盘基网柄菌细胞分化和凋亡的形态特征

本文用透射电镜和DAPI荧光染色法研究了盘基网柄菌(Dictyosteliumdiscoideum)细胞分化和柄细胞的凋亡特征,结果显示:细胞丘中绝大部分细胞的线粒体内出现一小空泡,随着发育进程,空泡逐渐增大,线粒体的嵴随之变少,直至线粒体完全空泡化,最后形成单层膜的空泡。据此我们推测前孢子细胞特有的空泡来源于线粒体,并且这种细胞器水平上的内自噬现象与前孢子细胞分化密切相关。在前柄细胞分化阶段,前柄细胞中出现数个自噬泡,最初吞噬的线粒体嵴结构完整;随着前柄细胞进一步分化,部分线粒体内出现类似于前孢子细胞中的内自噬现象,并且自噬泡只吞噬这种线粒体。在凋亡后期,细胞核内核仁消失,染色体固缩形成高电子密度斑块,自噬泡采用与细胞核膜融合的方式来完成核的清除,最后柄细胞完全空泡化且包被一层纤维素壁。作者认为前柄细胞凋亡过程实质上是一种分化过程,所以有其鲜明特点:细胞出现自噬泡,标志着凋亡开始,用自噬而不是凋亡小体来清除胞内各种细胞器,直到分化最后阶段才清除细胞核和形成纤维素壁。这些特点不仅是前柄细胞凋亡的形态学指标,也和细胞发育和分化相关。

Morphology of the apoptosis and cell differentiation during Dictyostelium discoideum development

Dictyostelium discoideum multiplies by binary fission under favorable conditions, once starving or depleting their food source, the cell will undergo differentiation and morphogenesis during its multicellular development. The cell differentiation leads to two main types of cells: prespore cell and prestalk cell, the prestalk cell is destined to go apoptosis for differentiating the stalk cell during the development. Although some changes in ultrastructure during the multicellular development such as food vacuoles and cellulose shell of stalk cell are observed before, the exact morphological characters during the cell differentiation and apoptosis, such as mitochondrion, autophagic vacuoles, nucleus and prespore cell vacuoles, remain unknown. To understand relationship between the morphological traits of the apoptosis and cell differentiation during development of Dictyostelium discoideum, electron microscopy and DAPI staining are applied. The result shows that the cristae near the outer membrane of the mitochondria was disrupted by a small vacuole that appears in mitochondria of the majority of cell at mound stage, cristae were eliminated gradually while the small vacuole grew larger, finally the mitochondria becoming vacuolated was enclosed with monomembrane. These results suggest that the prespore cell vacuoles is resulted from the disruption of cristea in mitochondrion, and this endoautophagy occuring at organelle level has something to do with the differentiation of prespore cell. In the prestalk cell, several autophagic vacuoles had emerged, the mitochondria engulfed by autophagic vacuoles kept their cristae intact at first. With further differentiation, these autophagic vacuoles fused, at this stage the cristae of some mitochondria commenced to disrupt in the form of endoautophagy just like in prespore cell mentioned above, only the mitochondria with cristae disrupted were engulfed by autophagic vacuoles,the other mitochondria whose critae stay intact were no longer engulfed until development was complete. During the culmination stage, the nucleus changed a lot and did not remain structural integrity any more, some high electronic spots of condensed chromatin appeared in the nucleus, the nucleolus diminished gradually, but the karyotheca remained intact. The autophagic vacuoles continued to engulf organelles such as mitochondria, eventually, karyotheca fused to the membrane of autophagic vacuoles to fulfill the elimination of nucleus. In stalk cell, almost all the organelles including nucleus had been engulfed by autophagic vacuoles, the whole stalk cell was high vacuolated and was wrapped by a cellulose shell. The author presumes that the whole process of apoptosis is substantially the differentiation of prestalk cell, so it shows its own characters: the onset of the apoptosis of prestalk cell is marked by the emergence of autophagic vacuoles, and cell disposes the damaged organelle such as mitochondria even nucleus eventually by the means of autophagic vacuoles rather than apoptotic body, while the endoautophagy of mitochondrion can be regarded as the morphological trait at the level of organelle. Changes in ultrastructural level during the apoptosis of prestalk cell is not only the morphological traits of apoptosis but also the results of cell differentiation.