What Was Observed? (Introduction)

• Researchers studied how the planarian flatworm adapts its body structure after regenerating a new head and tail. This adaptation involves changes to the body’s polarity, which is how cells are arranged in relation to the body’s front (anterior) and back (posterior).
• In double-headed planarians, the body undergoes significant reorientation as new body parts are formed, and the nervous system and cilia (tiny hair-like structures) adjust to these changes over time.
• The study aimed to explore how the polarity of tissues, like the cilia on the surface of the planarian, changes in response to the new body structure, and how signals from the brain drive these changes.

What is Planarian Regeneration?

• Planarians can regenerate lost body parts. They have the ability to regrow complete heads, tails, and other tissues after injury.
• The regeneration process involves changes at both the cellular and body-wide levels to restore the original body plan.

Key Process: Cilia Reorientation

• Cilia are tiny, hair-like structures that beat in a coordinated motion to help with movement. In planarians, the cilia on the underside of the body are responsible for their gliding movement.
• In double-headed planarians, the cilia initially beat in two opposing directions but gradually reorient to align with the new body axis, which shifts as the animal regenerates.
• This cilia reorientation happens over weeks and involves the slow adaptation of existing cilia rather than the formation of new cilia cells.

Who Were the Subjects? (Material and Methods)

• The researchers used Dugesia japonica planarians, which were kept in cold water and starved before being used in experiments.
• Double-headed planarians were created by cutting a single-headed planarian in half and treating the fragments with a chemical solution, allowing them to regenerate new heads.
• Different methods, like irradiation, were used to study how the absence of certain body parts (such as the brain or cilia) affects the regeneration process.

How Was the Experiment Conducted? (Methodology)

• To track the flow driven by the cilia, the planarians were placed in water with carmine powder, and their movement was observed under a microscope.
• The researchers used different techniques to remove parts of the planarians (such as the heads or specific tissue areas) and then observed how the cilia and nervous system adapted to these changes over time.

What Happened During Regeneration? (Results)

• Initially, the two heads of the double-headed planarians were different in size and control over movement.
• Over time, the two heads became more symmetrical, and both heads took equal control of the animal’s movement.
• The cilia on the ventral surface of the planarians gradually changed direction to align with the new body plan, moving from the tail to the middle of the body.
• The process of cilia reorientation took weeks to complete, with the flow of particles gradually shifting from the secondary head to the midpoint of the body.

What Did the Researchers Find About the Cilia? (Cilia Reorientation Mechanism)

• The cilia reorientation happens over a long period, from Day 7 to Day 42, even if new cells are not produced.
• The researchers found that removing or irradiating parts of the planarian’s body did not prevent cilia reorientation, suggesting that the reorientation is controlled by molecular signals within the existing cells, not new cell growth.
• When external cilia were removed or blocked, the cilia reorientation still occurred, indicating that the process is molecular and not dependent on the cilia’s ability to beat.

How Does the Head Influence Cilia Reorientation?

• The presence of the heads plays a crucial role in controlling the speed of cilia reorientation. Removing the primary head speeds up the process, while removing the secondary head slows it down.
• In double-headed planarians, the secondary head seems to drive the reorientation process, while the primary head has an opposing effect.
• Even after the heads were removed, the cilia reorientation continued, but at a slower pace, suggesting that the heads play a central role in the initiation of the process.

What About the Nervous System?

• The nervous system adapts to the new body morphology over time. In double-headed planarians, the symmetry of the nervous system gradually shifts to match the new body structure.
• The transport of signals in the nervous system is crucial for the reorientation of the cilia, as cutting the nerve cords affects the speed of cilia reorientation.
• The researchers hypothesize that the nervous system’s polarity adapts to changes in body structure, influencing how tissues, like the cilia, align with the new body plan.

Key Conclusions (Discussion)

• The regeneration process in double-headed planarians involves dynamic changes in the polarity of tissues and the nervous system.
• The nervous system plays a central role in driving the reorientation of cilia and other tissue structures, and this process occurs over an extended period.
• The study sheds light on how the brain and nervous system coordinate the regeneration of complex body structures, and how signals are transmitted to guide this process.
• These findings have important implications for understanding tissue polarity in regeneration and could inform research in bioengineering and regenerative medicine.

