Tetley L, Vickerman K
J Cell Sci. 1985 Mar;74:1-19. doi: 10.1242/jcs.74.1.1.
Acquisition of the variable antigen-containing surface coat of Trypanosoma brucei occurs at the metacyclic stage in the salivary glands of the tsetse fly vector. The differentiation of the metacyclic trypanosome in the gland has been studied by scanning electron microscopy and by transmission electron microscopy of thin sections and freeze-fracture replicas. The uncoated epimastigote trypanosomes (with a prenuclear kinetoplast) divide while attached to the salivary gland epithelium brush border by elaborate branched flagellar outgrowths, which ramify between the host cell microvilli and form punctate hemidesmosome-like attachment plaques where they are indented by the microvilli. These outgrowths become reduced as the epimastigotes transform to uncoated trypomastigotes (with postnuclear kinetoplast), which remain attached and capable of binary fission. The flagellar outgrowths disappear but the attachment plaques persist as the uncoated trypomastigotes (premetacyclics) stop dividing and acquire the surface coat to become 'nascent metacyclics'. Coat acquisition therefore occurs in the attached trypanosome and not, as previously believed, after detachment. Coating is accompanied by morphological changes in the glycosomes and mitochondrion of the parasite. Freeze-fracture replicas of the host-parasite junctional complexes show membrane particle aggregates on the host membrane but not on the parasite membrane. It is suggested that disruption of the complex occurs when maximum packing of the glycoprotein molecules has been achieved in the trypanosome surface coat, releasing the metacyclic trypanosome into the lumen of the gland.
布氏锥虫含可变抗原的表面被膜的获得发生在采采蝇媒介唾液腺的循环后期阶段。通过扫描电子显微镜、薄切片透射电子显微镜以及冷冻蚀刻复型技术，对循环后期锥虫在唾液腺中的分化进行了研究。未被膜覆盖的上鞭毛体锥虫（带有核前动基体）通过精细的分支鞭毛生长物附着在唾液腺上皮刷状缘上进行分裂，这些生长物在宿主细胞微绒毛之间分支，并在微绒毛使其凹陷处形成点状半桥粒样附着斑。随着上鞭毛体转变为未被膜覆盖的锥鞭毛体（带有核后动基体），这些生长物会减少，锥鞭毛体仍保持附着并能够进行二分裂。当未被膜覆盖的锥鞭毛体（前循环后期）停止分裂并获得表面被膜成为“新生循环后期锥虫”时，鞭毛生长物消失，但附着斑仍然存在。因此，被膜的获得发生在附着的锥虫上，而不是像之前认为的那样在脱离之后。被膜形成伴随着寄生虫糖体和线粒体的形态变化。宿主 - 寄生虫连接复合体的冷冻蚀刻复型显示宿主膜上有膜颗粒聚集，而寄生虫膜上没有。有人提出，当锥虫表面被膜中的糖蛋白分子达到最大堆积时，复合体就会被破坏，从而使循环后期锥虫释放到腺腔内。
