Medicine LibreTexts - Characteristics of Connective Tissue
A basic picture of the structural composition of connective tissue can get left out of undergraduate study. Given its importance, it's well worth taking a few moments to get oriented.
GA Tech - Fibrous ProteinsBrief descriptions of collagen, keratin, and fibroin.
Biology LibreTexts - CollagenA basic introduction from a Bio 101 perspective.
WikiPedia - Elastin