It is the main protein of connective tissue. It is the most abundant protein in mammals

Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals. [1] It is the main protein of connective tissue. It is the most abundant protein in mammals, [2] making up about 25% to 35% of the whole-body protein content.Collagen has been widely used in cosmetic surgery, as a healing aid for burn patients for reconstruction of bone and a wide variety of dental, orthopedic and surgical purposes. Some points of interest are:1.when used cosmetically, there is a chance of allergic reactions causing prolonged redness; however, this can be virtually eliminated by simple and inconspicuous patch testing prior to cosmetic use, and2.most medical collagen is derived from young beef cattle (bovine) from certified BSE ( Bovine spongiform encephalopathy) free animals. Most manufacturers use donor animals from either "closed herds", or from countries which have never had a reported case of BSE such as Australia, Brazil and New Zealand.3. porcine (pig) tissue is also widely used for producing collagen sheet for a variety of surgical purposes.4.alternatives using the patient's own fat, hyaluronic acid or polyacrylamide gel are readily available. 

Collagen