The purpose of this review is to outline our current understanding of cardiac fibroblast subtypes and describe how these cells potentially contribute to cardiac repair and disease. Three different means of classification are outlined: anatomic location, embryonic origin, and gene expression/function.
❤ 成纤维细胞可以根据解剖学定位，胚胎来源，基因表达及功能这三方面来进行分类，单细胞测序就可以从这三方面进行分析，这样就可以增加成纤维细胞的分析度
MESENCHYMAL CELLS IN THE UNINJURED HEART
成纤维细胞是未受损心脏中的间充质细胞。还有其他的间充质细胞，包括Cardiac fibroblasts express discoidin domain–containing receptor 2 (DDR2) (5), platelet-derived growth factor receptor alpha (Pdgfrα) (4–8), Tcf21 (4, 8–10), and vimentin (11). Pericytes express NG2 (12), platelet-derived growth factor receptor beta (Pdgfrβ) (12), and Tbx18 (13). VSMCs express α-SMA (14) and Myh11 (15). Some of these markers exhibit overlap between these three cell populations. For example, α-SMA can be found in subsets of pericytes and activated cardiac fibroblasts
CLASSIFYING FIBROBLAST POPULATIONS IN THE HEART
Cardiac Fibroblast Subtypes by Location
Figure 1 Anatomic location of cardiac fibroblasts. Fibroblasts reside in distinct anatomic locations in the heart, and each population of fibroblasts is likely to express a different transcriptional profile. Atrium-transverse section of atria where cardiomyocyte fibers are thinner and less dense compared to the ventricles. The fibroblast to cardiomyocyte ratios in atria have not been clearly documented. Adventitia-transverse section through a coronary artery. The number of vascular smooth muscle cells (VSMCs) can vary depending on vessel diameter. Cell types other than fibroblasts have also been noted in the dense collagen surrounding the vessel. The annulus fibrosus is a collagen-rich area surrounding the conduction system. Fibroblasts are present in these regions. Valves: Fibroblasts reside in three layers (fibrosa, spongiosa, and ventricularis). These are composed of collagen, proteoglycans, and elastin, respectively. Interstitium: This longitudinal section of myocardium would be found in the ventricles or ventricular septum.
- 心房（心肌细胞和成纤维细胞比例不清楚）
- 冠状动脉周围
- 纤维环（这里富含胶原纤维）
- 瓣膜（在这里成纤维细胞分为三层）
- 间质（心室或者室间隔）
成纤维细胞可能不仅取决于它们的位置（心室或隔膜），而且还可能由于靠近血管或神经纤维而有所不同
❤没有空间转录组，仅用单细胞转录组可以达到分析第3,4,5的效果吗，感觉挺有意思的如果做出来。主要是空间转录可以把成纤维细胞弄出来，但是普通的单细胞转录组还要知道标记基因是什么
Cardiac Fibroblasts Subtypes by OriginEpicardial fibroblasts.
Epicardial fibroblasts
大多数成纤维细胞起源于心脏的外间皮覆盖，即心外膜，但室间隔和左心室内的一些成纤维细胞来源于心内膜。这两个群体之间没有明显的行为差异，但在根据发育起源进行谱系追踪和基因缺失时必须谨慎，因为Cre重组酶可能不会在两个群体中表达。
a fraction of epicardial cells undergo an epithelial to mesenchymal transition to form a majority of coronary VSMCs and cardiac fibroblasts (35, 36). These epicardial derivatives contribute to the fibroblasts of the ventricles and atria (8, 37), the cardiac annulus fibrosus (38), and atrioventricular valve leaflets (39). Development of epicardial-specific Cre recombinase mouse lines, including Gata5-Cre and Wt1CreERT2 (37), led to the discovery that loss of either Pdgfrα (6) or Tcf21 (8) resulted in an absence of cardiac fibroblasts within the ventricular walls.
Endocardial fibroblasts
两个成纤维细胞群体在基因表达方面趋同，刺激，而不是它们的起源，是控制其纤维化反应的主要因素。
New Fibroblast Subtypes
单细胞测序原则上是可以把细胞进行无限的分类的，这篇文章里面描述的是大家发现的一些具有共性的细胞群。
1.下面这两种通过不同的提取成纤维细胞的方法，找到了共性的细胞群体
characterize mesenchymal populations
two populations of Col1a1-expressing fibroblasts exist. One population expresses markers that have been reported in the past, including Pdgfrα and Tcf21.
