Severed sea cucumber appendages don’t seem to die

海参断肢似乎不会死亡

Organs, arms, appendages, and other complex tissues usually decay rapidly when they’re separated from their host. Over the years, biologists have seen some success with keeping them alive outside of the body—organ transplants depend on it—but it has always required germ-free environments and nutrient-rich mediums filled with growth factors. Now, though, scientists have discovered bits of tissue removed from a species of sea cucumber called Psolus fabricii can keep on living indefinitely if they’re left in ordinary seawater. 器官、肢体、附肢以及其他复杂的组织在脱离宿主后通常会迅速腐烂。多年来，生物学家在维持离体组织存活方面取得了一些成功（器官移植就依赖于此），但这始终需要无菌环境和充满生长因子的营养培养基。然而，科学家们现在发现，从一种名为 Psolus fabricii 的海参身上取下的组织碎片，如果放置在普通海水中，可以无限期地存活下去。

“This is naturally occurring tissue immortality,” said Sara Jobson, a researcher at Memorial University of Newfoundland and lead author of the study. “Having tissues that survive that easily is unheard of. We’ve never seen anything like this.” “这是一种自然发生的组织永生，”纽芬兰纪念大学的研究员、该研究的主要作者萨拉·乔布森（Sara Jobson）说。“拥有如此容易存活的组织是闻所未闻的。我们从未见过类似的情况。”

Psolus fabricii is a species of sea cucumber that lives in the cold waters of the Atlantic and Arctic oceans. Its bottom side, known as a sole, is soft and ringed by a band of tube feet that it uses to grip rocks. Once on a rock, it extends soft, branching tentacles into the water to feed on suspended particles. Because these sea cucumbers inhabit harsh environments, their feet and tentacles experience high rates of injury and loss. Evolution has therefore endowed these sites with an incredibly high capacity for regeneration. Psolus fabricii 是一种生活在大西洋和北冰洋寒冷水域的海参。它的底部被称为“足底”，柔软且环绕着一圈用于抓附岩石的管足。一旦附着在岩石上，它就会将柔软的分支触手伸入水中，以摄取悬浮颗粒。由于这些海参栖息在恶劣的环境中，它们的管足和触手受伤和脱落的频率很高。因此，进化赋予了这些部位极强的再生能力。

While sea cucumbers can easily regrow these parts, they don’t have whole-body regeneration like flatworms and some starfish do. Their severed bits don’t grow into new sea cucumbers. But it turns out they don’t die, either. 虽然海参可以轻松再生这些部位，但它们并不像扁形虫和某些海星那样具备全身再生能力。它们的断肢不会长成新的海参。但事实证明，它们也不会死亡。

“We didn’t set out to find immortal tissues,” Jobson said. “Our lab focuses on sea cucumbers, and this sea cucumber has been used in other studies. One of my collaborators happened to notice that its amputated tissue just kept living, and it seemed to be healing and surviving and she didn’t do anything special to keep it. It was a fortuitous discovery.” “我们并非刻意去寻找永生组织，”乔布森说。“我们的实验室专注于海参研究，这种海参曾被用于其他研究。我的一位合作者偶然发现，它被切除的组织竟然一直活着，而且似乎在自我修复和存活，而她并没有采取任何特殊措施来维持它。这是一个偶然的发现。”

This fortuitous discovery quickly turned into an organized long-term experiment. The researchers took excised tube feet, groups of tube feet called ambulacra, and tentacles from P. fabricii and found all of them survived when placed in natural, non-sterile seawater. 这一偶然发现很快演变成了一项有组织的长期实验。研究人员取下了 P. fabricii 的管足、被称为步带的管足群以及触手，发现它们在放置于天然、非无菌的海水中时全部存活了下来。

“We examined all of them, but we primarily focused on tube feet,” Jobson said. When tube feet were severed, the wound margin was a mess of missing or fragmented epidermal and connective tissue. Within two days, the explants began shedding this damaged tissue. Internally, a large influx of coelomocytes, the sea cucumber’s immune cells, rushed from the inner connective tissues toward the damaged spot, apparently to facilitate organismal defense and regeneration. By day six, the healthy tissue had curled inward, completely sealing the wound site; the severed organ was more or less restored to working order. “我们检查了所有样本，但主要关注管足，”乔布森说。当管足被切断时，伤口边缘是一片缺失或破碎的表皮和结缔组织。两天内，外植体开始脱落这些受损组织。在内部，大量的体腔细胞（海参的免疫细胞）从内部结缔组织涌向受损部位，显然是为了促进机体防御和再生。到第六天，健康组织向内卷曲，完全封闭了伤口；断裂的器官在很大程度上恢复了正常功能。

