Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals.  It is the main protein of connective tissue. It is the most abundant protein in mammals,  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, and
2.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.