## 4 Cellular Component of the CRC microenvironment
### Cancer-Associated Fibroblasts (CAFs)  
Tumor-infiltrating fibroblasts, known as CAFs, are an important component of the TME. Currently, they are increasingly being paid attention to in the context of cancer progression because they extensively infiltrate cancerous tumors, which is connected with a bad prognosis [137; 138]. CAFs are a valuable source of extracellular matrix components and proteolytic enzymes (metalloproteinases). Therefore, in the tumor environment, they can impact the structure of the tumor extracellular matrix and induce metabolic reprogramming of malignant cells [139]. The key markers of CAFs typical for CRC include \(\alpha\)-smooth muscle actin (\(\alpha\)-SMA), fibroblast-specific protein 1 (FSP1), fibroblast activation protein (FAP), podoplanin, and S100A4 [140; 141; 142; 143]. In CRC, CAFs mainly originate from fibroblasts. However, precursors such as mesenchymal stem cells (MSCs) or endothelial cells (ECs) have also been identified [140]. In the case of tumor epithelial origin, fibroblasts provide tumor cells with numerous factors promoting their development. This occurs through direct connections and also via EVs [143; 88; 144]. Additionally, as they are naturally involved in the wound healing process, these cells intensively infiltrate the tumor, sustaining its structure and promoting volumetric development. The conditions of hypoxia and acidification in the TME also favor the production of factors inducing tumor progression. The secretome of CAFs in CRC mainly includes metalloproteinases, proangiogenic factors such as VEGF, proliferation-stimulating factors such as EGF, PDGF, hepatocyte growth factor (HGF), and basic fibroblast growth factor (bFGF), interleukins inhibiting the immune response, and genetic material in the form of miRNA [145; 146; 147; 148; 143]. In the process of metastasis, CAFs play a significant role through several closely related actions. One of them is the initiation of the migratory phenotype (EMT) and angiogenesis through TGF-\(\beta\), of which CAFs are a valuable source. Their high percentage in the CRC microenvironment and, consequently, the production of TGF-\(\beta\) enhance the metastatic phenotype and the expansion of cancer. Moreover, TGF-\(\beta\) mediates the recruitment of additional CAFs from the pool of MSCs, intensifying fibroblast infiltration and the autocrine production of this pro-tumorigenic factor. The strong pro-tumorigenic properties of TGF-\(\beta\) released by CAFs are evidenced by the reduced metastatic ability of CRC cells after knockdown of its gene in a mouse model [145; 149; 1]. The role of transforming growth factor \(\beta\) (TGF-\(\beta\)) is indeed highly dependent on the stage of cancer. While this factor inhibits further tumor development in the early stages, in advanced disease, it promotes further progression [150; 151]. CRC-derived CAFs are a valuable source of IL-6, which induces the production of VEGF by these cells and further enhances angiogenesis [66]. Another process involves the release of metalloproteinases, which induce the dissociation of intercellular connections (E-cadherin digestion). This results in the loosening of the tumor structure and invasion of the local environment as well as distant sites [1; 140]. CAFs are also strong producers of fibronectin, whose deposits are localized in the tumor microenvironment and can influence its modification. Moreover, the presence of fibronectin in the premetastatic niche is a strong chemotactic signal for cancer cells. Therefore, an elevated level of this marker in samples collected from CRC patients is associated with a poor prognosis and shorter survival [152; 153]. CAFs, alongside vascular cells and inflammation in the CRC microenvironment, contribute to the malignant phenotype of cancer as well as drug resistance (chemoresistance) and disease recurrence [145; 154].