Various biotypes of endogenous small RNAs (sRNAs) have been detected in human circulation, including microRNAs, transfer RNAs, ribosomal RNA, and yRNA fragments. These extracellular sRNAs (ex-sRNAs) are packaged and secreted by many different cell types. Ex-sRNAs exhibit differences in abundance in several disease states and have, therefore, been proposed for use as effective biomarkers. Furthermore, exosome-borne ex-sRNAs have been reported to elicit physiological responses in acceptor cells. Exogenous ex-sRNAs derived from diet (most prominently from plants) and microorganisms have also been reported in human blood. Essential issues that remain to be conclusively addressed concern the (a) presence and sources of exogenous ex-sRNAs in human bodily fluids, (b) detection and measurement of ex-sRNAs in human circulation, (c) selectivity of ex-sRNA export and import, (d) sensitivity and specificity of ex-sRNA delivery to cellular targets, and (e) cell-, tissue-, organ-, and organism-wide impacts of ex-sRNA-mediated cell-to-cell communication. We survey the present state of knowledge of most of these issues in this review.

中文翻译：

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人体循环中细胞外小 RNA 的来源和功能。

在人体循环中已检测到各种生物型的内源小RNA (sRNA)，包括microRNA、转移RNA、核糖体RNA和yRNA片段。这些细胞外 sRNA (ex-sRNA) 由许多不同的细胞类型包装和分泌。Ex-sRNA 在几种疾病状态下表现出丰度差异，因此已被提议用作有效的生物标志物。此外，据报道，外泌体携带的 ex-sRNA 可在受体细胞中引发生理反应。人类血液中也有来自饮食（主要是植物）和微生物的外源 ex-sRNA 的报道。仍有待最终解决的基本问题涉及 (a) 人体体液中外源 ex-sRNA 的存在和来源，(b) 人体循环中 ex-sRNA 的检测和测量，(c) ex-sRNA 输出的选择性和(d) ex-sRNA 递送至细胞靶标的敏感性和特异性，以及 (e) ex-sRNA 介导的细胞间通讯对细胞、组织、器官和生物体的影响。我们在这篇评论中调查了对大多数这些问题的了解现状。