身体细胞的武士精神-细胞自杀是生命必需的一部分

Cell Suicide: An Essential Part of Life

　

　

在细胞层面，死亡对生命至关重要。

细胞凋亡，有时被称为“细胞自杀”，是一种正常的、程序化的细胞自我毁灭过程。尽管它涉及到细胞死亡，细胞凋亡在我们的身体中起着健康和保护作用。

美国国立卫生研究院(National Institutes of Health)资助的许多研究人员的研究告诉我们，细胞凋亡有助于在出生前塑造我们的身体特征和器官，并清除体内不必要的或可能有害的细胞。如果没有细胞凋亡，我们就不会有明显的手指和脚趾或脑细胞连接来理解本文中的词语。

细胞凋亡也有助于支持免疫系统。例如，它在病毒感染过程中扮演着重要的角色，在入侵细胞与病毒颗粒扩散之前杀死它们。这种自我牺牲的行为阻碍了病毒的传播，可以拯救整个有机体。

细胞具有细胞凋亡所需的指令和仪器。他们把这些被称为蛋白酶的工具小心地藏起来，就像一套有鞘的刀，直到细胞内外的信号触发它们释放。这引发了一系列精心协调的事件，最终导致细胞高效、无痛的切除。

在细胞凋亡过程中，细胞收缩并远离邻居。然后，细胞表面开始沸腾，碎片像从一壶热水中流出的气泡一样破裂并逃逸出来。细胞核中的DNA凝结成均匀大小的碎片。不久细胞核就解体了，接着是整个细胞。一个由吞噬细胞组成的细胞清理小组——吞噬和处理死亡细胞和残骸的免疫细胞——来到现场清理残骸。

死亡的第二种方式
细胞凋亡（Apoptosis)是细胞死亡的两种方式之一。第二种是坏死(Necrosis)，是对压力过大的意外反应，比如外伤或中毒。这是心脏病发作时心脏细胞发生的情况，严重冻伤的手指和脚趾细胞，以及肺炎发作时肺部细胞发生的情况。细胞凋亡巧妙地清除了选定的细胞，而坏死则无策略地破坏。

因此，坏死的过程更加混乱。细胞在失去了控制液体进出的能力后，坏死的细胞会膨胀并最终破裂，并将其释放到周围的组织中。虽然吞噬细胞仍然猛扑过去清除这一区域，但与坏死有关的化学物质会导致这一区域发炎并变得敏感。想想你不小心碰到热炉子后手指的红肿和疼痛。

平衡的生命
当细胞凋亡破坏不需要的细胞时，有丝分裂(细胞分裂)产生新的细胞。细胞凋亡和有丝分裂共同作用来保持我们的健康。例如，我们的皮肤和毛细胞通过细胞凋亡和有丝分裂的连续周期而更新。肠道内的细胞也是如此。因为新的细胞取代了旧的，磨损的细胞，我们的组织保持健康。
　

图示：细胞有丝分裂和凋亡之间的平衡对人类健康至关重要。
来源:国家卫生研究院

正如你可以想象的那样，细胞凋亡和有丝分裂之间失去平衡可能会带来危险的后果。如果细胞凋亡在不该发生的时候被触发，我们的身体就会浪费掉非常好的细胞。科学家们认为，过多的凋亡至少是一些神经退行性疾病的部分原因，如阿尔茨海默氏症、帕金森症和葛雷克氏症，而且它可能在全身性致命感染败血症中发挥作用。另一方面，不受控制的有丝分裂会导致癌症。

　

By Stephanie Dutchen and Kirstie Saltsman, National Institutes of Health | February 23, 2011 11:55am ET

　

　

A mitochondrion (blue) undergoes the death throes of apoptosis. Two proteins, Reaper and Hid (green), trigger the release of cytochrome c (red), which in turn activates a fatal chain reaction. The yellow areas show a mix of Reaper-Hid and cytochrome c.

Credit: Credit: Hermann Steller, Rockefeller University.

　

At the cellular level, death is essential for life.

 

Apoptosis, sometimes called "cellular suicide," is a normal, programmed process of cellular self-destruction. Even though it involves cell death, apoptosis serves a healthy and protective role in our bodies.

 

The work of many researchers funded by the National Institutes of Health has taught us that apoptosis helps shape our physical features and organs before birth and rids our bodies of unneeded or potentially harmful cells. Without apoptosis, we wouldn't have distinct fingers and toes or the brain cell connections to understand the words in this article.

　

 

　

Apoptosis also helps support the immune system. For instance, it plays a critical role during viral infections, killing off invaded cells before they spill over with virus particles. This act of self-sacrifice hampers the spread of viruses and can save the whole organism

 

Cells come equipped with the instructions and instruments necessary for apoptosis. They keep these tools, called proteases, carefully tucked away like a set of sheathed knives, until some signal — either from within or outside the cell — triggers their release. This initiates a cascade of carefully coordinated events that culminate in the efficient, pain-free excision of cells.

 

During apoptosis, the cell shrinks and pulls away from its neighbors. Then the surface of the cell appears to boil, with fragments breaking away and escaping like bubbles from a pot of hot water. The DNA in the cell’s nucleus condenses and breaks into evenly sized fragments. Soon the nucleus itself disintegrates, followed by the entire cell. A cellular cleanup crew made of phagocytic cells — immune cells that engulf and dispose of dead cells and debris—arrives on the scene to mop up the remains.

　

 

A Second Way to Die

 

Apoptosis is one of two ways cells can die. The second, necrosis, is an unplanned response to an overwhelming stress such as a traumatic injury or exposure to poison. It's what happens to heart cells during a heart attack, cells in severely frostbitten fingers and toes, and lung cells undergoing a bout of pneumonia.  While apoptosis neatly removes select cells, necrosis destroys without strategy.

 

　

Accordingly, the process of necrosis is much messier. Having lost the ability to control the flow of liquid in and out, cells experiencing necrosis swell up and eventually burst, releasing their contents into the surrounding tissue. While phagocytes still swoop in to clear the area, the chemicals involved in necrosis cause the area to become inflamed and sensitive. Think of the redness and pain in your finger after you accidentally touch a hot stove.

 

Life in Balance

 

As apoptosis destroys unwanted cells, mitosis (cell division) makes new cells. While they may seem to be at odds, apoptosis and mitosis work together to keep us healthy. For example, our skin and hair cells are renewed via a continuous cycle of apoptosis and mitosis. So are the cells lining our intestines. Because new cells replace old, worn-out ones, our tissues remain healthy.

　

　

A balance between mitosis and apoptosis is critical to human health.

Credit: NIH

As you can well imagine, loss of the balance between apoptosis and mitosis can have hazardous consequences. If apoptosis is triggered when it shouldn't be, our bodies squander perfectly good cells. Scientists believe that too much apoptosis is at least partly to blame for some neurodegenerative diseases, such as Alzheimer's, Parkinson's and Lou Gehrig's, and it may play a role in the fatal body-wide infection known as sepsis. On the other hand, unchecked mitosis can lead to cancer.

https://www.livescience.com/12949-cell-suicide-apoptosis-nih.html

　

Apoptosis is an innate defense function of macrophages against Mycobacterium tuberculosis https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3155700/