1.1.2.1　Collagen fibers

Among dermal connective tissues，collagen fiber is the most abundant.HE staining of the collagen fiber shows a light red color，with a diameter between 2 and 15μm.The collagen fibers in the dermal papilla layer，epidermal appendages，and the vicinity of blood vessels are fine and do not go in a certain direction，while collagen fibers in other parts are combined into bundles.In the collagen bundles，there are a few fibroblasts scattered，the nucleus of which is stained darkly and the longitudinal section of which is diamond-shaped.The collagen bundles in the dermis gradually thicken from top to bottom，and the directions of the lower and middle collagen bundles are almost parallel to the leather surface and interweave with each other，extending in all directions on a horizontal surface.The collagen bundles in the lower dermis are the thickest one.Collagen fibers are formed by polymerization of collagen fibrils with a diameter of 70-140 nm.The main component is typeⅠ collagen，and a few are type Ⅲ collagen.Collagen fibers are poorly stretchable，but are tough and resistant to parallel tension.(https://www.daowen.com)