Distribution of insulin-like growth factor peptides in the developing chick embryo

Growth factors are likely to be of major significance in developmental biology. Here, the distribution of insulin-like growth factor (IGF) peptides is described during development of the chick embryo. IGF was immunolocalised using a polyclonal antibody to human IGF I detected with a modified Vectastain ABC procedure. Under the conditions used, the antibody binds strongly to IGF I and weakly to IGF II; thus the distribution of IGF peptide, rather than the individual factors, is described. Muscle, peripheral nerve and the notochord were labelled whenever present. Muscle label was associated with the myotubes and neural labelling with neurons; Schwann cells were unlabelled. IGF distribution changed during differentiation of connective tissues. Regions of mesenchyme destined to form cartilage labelled weakly or not at all, and cartilage condensations were unlabelled. In the limb, chondrocytes became labelled once cartilage rudiments had formed; however, in later development, label was absent in zones of rounded and flattened chondrocytes and appeared strongly at the onset of hypertrophy. Early osteogenic mesenchyme was also unlabelled, although later bone cells were strongly stained. In the neural tube, label was associated with differentiating neuroblasts and cell bodies and with axons, especially in the developing dorsolateral tracts. These results show a possible correlation between IGF label and cell division in early mesenchyme; cartilage condensations, which have reduced mitotic indices, do not label. In other tissues, notably muscle and nerve but also later connective tissues, label is associated with differentiating, rather than dividing, cells.