Summary In order to investigate the effects of collagen peptide ingestion on fibroblasts and the extracellular matrix in the dermis, collagen peptide was administered orally to pigs at 0.2 g/kg body weight/d for 62 d, and its effects were compared with those of lactalbumin and water controls. Fibroblast density, and diameter and density of collagen fibrils were significantly larger in the collagen peptide group than in the lactalbumin and water control groups. The two major components of dermal glycosaminoglycans, hyaluronic acid and dermatan sulfate, which are present in the inter-fibrillar space, did not differ significantly among the three groups. However, the ratio of dermatan sulfate, which is derived from fibrilbound decorin, was largest in the collagen peptide group. These results suggest that ingestion of collagen peptide induces increased fibroblast density and enhances formation of collagen fibrils in the dermis in a protein-specific manner. Key Words collagen peptide, dermis, collagen fibril, glycosaminoglycan, electron microscopyThe skin is the largest organ in the human body and protects the body from various external insults. Skin comprises two layers, the epidermis and the dermis, consisting of stratified squamous epithelium and connective tissues, respectively. The dermis contains large amounts of extracellular matrix (ECM) components, such as collagen and glycosaminoglycans (GAG), mainly produced by fibroblasts ( 1 ). Collagen is the most abundant protein in the vertebrate body, comprising approximately 30% of the total protein. To date, over 20 types of collagen genes have been identified, and these are divided into three groups: fibrous collagen, fibrilassociated collagen and basement membrane collagen ( 2 ). In the dermis, type I collagen forms collagen fibrils that are further organized into collagen fibers in association with other types of fibrous or fibril-associated collagen ( 3 ). The size of collagen fibrils is an important factor that determines the physical nature of tissue, as collagen fibrils/fibers form the framework of the vertebrate body ( 4 ). The size of collagen fibrils varies depending on tissue type and physiological conditions ( 5 ). Their diameters are reportedly regulated, for example, by the content of collagen types III or V or other types of non-fibrous collagen. Rates of synthesis and degradation of collagen also probably determine the size of collagen fibrils ( 6 , 7 ).GAG consists of repeating two-sugar units and, with the exception of hyarulonic acid (HA), exists in tissue as proteoglycans that form covalent bonds with core proteins ( 8 ). The dermatan sulfate (DS) proteoglycan decorin is located on the surface of collagen fibrils and regulates their size ( 9 ). On the other hand, HA is highly hydrophilic and forms a gel with large amounts of water, playing a role in resisting external mechanical stresses ( 8 , 10 ). The change in the ratio of HA affects the diameter of the collagen fibrils ( 11 ). Thus, the size of collagen fibrils and the amount of GAG, such as ...