## 2 Direct Communication in the Tumor Microenvironment  
Direct interactions between cells include gap junctions, which connect cells located in close proximity. Hemichannels (connexons) made of connexin proteins connect each other within the intercellular space, facilitating the transport of small molecules (approx. 1 kDa) and ions. This mechanism in the tumor microenvironment also involves RNA exchange among interacting cells [5; 6]. Primarily, gap junctions can easily regulate intercellular communication within the tumor microenvironment. This is because the direct transport between the cytoplasm of adjacent cells allows a rapid exchange of substances and a quick cell response [7]. Connexins are subject to multifactorial regulation, which affects the number and function of gap junctions. While the homeostasis between their degradation and synthesis is maintained in normal cells, it may be disrupted in the tumor microenvironment [8]. Initially, connexin proteins were classified as cancer suppressors [9]; however, an increasing number of studies suggest their significant role in tumor progression through the induction of cell migration. In colorectal cancer (CRC), heterogeneity in gap junction connections has been identified. The site of their occurrence between stromal cells determines the prognosis. Their modulating role in carcinogenesis was thus indicated. The progression of CRC has been linked to the expression of connexin Cx37 on fibroblasts present in the TME [10]. Additionally, cancer cells can influence the expression of connexins, as well as the endothelium of blood vessels, suggesting the involvement of connexins in pathological angiogenesis [11]. Direct interactions between TME cells also include the interaction of ligands with membrane receptors, which involves various types of cells. While several interactions between two cell types (ligand-receptor pairs) have been identified, the complex interactions still remain unclear [12]. Recent studies have shown the impact of ligand-receptor interactions on the infiltration of immune cells in the CRC microenvironment. Specifically, two ligand-receptor pairs (intercellular adhesion molecule 1, ICAM-1, and interleukin 2 receptor subunit alpha, IL-2 RA; ICAM-1 and integrin subunit alpha M, ITGAM) have been identified as driving the infiltration of tumor mass by dendritic cells (DCs). This effect does not necessarily correlate with immune response support, as, similar to other types of immune cells in the tumor microenvironment, DCs may exhibit impaired functions [12; 13]. Another immunotherapeutic target is immune checkpoint inhibitors (ICI), such as programmed death receptor-1 (PD-1), programmed cell death-ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) [14; 15]. Elevated levels of thymocyte selection-associated high-mobility group box (TOX), T-cell immunoglobulin and mucin domain-3 mRNA (TIM-3 mRNA), CTLA-4, V-domain Ig-containing suppressor of T cell activation (VISTA), T cell immunoreceptor with Ig and ITIM domains (TIGIT), killer cell lectin-like receptor subfamily G member 1 (KLRG1), TOX2, silent information regulator 1 (SIRT1), proliferation marker (Ki-67), and T cell activation and proliferation marker (Helios) have also been demonstrated in advanced CRC tissue, indicating them as potential biomarkers [15]. Adhesive molecules such as integrins, selectins, and cadherins mediate cell-cell and cell-extracellular matrix interactions. The migratory ability of cells is closely associated with the regulation of these proteins, specifically integrins, which may occur at the transcriptional level in CRC [16; 17; 18]. The main role of such connections is to maintain interactions between neighboring cells, resulting in changes in the expression of these molecules under the influence of environmental factors. Also, restructuring of the cytoskeleton occurs, leading to a change in the cell structure. However, due to the cytoskeletal remodeling associated with adhesive molecules, they are also responsible for changing the cell structure.