Restricted expression of Xenopus midkine gene during early development.

Midkine (MK) is a heparin-binding growth factor and forms a novel protein family together with another member, pleiotrophin (PTN)/heparin-binding growth-associated molecule (HB-GAM). A cDNA clone isolated from Xenopus laevis specifies for the Xenopus counterpart of MK (XMK), and the mode of XMK expression was studied by in situ hybridization and Northern blot analysis. XMK was first expressed at stage 11 (middle gastrula) and was located in the neural anlage at stage 12 (late gastrula). Through stage 13 to 15 (early neurula), XMK expression was restricted to the neural folds. At stage 23 (tailbud stage), XMK was predominantly localized in the brain and neural tube. At the larva stage, XMK expression was again restrictedly observed in the brain, the optic vesicles, the otic vesicle, and the spinal cord, all of which are derivatives of the neural tube, as well as in the branchial arches, derivatives of the cranial neural crest. Comparing the mode of MK expression between Xenopus and the mouse, we propose that MK plays evolutionally conserved roles in neurogenesis and development of the craniofacial architecture of ectomesenchymal origin. We also found that XMK was expressed in various adult organs; strong expression was observed in the brain, the eye and the spinal cord, all of which showed intense MK expression at the larva stage.