观察到了什么？ (引言)

• 研究人员研究了再生双头平面虫如何适应新的身体结构，特别是在其重新生成新头部和尾部时。这一适应过程涉及到身体极性（细胞如何排列与身体的前端和后端的关系）的变化。
• 在双头平面虫中，随着新身体部位的形成，身体发生了显著的重新定位，神经系统和纤毛（微小的毛发状结构）随着时间的推移适应这些变化。
• 研究旨在探索组织极性（如平面虫表面纤毛）如何响应新的身体结构发生变化，以及大脑发出的信号如何驱动这些变化。

什么是平面虫再生？

• 平面虫可以再生失去的身体部位。它们能够在受伤后重新长出完整的头部、尾部和其他组织。
• 再生过程涉及细胞和全身水平的变化，以恢复原始的身体结构。

关键过程：纤毛重新定位

• 纤毛是微小的、像毛发一样的结构，通过协调运动帮助平面虫移动。在平面虫中，腹部表面的纤毛负责它们的滑行运动。
• 在双头平面虫中，纤毛最初在两个相反的方向上摆动，但随着动物的再生，纤毛逐渐重新定位，以与新的身体轴对齐。
• 这一纤毛重新定位的过程持续了几周，涉及现有纤毛的适应，而不是新纤毛细胞的形成。

实验对象是谁？ (材料与方法)

• 研究人员使用了Dugesia japonica平面虫，它们被保存在冷水中并在实验前禁食。
• 双头平面虫是通过切割单头平面虫并处理碎片，允许它们再生新头部来生成的。
• 不同的方法，如辐射，被用来研究缺少某些身体部位（如大脑或纤毛）如何影响再生过程。

实验是如何进行的？ (方法论)

• 为了追踪由纤毛驱动的流动，平面虫被放置在水中，表面上撒上胭脂粉，然后通过显微镜观察它们的运动。
• 研究人员使用不同的技术切除平面虫的身体部位（如头部或特定组织区域），然后观察这些变化如何影响纤毛和神经系统随时间适应。

再生过程中发生了什么？ (结果)

• 最初，双头平面虫的两个头部在大小和控制运动方面有所不同。
• 随着时间的推移，这两个头部变得更加对称，并且两个头部在控制运动方面达到了平衡。
• 平面虫腹部表面的纤毛逐渐改变方向，以与新的身体结构对齐，从尾部到身体的中部。
• 纤毛重新定位的过程持续了几周，纤毛驱动的粒子流从第二个头部逐渐移动到身体的中点。

研究人员关于纤毛的发现是什么？ (纤毛重新定位机制)

• 纤毛重新定位是在长时间内发生的，从第7天到第42天，即使没有产生新细胞。
• 研究人员发现，切除或辐射平面虫的某些部位并没有阻止纤毛重新定位，表明这一过程是由现有细胞中的分子信号控制的，而不是新细胞的生长。
• 当外部的纤毛被移除或阻止时，纤毛重新定位依然发生，表明这一过程是分子驱动的，而不是依赖于纤毛的摆动能力。

头部如何影响纤毛重新定位？

• 头部的存在在控制纤毛重新定位速度上发挥着重要作用。去除主头加速了这一过程，而去除副头则减缓了这一过程。
• 在双头平面虫中，副头似乎是驱动重新定位过程的主要力量，而主头则有相反的作用。
• 即使去除头部，纤毛重新定位依然继续，但速度较慢，表明头部在重新定位过程的启动中起着中心作用。

神经系统如何影响纤毛重新定位？

• 神经系统会随着时间的推移适应新的身体形态。在双头平面虫中，神经系统的对称性逐渐变化，以适应新的身体结构。
• 神经系统中的信号传输对纤毛重新定位至关重要，因为切割神经索会影响纤毛重新定位的速度。
• 研究人员推测，神经系统的极性会适应身体结构的变化，影响纤毛等组织如何与新的身体计划对齐。

主要结论 (讨论)

• 双头平面虫的再生过程涉及组织极性和神经系统的动态变化。
• 神经系统在驱动纤毛重新定位和其他组织结构的过程中发挥着中心作用，这一过程持续了很长时间。
• 这项研究揭示了大脑和神经系统如何协调再生复杂的身体结构，以及信号如何传输以指导这一过程。
• 这些发现对于理解再生过程中的组织极性有重要意义，并可能为生物工程和再生医学领域的研究提供启示。