Chitin

Chitin is a variety of sugars extracted from the shells of marine crustaceans and is widely found in shrimp and crabs in nature^[1]. Chitin is a linear polysaccharide of the amino sugar N-acetylglucosamine and is present in the extracellular matrix of various invertebrates, including sponges, mollusks, nematodes and arthropods, as well as fungi^[2]. The structure of chitin is comparable to cellulose, forming crystalline nanofibrils or whiskers, and is functionally comparable to the protein keratin^[3]. Chitin is a size-dependent regulator of macrophage TNF and IL-10 production^[4]. Chitin can induce M2 macrophage polarization in vivo by activating TLR2 and Dectin-1 receptor signaling pathways, promoting anti-inflammatory and tissue repair responses^[5]. Chitin can induce fibroblasts to release interleukin-8 (IL-8), which is involved in the migration and proliferation of fibroblasts and vascular endothelial cells^[6].

References:
[1] Arbia W, Arbia L, Adour L, et al. Chitin extraction from crustacean shells using biological methods–a review[J]. Food Technology and Biotechnology, 2013, 51(1): 12-25.
[2] Moussian B. Chitin: Structure, chemistry and biology[J]. Targeting chitin-containing organisms, 2019: 5-18.
[3] Ifuku S, Saimoto H. Chitin nanofibers: preparations, modifications, and applications[J]. Nanoscale, 2012, 4(11): 3308-3318.
[4] Da Silva C A, Chalouni C, Williams A, et al. Chitin is a size-dependent regulator of macrophage TNF and IL-10 production[J]. The Journal of immunology, 2009, 182(6): 3573-3582.
[5] Lee C G, Da Silva C A, Dela Cruz C S, et al. Role of chitin and chitinase/chitinase-like proteins in inflammation, tissue remodeling, and injury[J]. Annual review of physiology, 2011, 73(1): 479-501.
[6] Okamoto Y, Watanabe M, Miyatake K, et al. Effects of chitin/chitosan and their oligomers/monomers on migrations of fibroblasts and vascular endothelium[J]. Biomaterials, 2002, 23(9): 1975-1979.