Collagen IV

 The extended family of collagen IV contains six chains, which are expressed in different tissues at different stages of embryonic development.

All epithelial basement membranes early in human development are composed of interconnected triple-helical protomers rich in alpha-1.alpha-1.alpha-2(IV) collagen.

Some specialized tissues undergo a developmental switch replacing a1.a1.a2(IV) protomers with an a3.a4.a5(IV) collagen network; this switch occurs in the kidney (glomerular and tubular basement membrane), lung, testis, cochlea, and eye, while an
a5.a5.a6(IV) network appears in skin, smooth muscle, and esophagus and along Bowman's capsule in the kidney.

This switch probably occurs because the a3.a4.a5(IV) network is more resistant to proteases and ensures the structural longevity of critical tissues.