mRNA expression patterns of the IGF system during mouse limb bud development, determined by whole mount in situ hybridization

During limb development the primary limb bud requires various signals to differentiate. Insulin-like growth factor (IGF)-I and IGF-II serve as ubiquitous cellular growth promoters and are modulated by their binding proteins (IGFBPs), which inhibit or augment IGF bioavailability. This is the first study to give a complete overview of the mRNA expression patterns of Igf-1, Igf-2, type 1 Igf receptor (Igf1r) and six Igf binding proteins (IGFBP-1-6) in embryonic mouse limbs, at various stages of development, by whole mount in situ hybridization (ISH). Our results show that all the members of the Igf system, except Igfbp-1 and -6, have specific spatio-temporal mRNA expression patterns. IGFBP-2 and -5 are found in the apical ectodermal ridge (AER), and IGF-I and IGFBP-4 in the region of the zone of polarizing activity (ZPA). IGF-II and IGF1R are found in regions of pre-cartilage formation. At 13.5 days post coitus (dpc) the IGF system colocalizes with apoptosis areas, IGFBP-2, -4 and -5 are found in the interdigital zone, while IGFBP-3 and IGF-I border this region. Furthermore, IGFBP-3, -4 and -5 are found in the phalangeal joint areas, at an early stage of joint formation. This supports the hypothesis that the IGF system may be involved in chondrogenic differentiation of mesenchyme and the regulation of apoptosis in the developing limb.