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Different Types of Collagen

Almost 3 decades have passed since we first realized that all collagen fibers within a particular organism are not made up of identical molecules. The different collagen types are usually identified using Roman numerals, assigned as they are purified and characterized.

Since this paper focuses on ligaments, we shall only describe the major characteristics of the two collagen types present in the ACL, namely Types I and III. Type V collagen seems to be present at less than one percent.

Type I Collagen

Before 1969, Type I collagen was the only mammalian collagen known. It is composed of three chains: two identical, termed α1 chains, and one different from the other two, called α2. Type I collagen is most abundant in skin, tendon, ligament, bone, cornea, etc.; it comprises between 80% to 99% of the body’s total collagen. Bone matrix is essentially all Type I collagen.

Type III Collagen

When human dermis is digested with pepsin under conditions in which the collagen molecules retain their helical conformation, Type I molecules can be separated from Type III by differential salt precipitation at pH 7.5. The Type III molecules are composed of three identical chains. Characteristic of this collagen is the presence of intramolecular disulfide bonds involving two cysteine residues close to the C-terminal region of the triple helix. Because the ratio of Type I and Type III collagen changes with age, Type III being predominant in fetal skin, this type of collagen is many times referred to as fetal or embryonic collagen.

Formation of intermolecular disulfide bridges by Type III collagen could be of great advantage during early development and wound healing, where collagen is deposited at a rapid rate in order to fill a gap.

Normal bone matrix may be the only tissue containing Type I collagen that lacks Type III collagen. Blood vessels are particularly rich in Type III collagen.

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