Effects of the mesonephros and insulin-like growth factor I on chondrogenesis of limb explants

The mesonephros has been shown to have a growth-promoting influence in vivo on limb outgrowth. This influence has been studied in detail using an organ culture system. The results show that in the presence of the mesonephros limb explants formed larger cartilages than cultures without mesonephros. Furthermore, with mesonephros, morphology of the cartilages is comparable to that of skeletal elements in vivo while cartilages formed in cultures lacking mesonephros were amorphous. The mesonephric influence also promoted the formation of a well-organized extracellular matrix in the cartilage while cartilage in cultures without mesonephros formed an abnormal appearing matrix. Cartilage matrices in cultures with or without mesonephros were immunoreactive to type IX and type II collagens, cartilage proteoglycan PGH, and link protein although cultures lacking mesonephros had a very restricted distribution of type IX collagen immunoreactivity. Despite the different distribution of type IX collagen, long-form-type IX collagen transcripts appeared similar in both types of culture based on in situ hybridization. The mesonephric effect on limb explants could be partially duplicated by the addition of insulin-like growth factor I (IGF-I) to cultures without mesonephros. Furthermore, the mesonephric influence on cartilage growth and morphological differentiation could be blocked by the addition of a blocking antibody to IGF-I to cultures with mesonephros. The results support the hypothesis that IGF-I is one of the growth factors produced by the mesonephros which may play a role in early limb development and chondrogenesis.