Usually the dense extracellular structure in fibrotic tissues is described as extracellular matrix (ECM) or simply as collagen. However, fibrosis is not just fibrosis, which is already exemplified by the variant morphological characteristics of fibrosis due to viral versus cholestatic, autoimmune or toxic liver injury, with reticular, chicken wire and bridging fibrosis. Importantly, the overall composition of the ECM, especially the relative amounts of the many types of collagens, which represent the most abundant ECM molecules and which centrally modulate cellular functions and physiological processes, changes dramatically during fibrosis progression.

We hypothesize that there are good and bad collagens in fibrosis and that a change of location alone may change the function from good to bad. Whereas basement membrane collagen type IV anchors epithelial and other cells in a polarized manner, the interstitial fibroblast collagens type I and III do not provide directional information. In addition, feedback loops from biologically active degradation products of some collagens are examples of the importance of having the right collagen at the right place and at the right time controlling cell function, proliferation, matrix production and fate. Examples are the interstitial collagen type VI and basement membrane collagen type XVIII. Their carboxyterminal propeptides serve as an adipose tissue hormone, endotrophin, and as a regulator of angiogenesis, endostatin, respectively.

We provide an overview of the 28 known collagen types and propose that the molecular composition of the ECM in fibrosis needs careful attention to assess its impact on organ function and its potential to progress or reverse. Consequently, to adequately assess fibrosis and to design optimal antifibrotic therapies, we need to dissect the molecular entity of fibrosis for the molecular composition and spatial distribution of collagens and the associated ECM.

通常，纤维化组织中的致密细胞外结构被描述为细胞外基质（ECM）或简单地被描述为胶原蛋白。然而，纤维化不仅是纤维化，已经被病毒，胆汁淤积性，自身免疫性或中毒性肝损伤引起的纤维化的形态学变化特征所证实，并伴有网状，鸡丝和架桥性纤维化。重要的是，ECM的总体组成，尤其是代表最丰富的ECM分子并集中调节细胞功能和生理过程的多种类型胶原蛋白的相对含量，在纤维化进程中会发生巨大变化。

我们假设纤维化中胶原蛋白的好坏，而且仅改变位置就可以将功能由好变坏。尽管IV型基底膜胶原蛋白以极化方式锚定上皮细胞和其他细胞，但I型和III型间质成纤维细胞胶原蛋白不能提供方向性信息。另外，来自某些胶原蛋白的生物活性降解产物的反馈回路是在正确的位置和正确的时间拥有正确的胶原蛋白以控制细胞功能，增殖，基质产生和命运的重要性的例子。例子是间质性胶原VI型和基底膜胶原XVIII型。它们的羧基末端前肽分别作为脂肪组织激素，内啡肽和血管生成调节剂内皮抑素。

我们提供了28种已知胶原蛋白类型的概述，并建议在纤维化中ECM的分子组成需要仔细注意，以评估其对器官功能的影响以及其发展或逆转的潜力。因此，为了充分评估纤维化并设计最佳的抗纤维化疗法，我们需要剖析纤维化的分子实体，以了解胶原蛋白和相关ECM的分子组成和空间分布。