Expression of insulin-like growth factor I in developing lens is compartmentalized

We, and others, have recently reported that insulin-like growth factor I (IGF-I) mRNA is expressed in multiple tissues during embryogenesis and in whole embryos during early organogenesis. Therefore, it is likely that, in addition to any effect on embryo growth, IGF-I plays a paracrine/autocrine role in development. The embryonic chicken lens, an avascular organ composed by a single type of cell that undergoes differentiation in vivo and in vitro, is an ideal model to characterize the paracrine/autocrine action of IGF-I. The lens cells express IGF-I receptors, and respond to exogenous IGF-I with induction of fiber cells differentiation and stimulation of delta-crystallin gene transcription. Whether embryonic lens cells express IGF-I was uncertain. In the present study, we used a sensitive semiquantitative method (reverse transcription of RNA followed by amplification with the polymerase chain reaction) to analyze IGF-I gene expression. An amplified product of the expected length (209 base pairs) was found in days 8, 12, 15, and 19 lenses. At all embryo ages studied, the product was more readily detected in the lens than in the liver, while in eye tissues (excluding lens), IGF-I expression was relatively high. After microdissection of the epithelial cells from the fully differentiated fiber cells, IGF-I expression was detected exclusively in the epithelial cells. IGF-I immunoactivity was found using high performance liquid chromatography followed by radioimmunoassay in the days 8-19 lens extracts, and in primary cultures of isolated epithelial cells. Our previous and present findings show that the lens has all the elements for IGF-I autocrine/paracrine action in development.