纺锤体是真核细胞有丝分裂或减数分裂过程中形成的中间宽两端窄的纺锤状细胞结构，主要由大量纵向排列的微管构成。纺锤体一般产生于分裂前初期（Pre-Prophase），并在分裂末期（Telophase）消失。纺锤体主要元件包括极间丝、着丝点丝、星体丝及区间丝四种微管和附着在微管上的动力分子分子马达以及一系列复杂的超分子结构组成。
动物细胞内的纺锤体两端具有由中心粒构成的“星体”，所以动物细胞的纺锤体也称为“星纺锤体”。高等植物的细胞的纺锤体则不含中心体，称为“无星纺锤体”。而真菌细胞的纺锤体含纺锤极体（Spindle Pole Body），一般被视为中心体的同源细胞结构。
在含中心体的细胞中，纺锤体的形成开始于细胞分裂前初期，核膜破裂（Nuclear Envelope Breakdown，NEB）之前。初期的结构为两个相互独立的以中心体为核的星状体（asters）。当核膜消失后，星状体和原本位于细胞核内的染色体发生一系列复杂的相互作用。最终结果为所有的染色体在纺锤体的中央（赤道板）排列整齐，每一个染色体上的两个着丝点各被一束极性相同的微管（通常称为纺锤丝）附着。此时细胞处于分裂中期，纺锤体生成完毕。实验证明，中心体在这个过程中的作用不是必需的。动物细胞在中心体被激光捣毁后仍旧能够筑构纺锤体，但其位置通常不在细胞的大致几何中心，其后的胞质分裂也会受严重影响。
在不含中心体的细胞中，纺锤体的生成是由染色体本身主导的。此过程由一小分子量的GTP连接蛋白（Ran GTPase）控制。核膜破裂后，纺锤丝由染色体周围生成。其后这些纺锤丝会在动力分子与微管的协同影响下自动排列为极性相反、数目大致相同的两组，每组的极性相对于一组着丝点。同时在微管远端的动力蛋白会将这些微管束集中到一点，形成纺锤极区（Spindle Polar Zone）。与此同时，染色体会自动在赤道板排列整齐。纺锤体完成形成过程。
在细胞分裂中，纺锤体的主要功能可分为两个部分。
排列与分裂染色体。纺锤体的完整性决定了染色体分裂的正确性。纺锤体的正常生成是染色体排列的必要条件。纺锤体生成完毕后一般会有5-20min的延迟，以供细胞调整着丝点上微管束的极性，以及决定是否所有的着丝点都附着正确。此后细胞进入分裂后期，染色体分裂为两组数目相等的姐妹染色单体。同样，纺锤体的完整性决定这个分裂过程在时间和空间上的准确性。
纺锤体另一功能为决定细胞质分裂的分裂面。染色体分裂的同时，纺锤体中的一部分微管不随染色体分裂到两极，而停止在纺锤体中央， 形成中心纺锤体（central spindle）。在纺锤中体的中央为两组极性相反的微管交叠的区域，称为“纺锤中间区”（spindle midzone）。这个区域就是接下来的胞质分裂面。胞质分裂开始于分裂后期的较晚时期。胞质分裂一般结束于分裂末期之后的1-2h，此期间两个子细胞由中间体（midbody）连接。 一般认为纺锤体的分解发生在细胞分裂末期。
In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells. Depending on the type of cell division, it is also referred to as the mitotic spindle during mitosis and the meiotic spindle during meiosis.
The cellular spindle apparatus includes the spindle microtubules, associated proteins, and any centrosomes or asters present at the spindle poles.[1] The spindle apparatus is vaguely ellipsoid in shape and tapers at the ends. In the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells. Acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during gametogenesis in animals.[2] In fungi, spindles form between spindle pole bodies embedded in the nuclear envelope. Most plants lack centrosomes or spindle pole bodies and instead spindle microtubules are nucleated on their nuclear envelopes.[3] The spindles take the cell's chromosomes through most stages of mitosis. They start to form in early metaphase and are connected to the centrioles at each end of the cell.
Aurora A is required for proper spindle assembly and separation.[citation needed] There have been identified many proteins necessary for the mitotic spindle assembly. Lamin B is not essential protein for spindle assembly. It is only a component of the spindle matrix helping microtubule assembly, since proper mitotic spindle can be formed without it.[4]
Polo-like kinase, also known as PLK, especially PLK1 has important roles in the spindle maintenance by regulating microtubule dynamics.
The completion of spindle formation is a crucial transition point in the cell cycle called the spindle assembly checkpoint. If some chromosomes are not properly attached to the mitotic spindle by the time of this checkpoint, the onset of anaphase will be delayed.[6] Failure of this spindle assembly checkpoint can result in aneuploidy and may be involved in aging and the formation of cancer.[7] Abnormal mitotic spindles can produce tripolar mitosis. These are clearly abnormal cases and, if present, are considered definitive evidence that a tumor is malignant rather than benign. Such abnormalities are therefore often searched for in histological assays by pathologists when evaluating the potential malignancy of a tumor mass.
This video shows the beauty of spindle during cell division. As it is known, during cell division, microtubules are formed and these microtubules are attached to kinetechore region of chromosomes. Kinetechore is a special DNA sequence region. When microtubules binds to kinetechore, it seperated chromosomes from each other. As you see in the video, chromosomes are about to be seperated. Blue one is most probably DAPI staining showing the presence of DNA. Red color shows the location of kinetechore. And green color shows microtubules.细胞分裂中美丽的纺锤体
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