It turned out LiPfe explants weren’t just surviving; they were actively reorganizing their architecture to adapt to the new, severed state. First came the shrinking. During the first week, the tissue shrank by about 23 percent in diameter. Given more time, it stabilized and reversed this trend. Between 60 and 120 days post-excision, LiPfe grew back to their initial size, and after a year, they were 12 percent larger than when they were first cut from the host. The researchers have introduced these tissues as a completely new class of living material they called LiPfe—living immortal P. fabricii explants. 事实证明，LiPfe 外植体不仅是在存活，它们还在积极重组其结构以适应新的断裂状态。首先是收缩。在第一周内，组织的直径缩小了约 23%。随着时间推移，它趋于稳定并扭转了这一趋势。在切除后的 60 到 120 天之间，LiPfe 恢复到了最初的大小，一年后，它们比刚从宿主身上切下时还要大 12%。研究人员将这些组织定义为一种全新的生命材料，并将其命名为 LiPfe——即“活体永生 P. fabricii 外植体”。

The internals of a foot tube attached to a sea cucumber include a mix of epidermal tissue, connective tissue, a neural plexus, muscle tissue, and an inner lumen. The separated explants, though, got busy dismantling parts of themselves that were no longer useful. Muscle tissues, which initially made up 17 percent of the explant, were gradually invaded by coelomocytes that broke the muscle down into small pieces and destroyed its organization. After 180 days, the muscle tissue and the lumen had completely disappeared from the explant. 附着在海参身上的管足内部包含表皮组织、结缔组织、神经丛、肌肉组织和内腔。然而，分离后的外植体开始忙于拆解那些不再有用的部分。最初占外植体 17% 的肌肉组织逐渐被体腔细胞侵入，这些细胞将肌肉分解成小块并破坏了其组织结构。180 天后，肌肉组织和内腔已从外植体中完全消失。

In their place, connective tissue expanded to become the dominant structure. The collagen fibrils within it began bundling together, creating strong bands or striations that looked similar to the vanished muscles. By the end of the first year, connective tissue accounted for 74 percent of the explant, while the epidermal tissue thinned out to occupy just 20 percent. 取而代之的是，结缔组织扩张并成为主要结构。其中的胶原纤维开始束集在一起，形成强韧的带状或条纹状结构，看起来与消失的肌肉相似。到第一年年底，结缔组织占外植体的 74%，而表皮组织则变薄，仅占 20%。

The outward appearance changed, too. The explants shifted in color from red or orange to a lighter white or pink. Red-pigmented cells clumped into small aggregates and migrated toward the center of the tissue, leaving the outer edges of the explants increasingly transparent. In a year, LiPfe explants rebuilt themselves into alien-like translucent orbs with a large red cellular mass at their core. They didn’t develop any orifices, though, or anything even remotely resembling a digestive system. 外观也发生了变化。外植体的颜色从红色或橙色转变为较浅的白色或粉色。红色色素细胞聚集成小团块，并向组织中心迁移，使外植体的外缘变得越来越透明。一年内，LiPfe 外植体重建成类似外星生物的半透明球体，核心处有一个巨大的红色细胞团。然而，它们没有发育出任何孔口，也没有任何类似消化系统的结构。

“One of the first questions we had was how they were able to maintain cellular energy,” Jobson said. “我们最初提出的问题之一是它们如何维持细胞能量，”乔布森说。

To test how LiPfe sustains itself, Jobson’s team exposed its explants to isotopically labeled amino acids and ammonium. By six days post-excision, the tissues saw a significant spike in the absorption of dissolved amino acids. LiPfe was directly sucking nutrients from the surrounding seawater to fuel its tissue repair and survival. 为了测试 LiPfe 如何维持自身，乔布森的团队将外植体暴露在同位素标记的氨基酸和铵盐中。切除六天后，组织对溶解氨基酸的吸收量显著激增。LiPfe 正在直接从周围的海水中吸收营养，以维持其组织修复和生存。

And they could survive for a really long time. The scientists noted that some of the tube foot explants survived for years just sitting free at the bottom of holding tanks, covered in a layer of particulate matter and surrounded by other living organisms. Some were completely buried under 10 millimeters of mud and still displayed the same morphology: round shape, transparent margin, and a red core. The only thing that could apparently harm them was proximity to decaying tissues of other species. 它们可以存活很长时间。科学家们注意到，一些管足外植体在储水箱底部自由放置了数年，表面覆盖着一层颗粒物，周围环绕着其他生物，依然存活。有些被完全埋在 10 毫米厚的泥土下，仍保持着相同的形态：圆形、透明边缘和红色核心。唯一似乎能伤害它们的是靠近其他物种的腐烂组织。