## 2 Microplastic and Nanoplastic Uptake
### Intestinal Absorbtion  
Many studies have shown that plastics can be absorbed in the intestines--either in the small or large intestine. The uptake of plastic particles in the gut is influenced by their size, shape, concentration, charge, and chemical composition. Plastic particles need to pass through the mucus layer in order to reach the epithelium of the intestine. The intestinal epithelium is composed of enterocytes, mucus-producing goblet cells, and microfold (M) cells [77; 78]. Different mechanisms can cause the absorption of plastic particles, and it can be based on the size of the particles. These mechanisms include endocytosis mediated by enterocytes and transcytosis via microfold cells, also known as M cells, found in a subset of intestinal epithelial cells in gut-associated lymphoid (GALT) tissue, known as Peyer's patches. Persorption is the passage through gaps at the tips of the villi following the loss of enterocytes and paracellular uptake [79; 80]. Because Peyer's patches have a high proportion of M cells, they are the main sites of microplastic absorption. Furthermore, the intestinal absorption of particles can lead to toxicologically relevant systemic exposure. Due to the small size of nanoplastics, they can penetrate deeply into organs [81]. M cells are specialized epithelial cells that are missing the microvilli found on other intestinal epithelial cells. Conversely, they possess broader (micro-)folds and a more delicate luminal surface, facilitating the rapid absorption of particles within the intestine [82]. Figure 2 shows the pathologic pathways through which MPs enter the gut. The gastrointestinal mucosal layer serves as the primary barrier against foodborne pathogens, toxins, and antigens. Celiac disease, colon cancer, and inflammatory bowel disease are all examples of diseases that can result from barrier defects caused by GI mucosal damage [83]. The gut microbiome is an essential element for gut health. It plays a vital role in supporting digestion, nutrient absorption, barrier function, and immunity [84]. The gut microbiome is sensitive to diet, drugs, and pollutants. Changes in it are linked to many diseases, including cancer, autoimmune diseases, diabetes, obesity, and cognitive dysfunction [85; 86]. Malnutrition and diets high in saturated fat and high-fructose carbohydrates can lead to increased gastrointestinal permeability in the mucosa [87]. During inflammation, epithelial barriers become more permeable, leading to increased translocation [88]. [PERSON] et al. (2016) conducted a study on European sea bass (Dicentrarchus labrax, body size = 140 \(\pm\) 8.42 g) that were fed feed containing 0.3 mm PVC plastic particles for 30, 60, and 90 days. Observations of histological sections of the intestinal segments revealed several pathological lesions. These lesions were associated with inflammatory, circulatory, and structural changes, indicating that PVC had translocated into the intestine and caused harmful effects on the mucosal and submucosal musculature of the three sections of the intestine [89]. Another study was performed on zebrafish, Danio rerio, by [PERSON] et al. (2016), who monitored the effects of the absorption and accumulation of PS in the gut, gills, and liver. The researchers noted that PS particles with a size of 5 um were found to accumulate in the gut, gills, and liver, whereas PS particles of 20 um were found to accumulate only in the gills and gut. The team conducted a histological analysis of the liver and found 5 um and 70 nm PS particles in the tissue, along with some pathological lesions, inflammation, lipid accumulation, and alterations in oxidative stress proteins [90]. MPs/NPs accumulate in various organs, including the gastrointestinal tract, respiratory system, reproductive system, nervous system, hemolymph, embryos, liver, kidneys, and spleen [91; 92]. A team of researchers led by [PERSON] recently found microplastic particles in the placentas of six consenting volunteers.