We are applying large-scale electron tomography to analyze the ultrastructural organization of microtubule-based spindles. Recently, we have analyzed the first mitotic division in the early C. elegans embryo (Redemann et al., 2017) and found that chromosomes connect to the mitotic centrosomes by anchoring into the spindle network. We also analyzed acentrosomal spindles in oocyte meiosis in C. elegans. We discovered that chromosome to microtubule associations transition from lateral to end-on during early to mid-anaphase. Interestingly, the microtubule depolymerase KLP-7 is involved in this transition from lateral to end-on associations (Redemann et al., 2018). Currently, we are focusing on the 3D ultrastructure of mitotic spindles in mammalian tissue culture (HeLa) cells. The differences in centrosomal versus acentrosomal spindle organization will be discussed.