Midkine (MK) is a growth factor implicated in the development and repair of various tissues, especially neural tissues. However, its in vivo function has not been clarified.

Knockout mice lacking the MK gene (Mdk) showed no gross abnormalities. We closely analysed postnatal brain development in Mdk(–/–) mice using calcium binding proteins as markers to distinguish neuronal subpopulations. Intense and prolonged calretinin expression was found in the dentate gyrus granule cell layer of the hippocampus of infant Mdk(–/–) mice. In infant Mdk(+/+) mice, calretinin expression in the granule cell layer was weaker, and had disappeared by 4 weeks after birth, when calretinin expression still persisted in Mdk(–/–) mice. Furthermore, 4 weeks after birth, Mdk(–/–) mice showed a deficit in their working memory, as revealed by a Y‐maze test, and had an increased anxiety, as demonstrated by the elevated plus‐maze test.

Midkine plays an important role in the regulation of postnatal development of the hippocampus.