A second group expressed Col1a1 transcripts but lacked Pdgfrα and Tcf21. The most distinguishing genes for this subset of fibroblasts were Wnt pathway genes Dkk3, Wif1, Tbx20, and Frzb.
using Pdgfrα expression to identify cardiac fibroblasts
also identified a subset of fibroblasts with a similar transcriptional profile. This population was termed Wntx, and further examination suggested that* Wnt pathway–related genes were more highly expressed in this population than the other fibroblast subsets.* Periostin expression, *usually associated with fibroblast activation, was also present in this population. *At baseline and three days post–myocardial infarction, these cells constituted 6% and 10% of the Pdgfrα-expressing fibroblast population, respectively
2. 静息成纤维细胞的表达共性
静息成纤维细胞群之间的第二个区别是Sca1表达。在基线时，大约一半的表达Pdgfrα的成纤维细胞具有低水平的Sca1表达，而大约30%的细胞表达高水平的Sca1（43).基因个体发生分析表明，高Sca1群体富集了增殖和干性基因，包括Thy1和Cd34，而低Sca1细胞富集了Bmp4和ApoE等细胞信号基因。先前的报告表明，高Sca1群体含有多能祖细胞，这些祖细胞能够在体外自我更新和分化为脂肪细胞，平滑肌细胞和心肌细胞（44,45).类似的成纤维细胞群，称为纤维脂肪细胞祖细胞，或FAPS
3·损伤后成纤维细胞类型
增殖成纤维细胞：Mki67 是一种常见的增殖基因。
炎症中的成纤维细胞： Fibroblasts also secrete a variety of cytokines, growth factors, and chemokines that modulate inflammatory cells. For example, interleukin (IL)-1ra (60), IL-1β (61, 62), IL-6 (61–65), INF-γ (60), TNF-α (61, 62, 65), MCP-1 (62, 64, 65), GM-CSF (66), IL-9 (60), IL-10 (60, 64, 65, 67), IL-11 (55), IL-12 (65), IL-23a (68), CCL5 (68), and TLR4 (69) are upregulated in fibroblasts after in vitro activation or cardiac injury, suggesting that fibroblasts directly control inflammatory cell activity within the infarct region (reviewed in 70).
❤除非成纤维细胞是唯一分泌蛋白质的细胞，否则这些研究仅表明信号通路的重要性，而不是成纤维细胞在炎症反应中的特定作用
成纤维细胞和细胞外基质产生：
Historically, the term for an activated fibroblast has been myofibroblast. This cell nomenclature is rooted in the observation that populations of cells within a zone of injury or inflammation establish actin stress fibers that contain** α-SMA or SM22**. While expression of these contractile proteins is one reliable means to identify myofibroblasts, VSMCs also express these proteins, and distinguishing between the two cell types may be difficult.
Periostin is a matricellular protein that is not present in resting or quiescent fibroblasts, but after injury it is rapidly, specifically, and robustly expressed by cardiac fibroblasts in regions of matrix reorganization.
表明每个纤维化模型可能有自己的时间和空间成纤维细胞基因表达谱
one population expressed cell cycle genes denoting proliferative cells.
A second population that clustered with the cycling fibroblasts* expressed activation markers such as periostin and α-SMA but no cell cycle genes*. These cells were thought to be transitioning toward the proliferative state.
Seven days after myocardial infarction two additional populations emerged that were thought to be myofibroblasts. One population was considered antifibrotic while the other population expressed ECM-related genes and was termed profibrotic. Additional analyses with more limited cell numbers did not detect these specific subsets, but similar gene profiles were observed in all of the activated populations (73).
In a spontaneous mouse model of fibrosis, single-cell sequencing also identified subpopulations of fibroblasts that could be distinguished as ECM remodeling, Tgfβ1 signaling, Wnt signaling, and proliferation.
基质修饰基因的成纤维细胞表达可在成纤维细胞增殖期后数周发生。一种表达谱被描述为一种特殊的基质产生细胞，即母纤维细胞。这些细胞被鉴定出来，因为它们在大多数活化的成纤维细胞发展出基因表达的静止谱或经历程序性细胞死亡（54).该母纤维细胞群强烈表达先前与骨和软骨重塑相关的基因，包括Chad，Cilp2和Comp。
成纤维细胞衰老:与炎症和衰老的关系
与纤维化有关的其他细胞类型：
心外和内皮细胞 已经提出了肌成纤维细胞的几种来源，包括纤维细胞、巨噬细胞、骨髓来源的细胞（79–81）和内皮细胞（82).现在多条证据表明，这些来源可能不会对受损心脏中的基质沉积有显着贡献。
血管周围细胞 间充质基质/干细胞
成纤维细胞亚群可能因心脏损伤的类型和程度而异
图2 成纤维细胞在休息时和受伤后分期。几项研究发现了静息心脏成纤维细胞的三个亚群。损伤或感染后或衰老过程中，成纤维细胞响应炎性细胞因子或心肌细胞分泌因子而被激活。成纤维细胞进入增殖阶段，这可能与细胞外基质（ECM）的产生重叠。成纤维细胞的扩大群体可以分为至少四个不同的亚群。肌成纤维细胞是α-SMA，而α-SMA+−细胞可能促进血管生成。扩增的成纤维细胞有多种拟议的命运。一些恢复到静止的成纤维细胞，而另一些则表现出独特和持续的ECM产生阶段。其他人要么经历细胞凋亡，要么变得衰老。尽管多项研究暗示成纤维细胞在调节炎症方面的作用，但单细胞测序实验尚未描述炎症基因表达谱。在每个成纤维细胞期下方是一些可能由该阶段的大多数细胞表达的标志物。
