not correct to strictly divide oocysts into thick- or thin-walled in cryptosporidia. This study suggest to re-evaluate the putative absence of sporocyst, an important taxonomical feature used for identification of cryptosporidia.|