One such system has been developed to study skin development and basal membrane formation between keratinocytes and the stroma. This system is used to delineate the development of basement membrane between fibroblasts in the stroma (in this case fibroblasts in a type-I collagen gel) and keratinocytes grown on top of the gel. Perlecan expression and thus basement membrane maturation is dependent on nidogen crosslinking of collagen IV and laminin γ1 chain in this system. This effect also led to a lack of hemidesmosomes in the developing tissue. Another system using a disorganized hydrated collagen I gel has been used to demonstrate that primary human corneal fibroblasts will eventually invade the gel and create a matrix consisting of collagen type I and perlecan, as well as several other sulfated matrix glycoproteins. This mimics the in vivo corneal fibroblast's developmental program and response to injury.